connection with the shock has been gotten rid of, which is a more or
less gradual process.

CHAPTER XIII

THE BODY FLUIDS

We have learned to think of the cells which make up the body as
dependent on the fluid which surrounds them for the various materials
they require, and as a place into which they discharge the products of
their metabolism. We have seen furthermore that the fluids which bathe
the cells directly must be constantly renewed. The renewal is
accomplished by interchanges between this fluid and the blood, which
constantly flows through the tiny blood vessels that are everywhere
present in the body. In its course, in turn, it passes through the blood
vessels of the organs in which it is to be itself renewed; the digestive
organs for food supplies, the lungs for oxygen, the kidneys for the
discharge of waste material. We must now look further into the nature
and action of the various body fluids. Of course the foundation of all
of them is water. In this water must be dissolved everything that is
used by any of the cells for food or anything that any of these cells
produces. Under this latter head we have the waste products of ordinary
metabolism, or in the case of some cells special products of functional
metabolism. The presence of all these various materials would be bound
to make the body fluid an extremely complex mixture. In addition to
these various materials there are certain substances present besides
water to make up what we may call the structure of the body fluids as
distinguished from the materials which they are carrying from one place
to another. These structural materials include a number of salts of
which ordinary table salt (sodium chloride) is the most abundant as well
as the most familiar. In addition there are salts of lime and potash and
magnesium; all these latter in very small proportions. Just why the body
fluids should contain these salts is not very clear. We know that if
they were not present the cells of our bodies could not live, yet it is
true that there are a great many kinds of living cells that get along
perfectly well in fresh water, which may have no salts dissolved in it
at all. On the other hand there are more kinds living in the ocean and
exposed to the rather strong concentration of salts which make up ocean
water, and there are even some kinds of animals that live in strong
brine, so that evidently living protoplasm can adjust itself to
surroundings in which the strength of the salt in solution is widely
different. It is true that very few kinds of animals can endure being
changed suddenly from ocean water to fresh water or the reverse. One of
the best ways to clean the bottom of a ship that has become foul through
long sailing about the sea is to transfer it into a fresh water lake or
stream, where the accumulation of living animals and plants will be
killed and will drop off. Most kinds of ocean animals die rather
promptly if changed to fresh water, or fresh water animals if put into
the ocean. There are a few kinds of fish, like the salmon and shad,
which live in the ocean but lay their eggs in rivers, and these are able
to endure the change from the one kind of water to the other without
being destroyed. They, to be sure, make the transition gradually,
swimming up from the ocean into water that is less and less salty, until
they finally reach the fresh water stream itself. There are other kinds
of living things which can endure a much more abrupt change from salt to
fresh water and back again. In various parts of the world are large
rivers emptying into the ocean and so situated relative to towns and
cities that steamers make regular round trips from the ocean up to the
fresh water of the river and back again, and it will be found that on
the bottoms of these steamers are various plants and water animals which
endure the frequent shift from salt water to fresh and back again
without harm.

The percentage of salt in the body fluids of all the higher animals
including ourselves is only about one-fifth that in the ocean.
Furthermore the fluids of most kinds of land animals have about the same
percentage of salts dissolved in them. Naturally there has been much
speculation as to why there should be this percentage of salt in
preference to any other. One ingenious theory supposes that back in the
beginning of things, when the earth cooled down below the boiling point
of water, so that it was possible for water to collect on the earth, the
water in all the oceans was fresh. This would have to be true, since
water must have fallen in the form of rain; but in course of time some
of the salts in the earth’s crust would be dissolved, making the water
salty, and as time went on the ocean would become saltier and saltier.
This is still happening, for every river that discharges into the ocean
carries with it materials that it has dissolved from the underlying soil
during its passage from its source, and such material, when it once
enters the ocean, must stay there until the ocean water becomes
saturated with that particular substance. According to this theory there
was a time, then, when the ocean water was just about as salty as our
body fluids at the present time, and it supposes also that that was the
time when the ancestors of present land animals crawled out of the ocean
and took up their abode on land. Of course there is no way to prove that
this is so, but it does account for the particular percentage of salts
in our bodies as well as any other explanation we know anything about.

In addition to these various salts our body fluids contain in solution
moderate amounts of very complex chemical substances belonging to the
class of proteins. A fact about proteins which has not yet been
emphasized is that they make liquids in which they are dissolved sticky
or gelatinous. An excellent example of this is ordinary raw white of
egg, which is a solution of protein in water, and which shows the
gelatinous character very strikingly. Because of the protein that is in
solution in the body fluids, they have also this gelatinous character,
although to a much less extent than in the white of egg, because the
amount of protein in solution is so much less. As we shall show later,
this sticky quality of the fluid is of a good deal of importance in its
actual use in the body. It may be that the protein in the body fluids
serves other purposes as well. One interesting fact that needs to be
emphasized is that it is not used for fuel or building material for the
cells. The protein that comes to them as part of their food supply is
entirely distinct from that about which we are now talking, which is
part of the permanent structure of the body fluids. We shall speak of
the protein that serves to nourish the cells in a moment, when we are
talking about the relations of the fluids to the transportation of food
material.

In addition to the salts and proteins we have also dissolved a great
many very complex materials which may be looked upon as permanent or
relatively permanent constituents of the fluid, but about which we know
practically nothing chemically. We are sure that they are present,
because of certain effects which they produce, but the substances
themselves have never been made out by chemical analysis. These are
materials which are concerned with the resistance of the body to
infectious disease, and it will be necessary to say just a word about
infection to make clear the part played by them.

What we call an infection is the invasion of the body from the outside
by minute living organisms, either plant or animal, and the establishing
of them within the body, so that they grow and multiply. They carry on
their metabolism just as do all other living cells and produce various
chemical products as a result. There are many organisms living within
our bodies whose metabolic products apparently do us no harm, and so we
serve as hosts for these unbidden guests year in and year out without
even knowing of their existence. The products from other kinds of
organisms are poisonous to us, and when some of these organisms multiply
within us we discover it, because we are poisoned and become ill. Only
organisms whose products of metabolism are poisonous are counted
ordinarily as causing infection. In the strict sense we might be said to
be infected by the harmless organisms of which so many thrive within us,
but in the usual use of the word we speak of one as having an infection
only when his body has been invaded by organisms that produce poison. In
the case of most kinds of injurious organisms their multiplication
within the body unchecked would lead finally to its destruction. It is
necessary, therefore, that the body have some means either of checking
the development of the organisms or of neutralizing the poisons which
they produce. The body has this power, and the machinery for it consists
very largely of substances or structures in the body fluids. The
detailed story of these is too complex to be told here. We shall content
ourselves by saying that when an infection becomes established, as of
scarlet fever, for example, the poisons that are produced are poured out
by the organisms into the body fluids of the part where they happen to
be located, and are taken up from there by the blood and distributed all
over the body. The fever, headache, and other disagreeable symptoms are
due to these poisons. The interesting thing about it is that the poisons
themselves act toward the body as chemical regulators or hormones,
exciting some or perhaps all of the cells of the body to a special kind
of functional metabolism, which results in the manufacture of materials
which neutralize the poisons. Thus, one of the very important properties
of living protoplasm is to respond to the poisons from the metabolism of
other cells by producing neutralizing material. Whether one dies from an
infection or recovers from it depends on whether the cells are able to
produce enough neutralizing material to prevent themselves from being
killed or whether the poison is so abundant or so malignant that the
cells are destroyed in spite of their activity in pouring out the
neutralizing material.

Still another interesting thing about this whole matter is that every
kind of infecting organism has its own kind of poison, which differs
from that of the other kinds, and so the chemical effect of the poison
upon the cells is not the same for one infection as for another. The
functional metabolism of the cells in turn is adjusted to the kind of
poison, so that the material they pour out is suitable to neutralize the
particular poison which aroused them to activity in the first place, and
in most cases no other. If one gets well from any infection, there is a
surplus of the neutralizing material left in his body fluids, and, as
long as it remains, he is secure from another infection of the same
kind. This condition is defined as immunity. Since the neutralizing
materials are different for different infections, immunity against one
is in most cases of no avail against another. One may be immune against
scarlet fever, but be just as likely to catch pneumonia as a person who
has never suffered from any infection at all. It follows that an
individual who has had and recovered from a great many infections has a
correspondingly large assortment of neutralizing materials in his body
fluids. Some of these appear to persist throughout life, others
disappear fairly soon.

The next group of permanent constituents to be described consists of
some materials which seem to have nothing at all to do as long as
everything is going well: the body fluids bathing the cells, or, in the
case of the blood, circulating about from part to part through the blood
vessels. These come into play only when, as the result of injury, the
fluids begin to escape; namely, in the case of bleeding, or, as it is
technically called, hemorrhage. It is clear that if any injury is
started sufficient to allow the fluid to escape, there would be no more
reason why it should stop running out, unless prevented, than there is
why water should stop running out of an open faucet of itself. Since, as
we know, we do not bleed to death every time we get a slight cut, but
after a long or shorter time the bleeding stops of itself, there must be
some automatic arrangement by which the opening is plugged. All of us
are familiar with the way in which this is done. We know that in the
course of a few minutes after the bleeding begins, the blood tends to
set into a firm jelly, which is called the _clot_. This clotting is the
result of a chemical transformation which goes on in the blood as the
result of its escape from the blood vessels and its exposure to the
outside. The details of the chemical processes are too complicated to be
described here; all we need to remember is that the blood within the
body contains certain special materials which are soluble and therefore
float in solution along with the other soluble materials. When any blood
vessel is injured, so that the blood begins to escape, a series of
chemical changes is started automatically by which this soluble material
is changed and becomes insoluble, so that instead of remaining dissolved
in the blood it is precipitated out. In this form it is called _fibrin_.
Fibrin is a very sticky, stringy mass which forms a spongy network,
spreads itself over the injury, and clings firmly to the edges, thus
plugging the opening, unless the rush of blood is so strong that it
keeps washing the fibrin away as fast as it is formed. Bleeding from
small wounds will presently stop of itself, but if the hemorrhage is too
great for this, it is necessary that the blood flow be slowed down or
stopped artificially. This is done by locating and pressing upon the
large blood vessel through which the blood is escaping. In order to do
this successfully, one has to know something about the course of the
blood vessels, and this will be described in a later chapter.

There are a few people whose blood lacks some of the necessary chemical
substances to enable it to clot; such persons are known technically as
“bleeders.” Even a slight injury in one of them will cause serious, or
even fatal, hemorrhage, unless the escape of blood is stopped
artificially, since it will not stop of itself. An interesting fact
about this condition is that it runs in families; in other words it is
hereditary.

In addition to all these constituents of the body fluids which are
_dissolved_ in them, there are in that part of the fluids confined to
the blood vessels, which we call the blood, three kinds of structures
floating; these we have next to describe briefly. The first of these are
the _red corpuscles_. They give the blood its red color, although if
looked at singly they appear yellowish rather than red. Red corpuscles
are almost inconceivably tiny. They are red flexible disks, a little bit
thinner in the middle than at the edges, about one three-thousandth of
an inch in diameter. Some idea of the enormous numbers in the body can
be gathered, when we say that a drop of blood the size of the head of a
pin would contain four or five million of them. The red corpuscles are
made up of a sort of framework of protein within which is inclosed a red
coloring matter or pigment, known as _hemoglobin_. It is this pigment
that gives the blood its color, and in some respects it is one of the
most important of the nonliving substances in

[Illustration: STRUCTURE OF A DROP OF BLOOD AS SEEN UNDER THE MICROSCOPE

Two white (colorless) corpuscles _a_ appear. The remainder are red
corpuscles sticking together, forming _rouleaux_. (From Martin’s “Human
Body.”)]

the body. This is because it is the means by which the cells obtain
sufficient supplies of oxygen. As we have already seen, every cell is
constantly drawing from the body fluids about it the oxygen which is
required for carrying on its metabolism. The fluids in turn get oxygen
from the blood. It is necessary, therefore, for the blood to convey
abundant supplies. Oxygen will dissolve in water, as is proven by the
fact that fish and other aquatic animals are able to get enough oxygen
from the water in which they live to serve their needs; but it is not
sufficiently soluble to supply the needs of an active body like that of
man. It is necessary, therefore, to have a special additional means of
conveying oxygen besides its simply dissolving in the blood. This
additional means is furnished by the hemoglobin, which is an
“oxygen-carrying” pigment. What this means is that the hemoglobin has
the property of taking up oxygen chemically, whenever it is exposed to a
region where there is oxygen in abundance, and of giving it up again
whenever it passes through a region where oxygen is scarce. It is thus
that oxygen is conveyed from the lungs to the active tissues of the
body. We shall have more to say about this in the chapter devoted
especially to the matter of the oxygen supply.

We said a moment ago that the red corpuscles consist of a protein
framework inclosing hemoglobin. They are not living. They must,
therefore, have been made by living cells and poured out into the blood
stream. We might suppose that this was done once for all and that the
same red corpuscles are floating in our blood now that started floating
there when the blood was first formed; but, as a matter of fact, this is
not the case. There is a continuous breaking down of red corpuscles
which must be made good by a continuous manufacture of new ones. Most of
the larger bones in our bodies have a sort of spongy framework by which
the ends, where the joints are, are made stronger. Within the space of
these frameworks is a kind of marrow, known as _red marrow_, because it
has such a very abundant blood supply. It is in this red marrow that the
manufacture of red corpuscles goes on. There are throughout the red
marrow living cells which are constantly dividing and subdividing,
forming more and more so-called _daughter cells_. Within these daughter
cells hemoglobin is presently deposited; a little later they lose the
nucleus and probably the remainder of the living protoplasm as well,
leaving just the framework of nonliving protein with its contained store
of hemoglobin. This is the finished red corpuscle, and it breaks loose
from the red marrow and floats out into the blood stream. The rate of
manufacture of red corpuscles is very rapid; undoubtedly millions of
them are formed daily in the various red marrow regions of the body.
The total number of red corpuscles does not increase correspondingly,
because they are broken down at the same rate as they are formed. It is
now believed that in the _spleen_, which is a large organ of the abdomen
whose function has always been obscure, those red corpuscles which are
destined for destruction are picked out of the blood and broken down. We
commonly suppose that this fate overtakes corpuscles that are worn out
and are no longer efficient oxygen carriers, but we do not know, as a
matter of fact, that the corpuscles do lose their effectiveness in
course of time, nor have we any idea how the spleen could select out of
the millions in the blood those particular ones which are no longer
useful.

What the spleen does to the corpuscle is to break it up so that the
protein and the hemoglobin in it are set free in the blood stream. We do
not know what becomes of the protein; probably it is taken up and
utilized. We do know that the hemoglobin is decomposed in the liver. One
constituent of hemoglobin, in fact the constituent which gives it its
ability to carry oxygen, is the element iron. Iron is not particularly
abundant in living things, and we find that the body is thrifty with
regard to it. When the liver decomposes the hemoglobin, the iron is
saved in some way which enables the blood to carry it back to the red
marrow, where it can be used over again. There are also some portions of
the hemoglobin which are valuable as food material; the remainder, which
is of no further use, is discharged from the body as a part of the bile.

Besides the red corpuscles the blood contains what are known as the
white or, better, the colorless corpuscles. These, instead of being dead
structures, as are the red corpuscles, are actual living cells, which
float along in the blood stream or have the power of clinging to the
walls of the blood vessels and crawling along them; in places where the
blood vessels are very thin, they work their way right through, and so
get out into the spaces among the tissues. There are not nearly so many
colorless as red corpuscles, in fact, the latter are eight or nine
hundred times as numerous as the former. A great deal of study has been
devoted to colorless corpuscles to find out just what they do. We are by
no means certain that we understand fully all their activities, but we
do know that one very important thing that they do is to absorb into
their own bodies tiny foreign particles that may be present in the blood
stream or in the spaces among the tissues. There are not many kinds of
foreign particles that can get into these places. In fact, about the
only sort that can are the tiny plant or animal cells which are
responsible for disease. When these foreign organisms invade the body,
the colorless corpuscles may engulf them into their own bodies and
destroy them. In this way a great deal of infection is prevented. Of
course it may happen that the invading organisms are so numerous that
the colorless corpuscles can not get rid of all of them, or the
corpuscles themselves may show a diminished activity. It is an
interesting fact that both the number of colorless corpuscles in the
body and the vigor with which each colorless corpuscle attacks foreign
organisms vary from time to time, so that we are much more secure
against infection at some times than at others. In general we may say
that when the body is in a condition of vigorous health the colorless
corpuscles will be efficient. When we are run down, on the other hand,
these cells share with all other body cells this state of low vitality.
This explains why people who allow themselves to become run down are
more likely to fall victims to infection than those who are in vigorous
health, and emphasizes, of course, the importance of a habit of life
that shall keep the body vigorously healthy at all times. It should be
understood that the colorless corpuscles do not show the same
effectiveness against all kinds of foreign organisms; some kinds of
disease germs are able to bring about infection in the body quite
regardless of the activity of the colorless corpuscles. From other
kinds, on the other hand, they give the body very complete protection.

The most familiar example of the action of the colorless corpuscles is
in the formation of what we all know as pus. There are a few kinds of
organisms that, instead of getting into the body and becoming scattered
through its fluids, establish themselves at certain points and by growth
and multiplication accumulate at those places in large numbers. Examples
are pimples and boils. In these cases the pus-forming organisms have
located just under the skin and are multiplying there at a great rate.
They produce poison which is absorbed from the place where they are and
distributed through the body. This poison appears to have some sort of
chemical attraction for the colorless corpuscles; at any rate the
corpuscles gather from all around to the place where these organisms are
located and engulf as many of them as they are able, but in so doing
they themselves are destroyed, and pus, as we know it, is simply a mass
made up of the dead bodies of the colorless corpuscles along with the
organisms which they have destroyed and which in turn have destroyed
them. In the ordinary pimple or boil the colorless corpuscles ultimately
win the combat and the infection is completely overcome. It happens
occasionally, however, that a pus-forming infection becomes established
in some place where it manages to maintain itself in spite of the
attacks of the colorless corpuscles. This happens very frequently in the
tonsils, so that persons who have infected tonsils may have pus
formation going on month in and month out. This may also take place at
the roots of the teeth. In fact it is now commonly believed that
whenever the nerve to the tooth dies, infection is certain sooner or
later to become established at the tip of the root. These places where
pus formation is going on continually are known as “pus pockets.” For a
long time very little attention was paid to them. Persons occasionally
suffered acute distress from gumboils or had attacks of sore throat
owing to the infected condition of the tonsils, but beyond these no
particular attention was paid to the pus-forming organisms, unless, as
occasionally happened, an especially virulent type became established
which brought on the condition commonly known as blood poisoning. Within
the last few years the discovery has been made that there is a steady
discharge of poison from every pus pocket into the body fluids and so to
the blood stream. The amount is ordinarily so small that the body as a
whole is not seriously affected, but now and then, either because of a
larger production of poison or because of a lower resistance on the part
of the body, serious ill effects are produced. Among them may be
mentioned acute (inflammatory) rheumatism. This is not only an extremely
painful condition, but it is very likely to leave serious after-effects,
as, for example, injured heart valves, to give trouble for the rest of
life. The discovery of these facts has given us great respect for pus
pockets, so we no longer treat them carelessly. Infected tonsils are
looked after, not simply because they bring about sore throat now and
then, but even more because of the poison which they are likely to send
through the body. It is probably not too much to say that the practice
of dentistry has been revolutionized since the significance of pus
pockets has been discovered. Formerly the killing of nerves to relieve
aching teeth or to insure them against future aching was a common
practice. The modern dentist, on the other hand, kills nerves to teeth
with the very greatest reluctance and only as a last resort, because he
knows full well that in so doing he is opening the way for the
establishment of pus pockets with the train of ills that is likely to
follow. At the present time methods of curing pus pockets at the base of
the teeth are not very adequate, so that the extraction of the tooth has
to be resorted to, but there is every reason to believe that the dental
profession will shortly find methods by which pus pockets can be
controlled without having to sacrifice the teeth.

The third kind of structures which are present in the blood stream are
much smaller than the red corpuscles, but are nowhere near so numerous.
They are called the _platelets_ and are disk-shaped bodies composed
chiefly of protein material, and probably, although not certainly,
living cells. Their presence in the blood remained unsuspected up to
about the end of the last century, not so much because of their very
small size, as because they go to pieces very quickly after the blood is
shed. By the time a drop of blood could be gotten under the microscope
they would be all gone. They were discovered only as the result of the
adoption of a method of treating the blood which preserved them long
enough so that they would still be present when the blood was looked at
under the microscope. They seem to have something to do with the changes
that take place in the blood by which it is caused to clot. Whether this
is their only function, we do not know.

We have now described the substances which are present in the blood and
in the other tissue fluids as a fairly permanent part of their make-up.
In addition to these there are present all the materials that are in
transit to the cells or from them. These include all kinds of foodstuffs
on their way from the digestive organs to the cells. As we shall see in
detail later, the digestive organs work over the food that we eat before
passing it on to the blood, so that the actual food materials that are
being transported by the blood are the digested products of the food
rather than the food itself. For example, there are to be found in the
blood, in addition to the regular blood proteins which were described a
moment ago, the digestion products of the food proteins on their way to
serve the needs of the various cells of the body. There will be found
also the digestion products of the other classes of foodstuffs. The body
fluids contain also the waste products of cell metabolism on the way to
be discharged from the body and the special products, such as the
hormones, which are manufactured by certain cells and carried through
the blood stream to act upon other cells. Of course we realize that not
all of the materials that are manufactured by cells are poured out into
the blood stream. Such materials as saliva, gastric juice and the like
are passed directly from the cells in which they are manufactured into
tubes by which they are conveyed to the digestive canal, where they
carry on their special work of digesting the food.

Among the things which are present in the body fluids should be
mentioned the two gases oxygen and carbon dioxide. We should expect
oxygen to be present in the body fluids, because it is necessary for the
metabolism of cells and can get to them only by being carried in the
blood stream. We have seen in the red corpuscles the special method by
which an abundance of oxygen is transported. Carbon dioxide is the
gaseous product of the oxidation of carbon and is found in large amounts
wherever there is burning, since carbon is the chief constituent of all
fuel and whenever carbon is burned, carbon dioxide is formed. Since the
fuel materials that are burned in the living cells consist largely of
carbon, carbon dioxide is produced in them as well. They have to get rid
of it, and the only way they can do so is by passing it out into the
fluids that surround them, which in turn pass it on to the blood. The
way in which the body handles these two gases makes up a special
subdivision of the subject of physiology and will be so treated in the
chapter on respiration.

CHAPTER XIV

THE CONVEYER SYSTEM OF THE BODY

In the last chapter we talked about the body fluids and saw that they
can be subdivided into the tissue fluids, which surround the cells, and
the blood, which is inclosed in a system of pipes and which carries the
materials to and from the tissue fluids. We now have to take up this
matter of transporting the material in more detail. The first step will
be to see how materials that are in the blood get from it to the tissue
fluids, and how materials that are in the tissue fluids get from them
into the blood. Unless these interchanges can take place freely, there
is no way in which the blood can serve as a conveyer system. In order to
see how the interchanges are carried on, we shall have to look first at
some features of the construction of the system of pipes through which
the blood flows. As we all know, the large blood vessels are either
arteries or veins, the arteries being blood vessels which are carrying
blood away from the heart, and the veins vessels that are carrying blood
toward the heart. If we start with an artery and trace it through the
body, we find that it is continually giving off branches which in turn
give off smaller branches, until finally the subdivisions are so small
that we cannot trace them any further with the naked eye. In the days
before the microscope was discovered, there was a great deal of question
as to how these finest branches ended. At first no one suspected that
there was connection between the fine subdivisions of the arteries and
the fine subdivisions of the veins through which the blood could pass.

About the beginning of the seventeenth century William Harvey became
convinced that there must be fine vessels leading across from the
smallest arteries to the smallest veins and that the blood must pass
through these. He came to this conclusion without ever having seen these
small vessels, since at that time there was no microscope by which such
tiny structures could be seen. Before Harvey’s time it was supposed that
the blood ebbed and flowed in the arteries and in the veins. He showed
that the blood flows in one direction constantly, leaving the heart by
way of the arteries and coming back into it by the veins. Harvey was,
therefore, the discoverer of the circulation, one of the most important
discoveries in physiology. After the microscope was perfected the tiny
tubes connecting the finest arteries with the finest veins were made
out. They were found to be very small in diameter and to have very thin
and delicate walls. They were also found to be extremely numerous. The
finest subdivisions of the arteries that can be seen with the naked eye
are scattered very thickly through all the tissues of the body which
have a blood supply, and they go on subdividing microscopically, so that
the finest vessels are scattered more and more thickly through the mass
of living substance. The very finest of all, which are the tubes
connecting the smallest arteries with the smallest veins, are called
_capillaries_, from a Latin word meaning a hair, to indicate their very
small size. They are so close together in most parts of the body that it
would be difficult to thrust a

[Illustration: Photo, Paul Thompson

THE TEST FOR BLOOD PRESSURE]

[Illustration: Photo, Paul Thompson

PART OF PROCESS OF URINALYSIS INDICAN TEST]

[Illustration: A NETWORK OF CAPILLARIES

The artery _a_ and vein _v_ (highly magnified). (From “The Human
Mechanism,” Hough and Sedgwick)]

pin in anywhere for any distance without striking against one or more of
them. The capillaries are spread so thickly that there are not many
places in the body where living cells are more than a very small
fraction of an inch from one of them. The cells do not, however, lie
right against the capillaries, but are separated from them by tiny
spaces filled with tissue fluid. In order for material to get from the
blood to any living cell, then, it must pass through the wall of the
capillary into the fluid which fills the tissue space and from that in
turn to the cell itself. The wall of the capillaries is so delicate that
if the blood flowing through any capillary contains more of any
substance than is present in the tissue fluid surrounding that
capillary, some of it will pass out through the wall and into the
tissue fluid, just about as freely as though there were no wall there at
all. The arrangement can be illustrated by a familiar example; if a drop
of ink is allowed to fall into a glass of water, it will color only a
small part of the water at first, but quickly spreads out until each
part of the water is as deeply colored as any other part. If the glass
of water were to be divided in half by a very delicate membrane, and the
ink dropped in on one side, it would spread out in the same way, passing
through the membrane in so doing, until again all the water in the glass
was equally colored. Of course, the quickness with which the ink could
pass through the membrane would depend on how delicate the membrane was.
We could imagine membranes which would not let any ink at all through,
and every degree from that up to membranes so delicate as to offer no
obstruction at all to the passage of the ink. The walls of the
capillaries rank as membranes of such delicacy as apparently to offer
almost no obstruction to the passage of materials through them. They
hold back the red corpuscles and the platelets fairly well, so that they
do not pass out of the blood and into the tissue spaces, unless the
capillaries are actually injured. The colorless corpuscles are able to
make their way through the capillary walls and so also do nearly all the
substances that are dissolved in the blood. It is an interesting fact
that the blood proteins do not pass freely through the capillary walls,
although the digestion products of food proteins, do. It will be
remembered that in the chapter on Body Fluids the importance of the
sticky quality of the blood proteins was spoken of. It is now believed
that it is because of this gelatinous nature that the blood proteins
are not able to pass out through the capillary walls, and this is
supposed to be important in the proper working of the circulation. In
fact there is a condition of greatly lowered vitality to which the name
“shock” is applied, in which the blood proteins escape through the
capillary walls to so great an extent as to interfere with the proper
working of the body. It has been found possible to prevent this in a
very large measure by the simple expedient of injecting some substance
like mucilage directly into the blood stream. We are to think of the
capillary walls, then, as allowing materials to pass freely through them
in either direction, from the blood into the tissue spaces or from the
tissue spaces into the blood, with the exception of the red corpuscles,
platelets, and the blood proteins, and as thus keeping the tissue fluids
supplied with whatever materials the blood contains or taking from the
tissue fluids the waste products of cell metabolism, which the cells are
pouring out. With this arrangement clearly in mind, all that remains for
the understanding of the conveyer system is to see how the blood is kept
in motion and distributed among the various organs of the body, and then
to consider where the blood in turn gets its supplies of materials which
it can pass on to the tissue fluids, or how it gets rid of the
substances which the tissue fluids have passed on to it from the cells.

At the beginning of the chapter we said something about the arteries and
veins and their branching into smaller and smaller subdivisions with the
final connecting link between the smallest arteries and the smallest
veins in the form of capillaries. We are now to consider in detail the
movement of the blood through these tubes, and to do that it will be
necessary to speak again of this arrangement. In describing the
circulation we usually begin with the heart. The heart itself will be
taken up presently. First let us trace the blood vessels from the heart
through the body and back to it again. The large main artery leading
from the heart is known as the _aorta_. This springs from the upper side
of the heart, bends over in an arch, and passes down through the chest
into the abdomen.

[Illustration: THE HEART AND BLOOD VESSELS DIAGRAMMATICALLY REPRESENTED

_L_, lung; _M_, intestine; _P_, liver; dotted lines lymphatics.
(Martin’s “Human Body”)]

Branches are given off from the aorta all along its length. The very
first of these come off before the aorta gets away from the heart, and
are the arteries by which the tissues of the heart itself are supplied
with blood. A little farther on are large arteries, one for the left arm
with a large branch running up the left side of the neck, another for
the right arm with a large branch running up the right side of the
neck. Each of these in turn gives off branches all along to provide for
the tissues of the arms, neck, and head. It is worth while noting that
the arteries running up the neck to the head are large in proportion to
the size of the head itself; this is because the brain, as the most
important organ in the body, requires and receives a disproportionately
large blood supply. Besides the brain the head contains numerous muscles
and also the salivary and tear glands, all of which carry on active
metabolism and therefore require abundant blood supply. The main
branches of the aorta to the head and arms are given off from the arch;
as the aorta passes down through the chest it gives off small branches
to the muscles of the chest wall, and then passes into the abdomen. Here
are located two of the three important arrangements for renewing the
blood; namely, the digestive organs and the organs of excretion
(kidneys). Large branches from the aorta pass to the digestive organs
and others to the kidneys; smaller branches lead to the muscles of the
abdominal wall, and also to the various secreting glands that are
located in the abdomen. At the lower end of the abdomen the aorta
divides, giving one large branch for each leg. As we have already seen,
if we follow any of these subdivisions through its finer and finer
branchings, we shall finally be able, with the aid of a microscope, to
trace it to capillaries, where the interchanges between blood and tissue
fluid occur, and beyond the capillaries to tiny veins which unite with
other tiny veins from other capillaries into larger veins. These again
continue to come together into main veins corresponding in every part of
the body with the main arteries. All these veins finally unite into two,
one for the lower part of the body, called the _inferior vena cava_,
and one for the upper part of the body, called the _superior vena cava_.
These two come together just at the entrance to the heart. One special
feature of the blood supply to the digestive organs may as well be
mentioned here; it is that the blood which flows through the capillaries
of the stomach and intestines is all reassembled into a vein known as
the _portal vein_, which instead of passing directly into the inferior
vena cava goes first to the liver, where the vein breaks up into another
set of capillaries, the liver capillaries, beyond which is another vein
which leads into the inferior vena cava. The result of this arrangement
is that all blood passing into the capillaries of the stomach or
intestines is obliged to pass again through the capillaries of the liver
before going on into the main stream of the circulation. This is an
important feature of the renewal of the food supplies of the blood.

We have now traced the blood from the heart through the body back to the
heart again, and have seen how in its course some of it will pass
through such active tissues as muscles or brain or glands, so that the
interchanges can go on by which the fluids in these active tissues can
take up needed materials and give off wastes. Also a part of it flows
through the digestive organs, where food materials can be taken up, and
another part flows into the kidneys, where wastes can be gotten rid of.
This leaves us to consider only the passage of the blood through the
lungs, where the supply of oxygen is to be taken up and the gaseous
waste product, carbon dioxide, is to be disposed of.

The most urgent requirement of the body is the requirement for oxygen.
There is under ordinary circumstances at all times some surplus of food
materials stored in the cells, so that even though the renewal of their
surrounding fluids from the blood should stop, they could keep on going
for a time on the material that is stored within them; but there is no
such storage of oxygen. The cells in the body lead almost a
hand-to-mouth existence so far as their oxygen supply is concerned. They
are constantly withdrawing oxygen from the tissue fluids surrounding
them, and these fluids are just as constantly withdrawing it from the
blood; therefore any failure of the blood to be properly supplied with
oxygen results very promptly in a condition of oxygen hunger in the
cells. This means prompt cutting off of metabolism, since metabolism is
a matter of oxidizing fuel, and oxidation cannot go on unless the oxygen
is provided. This urgent need for oxygen is met in the body by having
the arrangement for supplying it to the blood much more perfect than the
other renewal arrangements. We saw a moment ago that only part of the
whole blood stream passes through the digestive organs at any given time
and only part of the stream passes through the kidneys. The whole
stream, on the other hand, passes through the capillaries of the lungs.
This is brought about by having an arrangement whereby the combined venæ
cavæ after entering the heart communicate with an outlet in the form of
an artery leading to the lungs. This artery, which is called the
_pulmonary artery_, breaks up into capillaries in the lungs, which
reunite into the _pulmonary vein_ which comes back to the heart again.
It is from the pulmonary vein that connection is made with the aorta,
starting the blood on its course through the body again. We see then
that the blood passes through the heart twice in each complete round,
once as it comes in from the body at large on its way to the lungs, and
again as it comes in from the lungs on its way to the body at large.

We have spoken of the heart thus far as a single organ; it is actually
two hearts side by side, and these would work just as well if they were
at a distance instead of being built into one organ. There has probably
been more misunderstanding of the heart by people in general than of any
of the other parts of the body. This is because from the earliest times
the heart has been looked upon as the seat of the affections, and so
powers and properties have been attributed to it to which it is not at
all entitled. As we tried to make perfectly clear in a former chapter
all intelligence and all feelings are located in the brain; the heart
cannot possibly take any more active part in these than can the stomach,
liver, kidneys or any of the other parts of the body which are concerned
with the maintaining of the tissues in good working order. Probably no
one really knows how it came about originally that the heart was endowed
with these peculiar gifts. It is true that in time of strong emotion
there are changes in the activity of the heart which we can perceive.
These occur because the heart is under the same kind of nervous control
as are the smooth muscles and glands, and shares with them in the
disturbances which accompany emotion; but the real seat of these
emotions is, of course, the brain. As a matter of fact the heart is
nothing but a muscular pump whose sole function is to keep the blood in
motion. From what has already been said it is clear that the heart
cannot relax its activity for more than an instant without disastrous
results; the pressing need of the tissues for oxygen requires that the
blood be kept moving. If there were any other way in which the needs of
the cells could be supplied except through the movement of the blood,
the heart could be dispensed with perfectly well. We have emphasized
this about the heart because it is much easier to understand its working
if one thinks of it simply as a pumping organ than if one is attributing
to it mystic functions connected with our higher emotions.

[Illustration: DIAGRAM SHOWING THE RELATION OF THE TWO HALVES OF THE
HEART

_ra_ and _rv_, right auricle and ventricle; _la_ and _lv_, left auricle
and ventricle; _ao_, aorta; _vc_, venæ cavæ; _pa_, pulmonary artery;
_pc_, pulmonary capillaries; _pv_, pulmonary vein. (Martin’s “Human
Body.”)]

We showed a moment ago that the heart is really a double pump. The
relation of the two halves is shown in the diagram. One of the two
pumps, that on the right side of the heart, receives the blood from the
body at large and pumps it out into the pulmonary artery and through
the capillaries of the lungs; the pump on the left side of the heart
receives the blood from the lungs through the pulmonary vein and pumps
it out into the aorta and so through all the other capillaries of the
body. Since the circuit of the body is much more extensive than the
circuit of the lungs, the work of pumping is correspondingly greater,
and we find the left part of the heart a much more powerful pump than
the right. The heart operates as a _reciprocating pump_, by which we
mean that it alternately fills and empties. In this respect it is like
ordinary pumps except those of the rotary variety. Any reciprocating
pump must have a chamber which will fill and which can then be emptied
forcibly. In order that it shall not empty itself back through the pipe
from which it filled there must be a valve in the intake pipe which
shall close as the pump is being emptied. If, as is the case in the
heart, it is emptying itself into a system which permits backflow, there
must be another valve in the outlet pipe to prevent the fluid that has
been expelled from running back in. Each of the heart pumps consists,
then, of a chamber, which alternately fills and empties itself, and two
valves, one on the intake and one on the outflow side. In ordinary pumps
the forcible emptying is performed by a piston which moves through the
pump chamber expelling the liquid ahead of it and then has to draw back,
making room for the chamber to fill again. In the heart the forcible
emptying is accomplished by muscular action. The wall of the heart
consists of a great many muscle fibers so arranged that when they
contract they pull the walls of the heart together, making the cavity
smaller, or even obliterating it completely. The contraction of these
fibers makes up what we are familiar with as the beat of the heart. The
frequency with which they contract varies a good deal in different
individuals. The average is about seventy-two a minute; but it may be as
slow as forty-eight or fifty, or may run up to one hundred and forty or
one hundred and fifty a minute. Whatever the rate, in every case there
is an alternation of contraction and relaxation; during the relaxation
the cavity is filling with blood through the intake valve, the outflow
valve being closed, so that no blood that has once been pumped out can
rush back in again. By the contraction of the muscles the heart is
emptied, the outflow valve being open, and the intake valve being closed
to prevent an escape of blood backward into the veins through which it
flowed in. The part of the heart that carries on this active pumping
work is known as the _ventricle_; that on the right side, which receives
the blood from the body and pumps it to the lungs, is the right
ventricle; and that on the left side, which receives the blood from the
lungs and pumps it to the body, is the left ventricle.

In addition to the ventricles, which are the active pumps, each side of
the heart has an additional chamber known as the _auricle_, whose
purpose is to serve as a reservoir into which blood can flow during the
time that the ventricles are emptying themselves. If it were not for the
auricles, the movement of blood into the heart would have to stop with
every beat, because while the ventricles are contracting the intake
valves are closed and there would be no place to which blood could flow,
but since each side has its auricle, the flow of blood goes on during
the beat of the ventricles, the auricles filling up. The intake valve,
in order to operate properly, should be located between the auricle and
ventricle, and this is where it is. The vein opens directly into the
auricle without any valve between; the auricle opens into the ventricle
with the intake valve at the point of junction. The intake valves are
given rather formidable names; they are sometimes spoken of as the
_auriculo-ventricular valves_; that on the right side of the heart
between the right auricle and the right ventricle is composed of three
flaps of membrane, and has therefore been named the _tricuspid valve_.
The intake valve on the left side of the heart, which is composed of but
two flaps, is known as the _mitral valve_. As soon as the beat of the
ventricles is over and the ventricular muscle relaxes, the blood which
has accumulated in the auricles presses the intake valve open and blood
begins to flow through it directly into the ventricle. Both the intake
and outflow valves are composed of stout but thin sheets of membrane, so
that very little pressure is required to operate them. The weight of the
blood that is accumulated in the auricles during the beat of the
ventricle is more than sufficient to force the valve open and allow the
blood to flow on through into the ventricle. In a heart that is beating
seventy-two times a minute, there cannot be much time occupied either in
filling or emptying. As a matter of fact both these intervals are
measured in tenths of seconds. If we take a heart that is beating at the
average rate of seventy-two times a minute the whole of a single beat
amounts to eight-tenths of a second. The beat of the ventricle takes
about three-tenths of a second or three-eighths of the whole time; the
period of relaxation of the ventricle, during which it is filling with
blood through the open intake valve, is about five-tenths of a second or
five-eighths of the whole time. The movement of blood is rapid enough
so that this five-tenths of a second allows the ventricle to fill; in
fact much less time than this is required, for in a heart that is
beating at twice the average rate, the ventricle still fills with blood
between beats.

A word remains to be said about the beat of the auricle. During most of
the period when the ventricle is relaxed the auricle is also quiet and
blood is pouring directly through it from the veins into the ventricle;
but just an instant before the ventricular beat begins, one-tenth of a
second to be exact, the auricle contracts, emptying what blood it
contains into the nearly filled ventricle; thus, when the ventricle
beats, which it does immediately, closing the intake valve at the same
time, the auricle is empty and so is able to accommodate the inflow of
blood from the vein during the three-tenths of a second that the intake
valve is shut. Both sides of the heart work exactly together, the two
auricles beating simultaneously, and the two ventricles. Of course it is
necessary that this be so, for if they did not keep pace exactly, one
with the other, there would be a piling up of the blood either in the
lungs or in the veins leading from the body to the heart, and the
efficiency of the circulation would be seriously impaired.

We can get a good deal of information about the way our hearts are
behaving simply by holding the hand against the chest directly over the
heart or by pressing the ear against the chest of some one else and
listening to the heart’s action. The physician makes use of a
stethoscope, which is merely an apparatus for conducting clearly the
sounds which the heart makes, so that it is not necessary to apply the
ear to the chest. When one listens thus to the heart he finds that with
every beat there are two distinct sounds: the first is a rather dull
sound which comes just at the beginning of the beat of the ventricle,
the second is a sharp sound occurring just at the end of the ventricular
beat. As we saw in Chapter IX, sound is always the result of vibrations,
and a great deal of study has been devoted to an attempt to find out
where the vibrations come from that cause the heart sounds. It is now
generally believed that the first sound is partly the result of
vibrations set up in the contracting heart muscle and partly due to
vibrations from the sharp closing of the intake valves. The second sound
is known to be wholly due to the sudden closing of the outflow valves.
The sounds are chiefly of importance in that they enable the physician
to determine whether the valves are holding tight or whether there is a
leakage of blood through them. In case the intake valve leaks, there
will be a backward jet of blood from the ventricle into the auricle with
every beat of the heart. This will cause a sort of hissing or murmur
which can be heard with the stethoscope in connection with the first
sound. If the outflow valve is the one that leaks, blood will squirt
back into the ventricle from the aorta, while the ventricle is relaxing.
The murmur in this case will come just after the second sound. The
skillful physician by comparing the loudness of the murmur when the
stethoscope is pressed at different points on the chest and back can
determine whether the leaky valves are on the right side or the left
side. Thus an accurate diagnosis of imperfect valves can be obtained. Of
course the heart will not work well if its valves are not tight any more
than will an ordinary pump, so that persons suffering from this trouble
cannot have as good a circulation as those whose valves have nothing
the matter with them. It is true that in most cases of imperfect valve
action there is a compensation in the form of an increase in the size
and strength of the heart muscle, so that the circulation is maintained
in spite of poor valve action by harder work on the part of the heart.
It is evident that in a case of this kind exceptional strains on the
heart are more dangerous than if the heart is normal to begin with, so
that persons with faulty valve action must avoid physical strains, such
as sharp running after street cars or trains, which would be borne with
impunity by ordinary individuals. Since faulty valves are a frequent
result of acute rheumatism, which in turn comes from pus pockets, and
since no way is known to cure a defective valve, once the trouble has
developed, it is evident that prevention is of the utmost importance.
Physical efficiency is very seriously hampered by poor heart action.

One feature of the heart action with which we are all perfectly familiar
is that both the rate and the vigor of the heartbeat vary greatly from
time to time. When one is lying quietly, the heartbeat is at its lowest
point. It becomes more rapid as one sits up, still more rapid upon
standing, increasing still more with the taking of any form of muscular
exercise, and in vigorous muscular exercise attains its greatest
rapidity and force. The rate in this latter case may be fully double
that of the quiet standing position, and, as the vigorous thumping tells
us, the force is also very much increased. As we saw in Chapter VII the
heart muscle works automatically, contracting and relaxing without being
stimulated through the nervous system. The _variations_ in rate and
force, however, are the result of nervous action. The heart muscle, as
we have already seen, is under the same sort of nervous control as the
smooth muscles and glands. It has passing to it two sets of nerves, one
to slow it down, the other to speed it up. Both these sets of nerves
arise from centers in the brain stem, and both these centers appear to
be discharging continuously. So it works out that the heart muscle is
under the constant influence of two opposing sets of nerves, and its
actual rate and vigor depend upon the balance between them. This has the
effect of making the heart extremely responsive to nervous influences.
The slightest relaxation on the part of the nerves whose function is to
cause slowing will lead to a prompt increase of rate, since the nerves
that tend to cause increase are active all the time. Various things may
bring about changes in the nervous balance governing the heart; chief of
these are muscular activity and emotional disturbance. Practically all
the changes in the heart action that we observe from moment to moment
can be explained as being due to one or the other of these causes. There
are, however, two additional points to be noted briefly; the first is
that after muscular exercise the heart slows down very gradually, not
returning to its ordinary resting rate for a half hour to an hour after
the exercise is over, depending on how long the exercise was kept up.
The explanation of this long-continued rapid beat is found in the great
outpouring of waste products as the result of the exercise. We have
already learned that the functional metabolism of muscular work involves
the oxidation of a large amount of energy-yielding material and
therefore brings about the production of large amounts of oxidation
products. Their presence in the blood serves as a stimulus to

[Illustration: THE MICROSCOPE IS NECESSARY TO UNDERSTAND THE STRUCTURE
OF THE BODY

In this laboratory the workers are examining blood smears]

[Illustration: Photo, Cornell University Medical School

MICROPHOTOGRAPH OF BRAIN TISSUE]

the nerve center in the brain stem, which acts to quicken the heart, and
this keeps the rapid beat going until these products are gradually
gotten rid of from the body. Another somewhat similar case is the
prolonged rapid heartbeat following a violent emotion. The explanation
of this we saw a couple of chapters ago in the outpouring of adrenalin
that accompanies emotion. One property of adrenalin, as already noted,
is to quicken the heart; so, as long as any adrenalin remains in the
blood stream, the heartbeat will be faster than normal.

In the above paragraphs we have tried to make clear that the blood is
kept in motion through the body by the work of the heart, and that the
heart’s activity varies in accordance with the needs of the body; in
muscular exercise there is a great increase in metabolism, which means a
greatly increased demand both for food supplies and for oxygen. To meet
this increased demand it is necessary that the blood circulate more
abundantly, and in the automatic speeding up of the heart through the
nervous system we have the means by which this is done. In the case of
strong emotion, as already emphasized, the bodily reactions are such as
put the body into the best possible condition for meeting the emergency.
Evidently a quickened heartbeat, by insuring abundant supplies of oxygen
and of foodstuffs, contributes to this end. The slowing of the
heartbeat, when one lies down, is evidently helpful in enabling the
heart itself to recover from any strains that may have been put upon it.
The heart is a muscle, and like any other muscle carries on its
functional metabolism, which means that it is oxidizing fuel materials
and producing waste products. Since it is absolutely necessary that the
heart go on beating regularly year in and year out for perhaps eighty
or a hundred years, any relief from activity that it can get by slowing
down during sleep is evidently an advantage. It has been calculated that
the heart muscle really enjoys an “eight-hour day,” by which is meant
that on the average the functional metabolism of contraction is going on
in heart muscle only about one-third of the time, eight hours out of
each twenty-four. During the active waking time the metabolism takes a
larger percentage than that, but during sleep enough less to even up.

The heart empties itself into the large arteries; the left heart into
the aorta, the right heart into the pulmonary artery. Both these
arteries, as well as their subdivisions, are highly elastic. The very
best quality of rubber tubing is not superior to our arteries as samples
of elastic tubes. The blood, as we have already seen, is quite sticky,
and the capillaries through which it must pass in its course around the
body are microscopically tiny. The heart pumps the blood out of itself
at the rate of four or five quarts a minute or more, according to
whether it is working moderately or at high speed. To force this amount
of the sticky blood through the tiny capillaries evidently requires very
considerable force. As a matter of fact, the force is sufficient so that
if it were applied to working a fountain it would force a jet to the
height of nearly eight feet. Evidently pumping blood into elastic
arteries with this force and against the resistance offered by the tiny
capillaries causes the arteries themselves to be not only filled but
overfilled, so that their walls are greatly stretched. This fact, that
our arteries are elastic and are kept on the stretch by the pressure of
the blood within them, is of the very greatest importance to the proper
flow of blood and this in turn is so important to our well-being that
some of our most serious chronic diseases are traceable to the loss of
elasticity on the part of the arteries.

We must remember that once every second, or oftener, the heart is
shooting a jet of blood into the large artery which is already stretched
with blood and which can empty itself only through the tiny capillaries
at the tips of its finest subdivisions. On account of the inertia of the
blood stream, room is made for this additional jet of blood by
stretching the arteries near the heart more than they were stretched
before. The result is that there is an inequality in the amount to which
the arteries are stretched, those near the heart being stretched more
than those farther along. As quickly as possible this inequality of
stretch will be equalized by a spreading of the additional tension out
over all the arteries in the form of a wave. This wave makes up what we
know as the pulse. It can be felt in any artery that is near enough to
the surface so that the finger tips can press upon it. The radial artery
at the wrist is the one commonly used by physicians for feeling it.
There is a large artery in the neck in which the pulse can also be felt
readily, and if one takes the pulse of another person in the neck with
one hand and in the wrist with the other he can easily satisfy himself
that the pulse in the neck always comes an instant earlier than that in
the wrist. This is simply because the pulse spreads from the heart as a
wave, and the distance to the neck is not so great as that to the wrist.
By the time the finest subdivisions have been reached, the tension is
equalized throughout the arterial system, and there is no more pulse.
The advantage of this is that the blood flows through the capillaries
in a steady stream and not in a series of jerks. This, in fact, is the
chief, but not the only, benefit we derive from having elastic arteries.
Since the heart operates as an intermittent pump, it is evident that
unless the arteries were elastic and so could take up the shock, the
blood would have to pass through the capillaries in a series of jerks,
exactly corresponding with the beats of the heart. There is abundant
proof, which we shall return to in a moment, that to have the blood move
through the capillaries in this jerky fashion would be disastrous.
Before taking that up, however, we wish to show that by having elastic
arteries the actual work of the heart is less than it would be if the
arteries were stiff. The reason is really very simple. As was stated a
few pages back, the heart is actually emptying itself only during
three-eighths of every beat. If the arteries were stiff tubes, and
therefore not able to take up any of the blood within themselves,
exactly as much would have to pass out through the capillaries during
this three-eighths of the beat as was pumped in by the heart. In other
words, if the heart were pumping five quarts a minute, five quarts would
have to pass through the capillaries in three-eighths of the minute
instead of having the whole minute in which to do it. Since the arteries
are actually elastic, they are able, by stretching a little more, to
make room for part of the blood and so spread the time of its passage
through the capillaries out over the whole time instead of confining it
just to the period when the ventricle is contracting. Evidently it would
take more work to pump five quarts of blood through the capillaries in
three-eighths of a minute than in a whole minute.

We measure the work of the heart by what we call blood pressure, about
which we are hearing so much these days, so that it will be well to
explain as clearly as possible just what is meant by it. The blood
pressure really means the pressure of the blood within the large
arteries. It could be measured with any ordinary pressure gauge, if it
were not for the fact that we cannot very well cut into our bodies to
apply gauges to the arteries. For this reason it has been necessary to
invent means of finding out what the blood pressure is from the outside.
The way it is done is to put a band around the arm, press this band down
upon the arm until it squeezes the large arm artery shut, and then, by
means of a suitable gauge, find out how much pressure was required. Of
course, it is necessary to be able to tell when the artery has actually
been squeezed shut, so that the determination of blood pressure in human
beings is the work of an expert. Furthermore, blood pressure, as should
be clear from what has already been said, varies with every heartbeat.
It is at its maximum the instant the heart finishes emptying itself into
the artery, and falls off steadily, reaching a minimum just before the
next beat comes. The more elastic the arteries, the less difference
there will be between the maximum and the minimum blood pressure. The
reason for this will be clear when we think that if the arteries were
entirely rigid there would have to be a very high pressure during the
time the heart was actually beating to force the blood out through the
capillaries, but that between beats the pressure would fall off to zero.
The more elastic the arteries are, the more nearly do they exert a
steady pressure on the blood within them, and so the less will be the
difference between the maximum and the minimum pressure. When the
physician determines blood pressure, he really determines both the
maximum and the minimum pressure, in order that he may be able to judge
whether or not the arteries are as elastic as they should be. High blood
pressure just by itself might not mean much more than that the heart was
beating more rapidly than it should, but a high maximum pressure and a
low minimum pressure means nonelastic arteries. This in turn means that
the blood is forced through the capillaries in jets rather than in a
steady stream, and we may judge of the importance of having a steady
flow through the capillaries when we recall the well-known medical
proverb that “a man is as old as his arteries.” It is an actual fact
that the chronological age of an individual need not have much to do
with his physical age. If his arteries continue elastic over a long
period of years, he will be physiologically youthful, while if his
arteries become rigid he will be physiologically aged, no matter how few
his actual years upon earth may have been. Unfortunately we do not know
as much as we would like to about the causes of loss of elasticity in
the arteries. It does appear, however, that self-indulgence of various
kinds is apt to lead to loss of elasticity. For example, even the
moderate use of alcohol is now generally recognized by the medical
profession as a cause of impairment of elasticity in the arteries. It is
probable that intemperance in the use of various foods and drugs leads
also to this same condition.

We have just seen that the heart is obliged to maintain high blood
pressure in order to force the blood through the tiny capillaries. It
will be clear that the actual amount of pressure will depend in part
upon how much blood is forced through in a minute and in part upon the
extent to which the capillaries offer resistance. It is a familiar law
of friction that the smaller the tube the greater will be the resistance
it will offer to the passage of liquid through it, so that if the
capillaries change in size their resistance to the flow of blood through
them is bound to vary. The walls of the capillaries are very sparsely
provided with muscle fibers, but the very finest subdivisions of the
arteries, which are really no larger in diameter than the capillaries,
have much more smooth muscle in their walls. These muscles, as we have
already seen, can by their contraction or relaxation make the tiny
vessels smaller or larger. We have examples of this in the flushing and
pallor of the skin. What we wish to do now is to show how the flow of
blood through different parts of the body is controlled by changes in
the caliber of these tiny tubes. The muscles in the vessel walls have
the double nervous control commonly found in smooth muscles, and both
sets of nerves trace back to centers in the brain stem. One of these
centers causes the muscles to contract and the vessels to become
smaller; the effect of this is, of course, to increase the resistance to
the passage of blood through them. The nervous center which brings this
about is called _vasomotor_, or, more properly, the _vasoconstrictor_
center. Vasoconstriction means literally causing contraction of the
vessels, which is exactly what this center does. Not all the blood
vessels in the body are acted upon through the vasoconstrictor center.
Those of the skin and of the abdominal organs are under its control, but
those of the skeletal muscles are not. The result of activity of this
center, then, is to make it more difficult for blood to flow through the
skin and through the abdominal organs, but the ease of flow of blood
through the muscles is not affected. Of course, it will follow
automatically that the blood stream will be diverted in a large part
from the former regions into the latter. The skin and abdomen together
make up so large a part of the whole body that marked constriction of
the blood vessels in these two regions is bound to cause a considerable
increase in blood pressure.

A fairly high blood pressure is necessary for bodily well-being, because
only thereby is the brain assured of sufficient nourishment. To see why
this is so, we have only to remember that the brain is unfavorably
situated for receiving ample supplies of blood. It is at the top of the
body, so that the influence of gravity has to be overcome in forcing
blood up to it. Also it is completely inclosed in the bony skull, which
it in turn fills so completely that there is almost no room for the
accommodation of extra blood in it. In all other parts of the body a
rise in blood pressure stretches the arteries and so leads to there
being actually more blood within them, but there is no room for the
arteries in the brain to stretch, so that the total quantity of blood in
the brain cannot vary greatly from time to time. The only way in which
an increased blood supply can be obtained is by causing the blood to
flow more rapidly. This is precisely what happens every time the blood
pressure in the body rises. Whenever the vasoconstrictor center sends
nervous discharges into the blood vessels of the skin and abdominal
organs, causing them to contract, there is a diversion of blood from
them directly into the skeletal muscles and also a rise in blood
pressure due to the increase in resistance to the circulation, which
causes blood to flow more rapidly through the brain. The net result,
then, is improvement in the blood supply to the skeletal muscles and to
the brain. So important is the blood supply to the brain that the
vasoconstrictor center discharges actively throughout the waking part of
the day. If for any cause the activity of this center diminishes, there
will be an increased circulation in the skin and in the abdominal
organs, the blood pressure will fall, the circulation through the brain
will decrease, and along with it there will be a decrease in brain
function. After this passes a certain point unconsciousness will result.
This is what happens when one faints. For some reason or other the
vasoconstrictor center becomes inactive, and the series of events just
described is set in motion. Fainting ordinarily cures itself
automatically, because when consciousness is lost, the individual falls
over; this brings his head down on the level with the rest of his body,
makes it easier for the blood to flow through it, and so in a moment or
two consciousness will be regained. It is a mistake to try to scramble
to one’s feet immediately, because until the vasoconstrictor center
recovers its ordinary activity, raising of the head above the level of
the rest of the body is bound to result in its failure to receive
sufficient blood, and so faintness will come on again. It has long been
a practice to dash cold water in the face of a fainting person. The
physiological value of this is in the sudden stimulation of the sensory
nerves in the face by which is set up a stream of nervous discharges
which will play upon the vasomotor center, and arouse it again to
activity. Almost any vigorous sensory stimulation may have the same
effect.

During sleep the vasoconstrictor center is usually not very active; the
cause of sleep is not completely understood, but one of the most
satisfactory theories regarding it is that during the period of waking
the vasoconstrictor center becomes gradually more and more fatigued, and
so requires more and more stimulation to keep it active. This
stimulation may come in part through the ordinary channels of the sense
organs and in part from the higher brain centers, as when one keeps
awake by an effort of the will. Upon going to bed sensory stimulation is
cut off to a very large extent, also the will to remain awake is no
longer present. Under these circumstances the fatigued vasoconstrictor
center is under a minimum of stimulation and tends, therefore, to lessen
its activity. The result is that the blood pressure falls and presently
the circulation through the brain drops below the level of consciousness
and the individual is asleep. During the period of sleep the fatigued
center recuperates, so that it becomes more susceptible to sensory
stimulations and, in course of time, is aroused by such stimuli as
accompany the returning day to sufficient activity to restore the brain
to consciousness. Of course it will be perceived that there are many
things about sleep which are not satisfactorily explained on this
theory; in fact no one at the present time pretends that we can account
for it completely on this basis of changes of circulation through the
brain. It is believed, however, that they have a good deal to do with it
and it is certainly true that in healthy individuals the course of sleep
follows very closely the activity of the vasoconstrictor center. Undue
wakefulness in persons not suffering from disease can nearly always be
explained on the basis of excessive activity on the part of this
center. The activity may be the result of chemical stimulation, as when
persons are kept awake by drinking coffee or strong tea in the evening,
or by the persistence of adrenalin in the blood following a period of
great excitement. Mental activity, itself, tends to keep the
vasoconstrictor center whipped up, so that one who allows his mind to
work actively during the time when he should be asleep is very apt to
find sleep refusing to come when it is desired. The center is often
stimulated from the digestive tract; gastric irritation, even though not
acute enough to be recognized as indigestion, may cause wakefulness by
arousing nervous disturbances which play upon the vasoconstrictor
center. Flatulence, namely the presence of large volumes of gas in the
digestive tract, frequently acts as a mechanical source of irritation by
which wakefulness is induced. Evidently the factors favoring healthy
sleep are the inducing of fatigue, preferably by physical exercise, the
avoidance of chemical irritation or of excitement in the hours just
before going to bed, and finally the adoption of dietary habits which
shall insure good digestion. If an individual in whom all these
precautions are combined still continues chronically wakeful, the
trouble is sufficiently deep-seated to call for competent medical
attention.

Besides the vasoconstrictor center about which we have just been talking
there is in the brain stem a center which relaxes the tension of the
muscles in the walls of the blood vessels. This is known as the
_vasodilator_ center, and it acts in opposition to the vasoconstrictor.
In most parts of the body it does not appear to have any very great
importance for the simple reason that the blood is under such high
pressure that any relaxation of effort on the part of the
vasoconstrictor center leads at once to a forcing open of the blood
vessels. There are a few regions, however, in which the action of the
vasodilator center is of real importance; one of these is in the
skeletal muscles. These, as should be perfectly clear by this time, are
the seats of our most active functional metabolism. When the muscles are
active, great amounts of oxygen and food are being withdrawn from the
blood and large amounts of waste material, including carbon dioxide,
poured out into it. Only a very rapid circulation will take care of this
situation. At the same time the skeletal muscles are the most compact of
our living tissues. The cells are crowded together in making up the very
strong muscular machine by which our movements are performed. As the
muscles contract, they squeeze hard on the blood vessels passing through
them. In view of this situation it is very important that the blood
vessels be opened as widely as possible during muscular activity, and so
we find that the vasodilator center acts to improve the circulation
through the muscles, while they are active. In time of special
emergency, as we saw a moment ago, the vasoconstrictor center is at the
same time engaged in cutting down the blood supply to the skin and to
the abdominal organs, thus insuring to the muscles the maximum possible
nourishment through the blood stream.

Besides the skeletal muscles there is active functional metabolism in
the various secreting glands; a feature of their activity is that it
must be very great at certain times, but falls to little or nothing at
others; thus during the actual taking and digesting of food the various
glands which secrete digestive juices are exceedingly active, but in
the intervals they may be doing little or no work. They require a very
copious blood supply when they are functioning, but need very little
between times. The blood vessels flowing through all these glands are
under the influence of the vasodilator center, so that they are opened
as widely as possible while the glands are active. They, therefore,
receive much more blood in proportion to their size than they would if
it were not for the action of this center. Between times their blood
supply falls off to that which suffices for inactive tissues generally.
In time of emergency the action of the vasoconstrictor center upon the
vessels through these glands is such as to cut off the blood supply to
them almost completely. This is well illustrated in the dry mouth of the
frightened man, showing that the salivary glands have suspended
activity, a suspension due largely, if not wholly, to the cutting down
of the blood supply through them.

In the section just completed we have tried to give some idea of the way
in which the circulating blood provides the various tissues with the
materials they require and is adjusted automatically to meet variations
in demand from different tissues. Just one more point needs to be noted
in completing the account of the conveyer system. The tissue fluids
which serve as the connecting links between the circulating blood and
the individual cells are necessarily full of waste products, because
these come directly from the cells to the tissue fluids and afterward
pass on from them to the blood. The result is that the cells are
constantly bathed in a solution of their own waste products. There is
only one way in which relief from this condition can be obtained and
that is by moving the used tissue fluid away bodily and letting it be
replaced by fresh fluid. As a matter of fact, this happens; there is an
oozing of fluid through the walls of the capillaries from the blood into
the tissue spaces; this of course pushes the fluid already in those
spaces on ahead of it. If there were no place to which the fluid could
go, the result would be a swelling, as the tissue became more and more
filled with fluid. This is avoided ordinarily by a drainage system
whereby the tissue fluids are carried off as fast as more fluid comes in
from the blood, but when liquid is poured out faster than it will drain
off, as from a bruise or wrench, we do get a swelling.

The drainage system consists of very delicate vessels known as the
_lymphatics_, which come together into larger and larger vessels, just
as the veins do, and finally empty into the vena cava just at the point
where it enters the heart. There is no back pressure of blood here, so
that the movement of fluid through the lymphatics is not hindered. There
is no pump for forcing the lymph to move along; the very gradual motion,
which is all that is necessary to keep the fluid from accumulating in
the tissue spaces, is brought about by pressure upon the lymphatics
resulting from the bodily movements. The lymphatic vessels are like the
veins in having valves here and there along them. Whenever by any bodily
movement either a lymphatic vessel or a vein is squeezed, the valves
insure that the liquid shall be passed along in the direction toward the
heart and never in the reverse direction. In the case of the veins this
action is not absolutely necessary, since the heart itself is able to
maintain the circulation, although it does help, particularly in
bringing back the blood from the extremities. In the lymphatics this is
the only way by which movement of fluid is brought about. The result is
that when the body is perfectly quiet, there is very little movement of
fluid through the lymphatics; muscular activity, on the other hand,
leads to rather active movement through these vessels. The fact is well
illustrated in ourselves. One who sits for a long time humped over a
desk finds himself feeling very dull; to obtain relief he stirs about,
stretches, and yawns. The dullness was merely the result of the
stagnation of fluid in his tissues, causing the cells to be more or less
poisoned by their own waste products. By making active movements, these
stagnating fluids were forced along to be replaced by fresh liquid
direct from the blood and the beneficial effect is felt immediately.
_Massage_ properly applied has very much the same effect, although it is
doubtful whether as good results can ever be obtained thus as by actual
vigorous exercise.

At various places along the lymphatics are little spongelike lumps of
tissue known as _lymph nodes_; the particular spongy substance of which
they are composed is called _adenoid tissue_. This adenoid tissue acts
as a filter for the fluid passing through it. Any foreign particles,
living or nonliving, that get into the stream are caught in the lymph
nodes and held there more or less permanently. Most of the nonliving
particles that get into our tissue fluids are from the dust that we
inhale, which works its way through the mucous membrane of the
respiratory passages and so into the body fluids. The lymphatics that
drain the lungs carry along these dust particles and they lodge in lymph
nodes at the base of the neck. Persons who have lived in dusty regions
or have pursued a dusty livelihood, such as coal heaving, will have by
the end of their lives lymph nodes which are literally black with dirt.

The tonsils are lymph nodes at the base of the tongue. Unfortunately
they are so near the surface of the throat that they frequently become
infected from the throat itself, and so become the seats of pus pockets,
as already noted. Closely related to the tonsils are the masses of
adenoid tissue at the back of the throat which frequently grow to an
undue size in children, and are then known as adenoids. The harm done by
adenoids is chiefly mechanical; they may block the Eustachian tubes, and
so cause deafness, as already mentioned in Chapter IX, or they may
interfere with the free movement of the air through the nasal passages.
Children in whom this condition exists are mouth breathers. Adenoids,
like tonsils, are subject to infection, and so may give trouble by
becoming the sites of poison formation. Adenoids represent always an
overgrowth and for that reason may be removed without any possibility of
hampering the proper working of the body. Experience has shown that
children whose development appears to be hindered by the presence of
adenoids are almost invariably benefited by having them removed. The
tonsils are normal parts of the bodily structure and as such undoubtedly
have a regular work to do, but here again experience has shown that harm
from persistent pus pockets is so much greater than harm from loss of
function following their removal as to justify taking them out, whenever
pus pockets develop in them. There are enough lymph nodes in the region
about the throat, so that if tonsils or adenoids are removed any foreign
matters that get into the body fluids will still be filtered out.

[Illustration: Copyright, Keystone View Co.

X-RAYING THE TEETH AND JAW. HEALTH OFTEN DEPENDS ON THEIR GOOD
CONDITION]

[Illustration: Photo, Paul Thompson

A THROAT EXAMINATION--IMPORTANT IN DETERMINING PHYSICAL FITNESS]

One more function of the lymph nodes must be mentioned; this is their
property in preventing the spread of cancer cells. We now know that
so-called secondary cancers are the result of the spread of cancer cells
from the original seat of the cancer to other parts of the body, and
that this spread is much hindered by the ability of the lymph nodes to
catch the cancer cells and hold them. Unfortunately sooner or later some
of the cells will escape beyond the lymph nodes and so spread the
malignant growth throughout the body, but until this happens the cancer
is confined to the region where it started, and it is during this period
that complete cure by surgical means is possible. It is because of the
imminent danger of the escape of cancer cells beyond the restraining
lymph nodes that relief by surgery should be sought at the very earliest
possible moment. Delay, whether due to carelessness or any other cause,
is as certainly fatal in the case of cancer as in any other disease for
which a cure is known.

CHAPTER XV

THE SERVICE OF SUPPLY OF FOOD

In order for the blood that circulates through the body to pass on to
the body cells the materials which they need, it is evident that the
blood itself must have some source from which to obtain the materials.
Our present task is to examine this source. In Chapter IV we talked
about food and its uses; here we are concerned with the way in which the
foodstuffs are taken into the body and prepared for use. We know that
the material which we take in is part solid and part liquid, giving rise
to the familiar distinction between food and drink. For our present
purposes this distinction has no importance and will not be made. What
we do have to note is that of the complex mixture of materials which
makes up any ordinary meal, some of the substances are ready to be taken
up at once by the blood and to be distributed around the body; others
have to go through a preliminary course of preparation. All of the
dietary accessories, so far as we know, pass from the digestive organs
into the blood without change. All or nearly all of the energy-yielding
foods, on the other hand, must have a preliminary preparation to which
we give the name of digestion; the operation of digestion being to break
down the complex foodstuffs that are eaten into simpler materials of
which the body can make use. Digestion is made up of a series of
chemical changes by which the large molecules of the original foodstuffs
are broken into smaller and simpler molecules. This breaking down of
large molecules into smaller is a very common kind of chemical process.
A feature of it is that under ordinary circumstances the breaking down
of the large molecule goes on very slowly, but if the right conditions
are provided the breaking down proceeds rapidly. The particular
condition which is necessary is the presence in the solution where the
large molecules are breaking down of something which will hurry up the
process. There are a good many kinds of substances which have this
ability to hasten molecular decomposition; those that do it in carrying
on digestion are given the name of _enzymes_. We do not know just how
the enzymes act; we merely know that when they are put along with the
substance which is to be decomposed, it goes to pieces very much more
rapidly than if none of them was present. The enzymes themselves are not
used up in the process, so if time enough is allowed, a very small
amount of enzyme can bring about the decomposition of a very large
amount of material. In our bodies the whole digestive process consists
of a succession of decompositions of complex materials into simpler ones
under the speeding-up influence of enzymes. We have a number of
different kinds of foodstuffs to be decomposed and a corresponding
number of enzymes.

The process begins in the mouth; here the food is chewed and moistened
with saliva before being swallowed; both the chewing and the moistening
are important to insure good digestion later on. The enzymes have no
particular ability to penetrate a mass of food material; what they do is
to attack it from the outside and work in as it decomposes. Since
enzyme action is thus a surface action, it is evident that the larger
the surface the more efficient will be the action. Chewing is nothing in
the world but a mechanical breaking up of the food to get the largest
possible surface. We have sufficient proof of its importance in the
digestive disturbances that arise as the direct consequence of improper
chewing. Of recent years it has been realized that undernutrition,
particularly in children, is often a result of the failure to chew the
food properly; it has been found, furthermore, that bad teeth or
improperly shaped mouths are very frequently responsible. For this
reason in most of our large cities dental clinics are being established
for the purpose of inspecting and, if necessary, caring for the mouths
of school children. The result of this work is to improve the general
average of health among children simply by increasing the extent to
which the food is chewed. This fact, together with that described in
Chapter XIII as to pus pockets, should impress upon both parents and
teachers the fundamental importance of proper care of the teeth. This
includes not only the prevention of decay by daily thorough cleaning and
the securing at intervals of not more than six months of dental
inspection with treatment where necessary, but also in the case of
children with deformed jaws the special treatment necessary to bring the
teeth into position for effective chewing. Too much stress can scarcely
be laid on the importance of these precautions.

The moistening of the food with saliva is likewise important to good
digestion. As a matter of fact, without it even the act of swallowing
would be impossible. One can easily prove that insufficient moistening
prevents swallowing by eating a rather large quantity of a very dry
food, like crackers. A common habit of people is to moisten the food
with water which they drink instead of waiting for it to be properly
moistened by saliva. Contrary to popular belief there is absolutely no
objection to drinking water at mealtime. In fact, the presence of the
additional liquid in the alimentary tract is probably beneficial rather
than the reverse; there is, however, a very serious objection to using
water for washing down half-chewed food. The best possible way to judge
whether the food is sufficiently chewed is by observing whether it has
been sufficiently moistened so that it will be swallowed easily. If so,
the chances are that the chewing has been sufficient. Between mouthfuls
as much water may be taken as one desires, although if ice water is
drunk, it should be held in the mouth until the worst chill is taken off
before being swallowed, so that it will not chill the stomach. Since the
comfort of drinking ice water is in the cooling of the mouth and throat
and not at all in the cooling of the stomach, this increases rather than
diminishes the enjoyment one gets from a glass of it on a hot day. So
much satisfaction can be obtained by proper drinking habits, that it is
a pity to allow the health to be injured by improper habits to such an
extent as to necessitate, as frequently happens, the complete
abandonment of water drinking with meals.

The saliva is primarily for the purpose of moistening the food, but
besides this it is also a definite digestive juice, because it contains
one of the digestive enzymes. This enzyme, to which is given the name
_ptyalin_, acts upon starch, changing it to sugar, but not to the
particular sugar of which the body makes use. Perhaps we should digress
for a moment to say that there are chemically a number of sugars. These
are in many respects alike, although they also vary a good deal among
themselves. For example, some are much sweeter than others; glucose,
which is a sugar that can be made from cornstalks and other plant
products, is not nearly so sweet as cane or beet sugar, although one is
fully equal to the other so far as nutritive value is concerned. Of all
the various kinds of sugar that exist, only those in the glucose class
can actually be used by the body, so that all other sugars have to be
changed by digestion into a glucoselike sugar before passing on into the
blood. The enzyme of saliva converts starch not into glucose but into a
more complex sugar to which is given the name of maltose. It, therefore,
starts digestion, but does not carry it through to completion. Starch
digestion begins during the course of chewing and mixing the food with
saliva; it goes on while the food is being swallowed, and for a short
time after it enters the stomach, being stopped there sooner or later by
means which will be described in a moment. No other real digestion
occurs in the mouth, although the chewing and moistening are of very
great importance in preparing the food for the digestion that is to come
further along.

The food that is swallowed passes down the esophagus and enters the
stomach. Before speaking of this it will be necessary to recall what was
said in Chapter VII about the behavior of the stomach walls. Between
meals these are in a relaxed and flabby state, with the opposite walls
lying more or less in contact. There is usually a little liquid and some
swallowed air in the stomach, so that it is not actually empty, even
when we speak of it as being so. Just before mealtime the walls of the
stomach draw up so that instead of a flabby bag we have a fairly tense
tubular organ. It is at this time that the contractions of the stomach
wall, which we recognize as hunger, begin to come on. There is a
sphincter muscle between the esophagus and stomach which closes the
opening tightly and so prevents the pressure within the stomach from
forcing gas or liquid back up into the esophagus. This sphincter opens
automatically only in connection with the act of swallowing. Every time
we make a swallowing movement a sort of wave passes down the esophagus,
and when this wave arrives at the stomach the sphincter relaxes,
allowing whatever was moving down the esophagus to enter. If one watches
a horse or cow in the act of swallowing, the rather deliberate progress
of this wave down the neck can be followed. What happens as we eat a
meal is that every mouthful, as it is swallowed, is passed through the
sphincter into the stomach and room is made for it by a gradual
relaxation of the stomach wall, so that, as we saw in Chapter VII, the
pressure of the stomach against its contents stays fairly steady, in
spite of the fact that more and more material is being taken into the
stomach from the esophagus. One result of this behavior of the stomach
is that the first food that is swallowed is nearest to the stomach
walls, and that which is swallowed afterward is nearer the middle, being
inclosed on all sides by the previously swallowed food. This is fairly
important because the gastric juice is secreted by glands in the wall
of the stomach and so will get in contact first with the food that is
swallowed first, and only afterward will reach the food that was
swallowed later. Occasionally the sphincter between the stomach and the
esophagus relaxes unexpectedly; this is said to happen more often in
smokers than in nonsmokers. The result is that some of the sharply acid
stomach contents are forced up into the esophagus and vigorous
swallowing is necessary to crowd them back down into the stomach. The
burning sensation which accompanies this is known as heartburn,
although, as we have just seen, it is really entirely a matter of the
esophagus and has nothing whatever to do with the heart.

Although the stomach carries on a certain part of the work of digestion,
its primary purpose, as we shall see presently, is to serve as a storage
place into which a considerable amount of food can be placed in a few
moments and so enable us to do our eating at three or four definite
meals instead of little by little throughout the day. When we rise from
table after any meal, we have in our stomachs roughly one-third of our
total food supply for the day, the exact proportion depending, of
course, on our individual habits as regards our distribution of
food-taking among the three meals. During the two to four hours
following the meal this accumulated material in the stomach will be
passed along little by little to the small intestine, which is the real
digestive organ of the body. In the course of this time there will be
some additional digestion within the stomach, but not enough to prepare
any foodstuffs for actual use by the tissues. The outlet from the
stomach to the small intestine is guarded by

[Illustration: THE ALIMENTARY CANAL

_A_, stomach; _J_, _I_, small intestine; near _CC_, vermiform appendix;
_AC_, _TC_, _DC_, large intestine; _R_, rectum. (Martin’s “Human Body”)]

another sphincter similar to that which closes the opening between the
esophagus and stomach. This sphincter is ordinarily closed tightly,
relaxing occasionally to let a little food through into the small
intestine. It is interesting to note, however, that the sphincter does
not hold against some materials; for example, water that is drunk
between meals passes rapidly from the stomach on into the small
intestine. It is said that raw oysters, raw white of egg, and other
materials of a similar consistency also pass the sphincter immediately.
Most other kinds of food material cannot pass unless the sphincter
actually relaxes.

Shortly after food begins to enter the stomach churning movements are
set up. These consist of regular contractions of the smooth muscle
beginning at about the middle of the stomach in the form of a ring and
so causing a deep depression to form right around the stomach, which
then travels along toward the sphincter at the outlet. This, of course,
crowds the food up toward that end; but if the sphincter does not relax,
the food instead of escaping simply squirts back through the space left
in the middle of the ringlike construction and so is actively churned. A
regular procession of these waves travels over this part of the stomach
during the whole time that gastric digestion is going on. The outer half
of the stomach, the side toward the esophagus, does not join in this
churning motion. The walls here remain quiet, merely pressing upon the
food to crowd it up into the part where the churning is going on as fast
as room is made for it there. At intervals during this process the
sphincter relaxes and a small mass of food is crowded through into the
small intestine; then the sphincter closes again, preventing more from
passing until the former mass has had time to be acted upon by the
digestive juices in the intestine. The way in which this sphincter is
controlled is one of the very interesting facts of physiology, but
before describing it we shall have to say something about actual gastric
digestion.

The juice which the glands in the stomach wall secrete is called gastric
juice. This contains three important constituents, first of which is
hydrochloric acid. From the standpoint of chemistry it is a very
interesting thing that the gastric glands should he able to manufacture
a mineral acid like hydrochloric acid. That they do so, however, is
proven by the facts of digestion in everybody. In addition to
hydrochloric acid gastric juice contains two enzymes; the first is
pepsin, which begins the digestion of protein foods, although it does
not carry it through to completion. The other constituent is the enzyme
rennin, which clots milk; this property appears to be useful in that
clotted milk will not pass the sphincter into the small intestine as
quickly as it would if it were liquid, and so the digestive processes
can go on in milk after clotting just as it goes on in any other of our
foods. Since milk is the chief, and in many cases the only, food of the
young it is of course very important that its digestion should be very
efficiently carried on. As the food is churned by the stomach muscles,
it becomes mixed with gastric juice. Any milk that is present will be
clotted and any protein will begin to be digested. The food will also be
mixed with hydrochloric acid. It happens that the salivary enzyme,
ptyalin, will not act in the presence of an acid, so that as soon as any
of the food in the stomach comes in contact with hydrochloric acid of
the gastric juice, the digestion of starch by ptyalin stops. Since
gastric juice is mixed only with that part of the food which is being
churned, the other portion, that in the part of the stomach toward the
esophagus, goes right on undergoing salivary digestion. So, contrary to
the old idea that salivary digestion is unimportant because there is not
time enough during the chewing and swallowing of the food for it to go
on effectively, we now know that the latter parts of the meal
particularly may be very thoroughly acted upon by ptyalin before the
action is stopped through the contact of this part of the food with the
acid of the gastric juice.

The sphincter between the stomach and small intestine is operated by
hydrochloric acid, making up a remarkably ingenious arrangement for
securing the emptying of the stomach as fast as the food is ready to be
passed on into the small intestine. In order that gastric digestion may
have full opportunity, it is necessary that the food should be
thoroughly mixed with the gastric juice. As soon as this has happened,
however, it is proper that the food should be passed along to be acted
on by the rest of the digestive juices. Until that part of the food
which is next to the outlet sphincter is thoroughly mixed with gastric
juice there will be no surplus of hydrochloric acid by which to operate
the sphincter, but as soon as the mixing is complete there will be a
surplus. We have here a simple case of stimulation by a chemical
substance; the mere contact of the acid with the part of the sphincter
which fronts on the stomach is sufficient to stimulate the smooth
muscles in it to relax, and, of course, as soon as relaxation occurs,
the pressure of the churning movements upon the food will crowd that
part of it which is nearest to the sphincter through into the small
intestine.

Now we encounter the second part of the action which is just as
interesting as the first part. The small intestine is a rather narrow
tube; its cavity is not much more than three-quarters of an inch or so
in diameter. If the outlet sphincter from the stomach were to stay
relaxed after it once let go, the rapid pressing of the food from the
stomach would fill the small intestine quickly to a distance of several
feet. It happens, however, that the next of the digestive juices to act
upon the food are the pancreatic juice and bile, which are secreted
respectively by the pancreas and liver, and are poured into the small
intestine by ducts from these organs just a few inches beyond the outlet
sphincter from the stomach. In order for these juices to mix well with
the food it is important that only a little food come into the small
intestine at a time. Otherwise a large part of it would pass the opening
of these ducts so quickly that there would be no chance for it to become
mixed with the juice from them. What actually happens is that the
sphincter relaxes under the stimulus of surplus hydrochloric acid and
then as soon as a little food has passed through closes again. The
closing as well as the opening is operated by hydrochloric acid, but
there is this difference: the opening is the result of the stimulation
of acid upon the stomach side of the sphincter; when the same acid comes
in contact with the intestinal side of the sphincter, its presence there
causes the sphincter to contract. Of course, this means that whenever
any food passes out from the stomach into the small intestine the
sphincter is stimulated by acid simultaneously from both sides, but
under that condition the stimulus which causes the sphincter to contract
is more potent than that which causes it to relax, so that whenever
there is acid on both sides the sphincter will be shut. The only way in
which it can be opened to allow more food to pass out is to get rid of
the acid on the intestinal side. This is fairly quickly accomplished,
because both the pancreatic juice and bile are strongly alkaline
liquids, so that as fast as they mix with the food they tend to
neutralize the acid; the result is, of course, that by the time these
digestive juices have impregnated the food mass thoroughly and so have
accomplished their purpose, the stimulus which keeps the sphincter
closed has disappeared and there is left only the stimulus to open it,
due to the presence of acid in the stomach. As soon as this condition is
reached, the sphincter will open again, another mass of food will pass
through bringing its acid with it, which, of course, closes the
sphincter promptly, and so the whole story goes on again. This action
has been described in detail because it is one of the very best examples
we have of the remarkably ingenious arrangements by which the complex
bodily functions are carried on automatically.

From what we have just said it is evident that the proper emptying of
the stomach and, therefore, the proper carrying on of the whole process
of digestion, depends upon the formation and outpouring of ample
supplies of acid-containing gastric juice. A failure of gastric
secretion is bound to be followed by a failure of the stomach to empty
itself. Instead of going on to the small intestine where digestion is
completed and the digestion products passed on to the blood stream to be
used by the body, the food mass lies inert in the stomach. After a few
hours, if not immediately, the acute symptoms of indigestion are certain
to develop. We are describing not a theoretical possibility but an
actual happening in the life of thousands of people, sometimes almost
daily. Indigestion is one of the great causes of misery and of impaired
efficiency throughout the civilized world. It is said that Napoleon lost
the Battle of Waterloo on account of an acute indigestion which befell
him on the morning of the crucial day.

There are several causes of indigestion. Among them should be mentioned
food poisoning; that is, the inadvertent taking of some material which
instead of being a food proves to be actively poisonous. A second type
about which we shall have something to say presently is the result of
poisoning by substances which are products of the putrefaction of the
intestinal contents. A third type, and the one in which we are
particularly interested at this moment, is the one which results from
the failure of gastric juice to be properly secreted. A fourth
occasional cause of indigestion, which should be mentioned in passing,
is that which results from eyestrain, and is avoided by properly fitted
glasses. There is comparatively little food poisoning; most persons who
are susceptible to poisoning from particular kinds of foods find it out
promptly and learn to avoid the article which poisons them. A good deal
of indigestion is the direct result of intestinal sluggishness; this
usually comes on gradually enough to give warning of its approach. Sharp
attacks for which there seems to be no justification are nearly always
the result of the failure of gastric secretion. Because of the practical
importance of this topic a very large amount of study has been devoted
to it, and its story makes one of the very fascinating chapters of
physiology.

Our space permits us to touch on it only very lightly. It begins with
the time back in the middle of the eighteenth century when a couple of
Italian investigators fed sponges inside perforated metal capsules to
birds and animals and after recovering and squeezing the sponges
demonstrated that the juice which had been soaked up would dissolve
meat. The scene then shifts from Italy in the middle of the eighteenth
century to our own country at the close of the first quarter of the
nineteenth, when a French-Canadian lad, by the name of Alexis St.
Martin, was brought in to the frontier army post on Mackinac Island with
a gaping gunshot wound in his side. The victim of this accident
recovered, but meanwhile a permanent opening into his stomach was left.
The army doctor on duty at that post, William Beaumont by name,
perceiving the unique opportunity for the study of digestion that was in
his hands, employed the young man in his family and for many years
studied the digestive process as it went on in his stomach. Next we turn
to Russia, where an eminent physiologist, who was still living at last
reports, demonstrated by making artificially an opening into the stomach
of animals, similar to the one which was made by accident in the case of
St. Martin, that the secretion of gastric juice is under the control of
the nervous system and, furthermore, that it is under the control of
that part of the nervous system which we have already learned regulates
the functioning of smooth muscles and glands. Our present knowledge of
the nervous system would lead us to expect nothing else, but at the time
when these discoveries were made much less was known than now about how
the glands, as well as the smooth muscles, are controlled. This Russian
investigator showed that it is not necessary for food to enter the
stomach in order for gastric juice to be secreted. The enjoyment that
accompanies the taking of food into the mouth and so getting its taste
and flavor is sufficient to arouse nervous disturbances which excite the
gastric glands to activity. On the other hand, unfavorable emotions such
as anger, fright, or worry, prevent the discharge of these nervous
disturbances and so the glands remain inactive. Hand in hand with this,
as we saw in Chapter XII, there will be a complete absence of muscular
activity in the stomach; not only does the food fail to be churned, but
gastric juice fails to be poured out. No digestion goes on in the
stomach nor is the food passed on into the small intestine to be
digested there. We can expect nothing else than that indigestion will
come on. It appears that in human beings, with their high nervous
organization, simple weariness may suffice to hinder the outpouring of
the nervous disturbances upon which gastric secretion depends, so that
one may not be worried at mealtime, but may be simply overtired and
still fail of digesting the food. Fortunately we have means of avoiding
this; it has been shown that meat extracts have a definite chemical
effect upon the gastric glands, arousing them directly to activity. A
hot, well-flavored soup at the beginning of a meal is a great aid to
digestion, both because it tends to arouse us to active enjoyment of the
meal and also because of the chemical influence it exerts directly upon
our gastric glands. Cheerful conversation at table is an aid to
digestion, because it contributes to our enjoyment of our food. Heated
or angry discussion is to be avoided because of the danger that
unfavorable emotional disturbances may be aroused, hampering or even
putting a stop to the digestive activities.

We have here a powerful argument in favor of good cookery; some may be
inclined to think that food is food and that one kind is about as
nourishing as another. Not only theoretically, but in practice, food
that is attractive both in appearance and in flavor benefits the body
more than unattractive and badly cooked food. Of course, we must
recognize that no statement of this sort has absolutely general
application. The old proverb which says that “hunger is the best sauce”
continues true. Hard-working manual laborers and actively playing
children enjoy and digest food that is indifferently attractive. Brain
workers and people of moderate health are better for an attractively
prepared dietary.

We have now traced the food into the small intestine and will remember
that up to this point the actual act of digestion has not gone very far.
The enzyme of saliva has decomposed part of the starch, or perhaps all
of it, into a complex sugar; the enzyme of gastric juice has split some
of the very complex proteins into simpler ones. Of the final products of
digestion none has yet been formed; their formation is the work of the
small intestine. Into the upper end of it, as we have already seen,
pancreatic juice and bile are poured from the pancreas and liver
respectively. The pancreatic juice contains three enzymes; one of them
is identical with that of saliva and completes the decomposition of any
starch that the saliva has failed to act upon; the second is an enzyme
which acts upon proteins, differing from pepsin of gastric juice in that
it carries the splitting of proteins to completion; the third is the
enzyme which acts upon fats. Fats are the most troublesome of all the
foodstuffs from the digestive standpoint, because they are insoluble in
water. Not only do they themselves offer difficulty, but they smear over
the other food masses and make it hard for the digestive juices to get
at them. Although the enzyme for digesting fat is secreted by the
pancreas, its successful working depends on the presence of bile from
the liver. If there is an obstruction of the bile passages, so that bile
cannot be poured out into the small intestine, the digestion of fats
stops and, along with it to a considerable extent, all the other
digestive processes of the intestine. There is a familiar common
condition known as jaundice, in which the bile passages become stopped,
usually as the result of inflammation. When one is suffering from this
condition, it is very important that not much fat be included in the
diet; otherwise there is likely to be trouble with indigestion.

In addition to the pancreatic juice and bile there is a secretion known
as intestinal juice which is poured out by small glands scattered along
the wall of the small intestine. These secrete enzymes which convert the
various kinds of sugar into the particular sugar, glucose, which is the
one the body can use. The glands also secrete an enzyme which can carry
on to completion protein digestion. Thus there is ample insurance that
all the foodstuffs shall be made ready for the use of the body.

The next thing in order is for the digested foods to pass from the
digestive cavity into the blood. This process is known as absorption.
The first part of the alimentary tract from which absorption might occur
is the stomach, but, as we have just seen, the process of digestion is
not carried far enough in the stomach to fit the foods for going on into
the blood stream. Absorption from the stomach is, therefore, undesirable
rather than desirable. This is interesting because many condiments,
particularly the hot spices and mustard, irritate the stomach lining in
such a way as to promote absorption through it. Formerly this was looked
upon as an argument in favor of the use of these condiments, but we now
realize that it is rather an argument against them. The same applies to
alcohol. Alcohol irritates the lining of the stomach and produces
absorption through it. Unfortunately for those who desire a
physiological argument in favor of the use of alcohol at meals this is a
disadvantage rather than an advantage. The great region of absorption is
the small intestine, where the digestive process is completed and where,
therefore, absorption can properly go on. As to the method of absorption
not a great deal needs to be said; there is a very delicate membrane
lining the intestine, and just beneath this are the very numerous
capillaries through which the blood is flowing; also in the spaces
between the capillaries is the tissue fluid which drains into the
lymphatic system. The products of protein digestion and glucose pass
through the lining membrane of the intestine and through the capillary
walls directly into the blood stream and are carried along in it. The
digested fats, on the other hand, after passing through the intestinal
lining enter the tissue fluids instead of the blood. Incidentally it
should be noted that they are rebuilt into tiny fat droplets during the
process of absorption, so that what we find in the tissue fluids of the
intestine are true fats and not digestion products of fat. These tiny
fat droplets pass away from the intestine by way of the lymphatic
system. This, as we saw in the last chapter, drains finally through
large lymphatic vessels into the great veins just at the entrance of the
heart, and so the fat finally reaches the blood stream, but not so
directly as do glucose and the digestion products of protein.

There is a reason for this difference in the handling of fats and the
other foodstuffs which will be clear when we recall the point made in
the last chapter that all the blood which flows through the walls of the
intestine is collected in the portal vein and must pass again through
the capillaries of the liver before entering the circulation at large.
What happens is that all the digestion products, except the fats, pass
through the capillaries of the liver, but the fat gets into the blood
stream by another way and does not go through the liver en route; for
some reason or other it seems to be to the advantage of the body that
the fat should not be allowed to circulate through the liver
capillaries. It is, as we shall see in a later chapter, a distinct
advantage to have the glucose pass through the liver, and there is
probably also a good reason for having the digestion products of protein
take that course.

The digesting food is moved along the small intestine by contractions of
its muscular wall, which travel along slowly in the form of a wave, very
much as the wave of swallowing passes down the neck of a cow or horse.
These waves do not occur regularly but only now and then; what happens
is that several inches or two or three feet of the small intestine will
be filled with a food mass which stays for a while at that place,
digestion going on in it, and the digested food being absorbed through
the wall. Both the digestion and the absorption are aided by a kind of
churning motion made up of a series of contractions spaced about an inch
apart along the part of the intestine where the food mass lies. These
contractions form and disappear quite regularly at the rate of several a
minute. They have the effect of squeezing the food mass rhythmically,
but do not move it away from the place where it is lying. After this
churning has been going on for a while it subsides, leaving that part of
the intestine entirely quiet; then a contraction wave sets in at the end
of the food mass toward the stomach and pushes the mass bodily along the
intestine to a new section where the wave dies out and the churning
motion begins again. This process is repeated with every food mass as it
comes through from the stomach into the intestine until, finally,
usually three or four hours after the first food has commenced to come
into the intestine from the stomach, the whole mass has been propelled
the length of the small intestine and what is left has been passed on
through into the large intestine. The small intestine is about
twenty-five feet long in man; flesh-eating animals of about the same
size have intestines only about one-half as long; grazing animals the
same size, on the other hand, have intestines two or three times longer
than those of man; this intermediate length of the human intestine is
ordinarily looked upon as indicating that man is adapted to a mixed
diet, being neither strictly a flesh-eating nor strictly a
vegetable-eating animal.

During the slow passage of the food mass through the small intestine the
processes of digestion and absorption are completed, so that very little
material enters the large intestine except the indigestible parts of the
original food. These are made up mostly of cellulose and similar
indigestible vegetable materials, but they include also indigestible
fragments of gristle and other meat remnants. It is also worthy of note
that the amount of water does not decrease much during the passage of
material through the small intestine; in other words the material that
enters the large intestine is about as liquid as that which passes from
the stomach in to the small intestine. This does not mean that there is
no absorption of water from the small intestine; in fact, water that we
drink is probably absorbed quite rapidly; what it does mean is that
enough water is secreted into the intestine from the blood in the lower
parts to keep the whole mass liquid; this is of course important as an
aid both to digestion and absorption.

The large intestine, as its name implies, is a tube of much larger
diameter than the small; it begins low down on the right-hand side,
passes up to about the level of the stomach, then across the body and
down on the left-hand side, making thus a sort of inverted “U” in the
abdomen; the space within the “U” is filled up by the loops and coils of
the small intestine. The first part of the large intestine is devoted to
the absorption of water; the result of this absorption is to leave the
contents of the large intestine in a semidry condition; absorption of
water goes on continuously, so that the longer the material remains in
the large bowel, the drier will it become. In the ordinary course of
events the material, as it is dried out, is passed along through the
large intestine and at intervals, which should not exceed twenty-four
hours, the accumulated material is discharged from the body.

Various facts about the functioning of the intestine which have seemed
to many people mysterious are easily explained on the basis of what we
have next to talk about, which is the presence both in the small and in
the large intestine of flourishing colonies of bacteria. It has perhaps
not occurred to most of us that we are in the position of involuntary
hosts to enormous numbers of these microscopic plants, but that is
exactly what we are. There is no possibility of preventing this, since
the combination of warmth, moisture, and abundant food, which is
afforded within the intestine, is the most favorable possible situation
for many kinds of bacteria, and no effort on our part would enable us to
be free from them. Fortunately, in the ordinary course of events, we are
not affected one way or the other by their presence. In the small
intestine their chief diet consists of the sugars which are produced as
the result of starch digestion or are eaten directly. In connection with
nourishing themselves from these sugars, they bring about what is known
as alcoholic fermentation; that is, some of the sugar is converted into
alcohol with a by-product of carbon dioxide. Exactly the same thing
happens when yeast develops in a mass of dough; in the latter case the
bubbles of carbon dioxide are what we are after, since they are what
makes the bread light, the alcohol being driven off by the heat of
cooking. In the alcoholic fermentation that goes on in our small
intestines the carbon dioxide is a waste product and is passed on out of
the body, but the alcohol is absorbed and used as fuel. Little has been
said thus far about the physiological effects of alcohol. At this point
all we care to say is that alcohol can be oxidized by the body tissues
with the liberation of energy and so is a perfectly good fuel food. In
the minute amounts in which it is absorbed from the small intestine it