WHY WE HAVE A STOMACH
WHAT KEEPS US ALIVE
The Energy in Food and Fuel. The first question that arises in our
mind on looking at an engine or machine of any sort is, What makes it
go? If we can succeed in getting an answer to the question, What makes
the human automobile go? we shall have the key to half its secrets at
once. It is fuel, of course; but what kind of fuel? How does the body
take it in, how does it burn it, and how does it use the energy or power
stored up in it to run the body-engine?
Man is a bread-and-butter-motor. The fuel of the automobile is gasoline,
and the fuel of the man-motor we call food. The two kinds of fuel do not
taste or smell much alike; but they are alike in that they both have
what we call _energy_, or power, stored up in them, and will, when set
fire to, burn, or explode, and give off this power in the shape of heat,
or explosions, which will do work.
Food and Fuel are the Result of Life. Fuels and foods are also alike
in another respect; and that is, that, no matter how much they may
differ in appearance and form, they are practically _all the result of
life_. This is clear enough as regards our foods, which are usually the
seeds, fruits, and leaves of plants, and the flesh of animals. It is
also true of the cord-wood and logs that we burn in our stoves and
fireplaces. But what of coal and gasoline? They are minerals, and they
come, as we know, out of the depths of the earth. Yet they too are the
product of life; for the layers of coal, which lie sixty, eighty, one
hundred and fifty feet below the surface of the earth, are the
fossilized remains of great forests and jungles, which were buried
millions of years ago, and whose leaves and branches and trunks have
been pressed and baked into coal. Gasoline comes from coal oil, or
petroleum, and is simply the "juice" which was squeezed out of these
layers of trees and ferns while they were being crushed and pressed into
coal.
How the Sun is Turned into Energy by Plants and Animals. Where did the
flowers and fruits and leaves that we now see, and the trees and ferns
that grew millions of years ago, get this power, part of which made them
grow and part of which was stored away in their leaves and branches and
seeds? From the one place that is the source of all the force and energy
and power in this world, the sun.
That is why plants will, as you know, flourish and grow strong and green
only in the sunlight, and why they wilt and turn pale in the dark. When
the plant grows, it is simply sucking up through the green stuff
(_chlorophyll_) in its leaves the heat and light of the sun and turning
it to its own uses. Then this sunlight, which has been absorbed by
plants and built up into their leaves, branches, and fruits, and stored
away in them as energy or power, is eaten by animals; and they in turn
use it to grow and move about with.
Plants can use this sun-power only to grow with and to carry out a few
very limited movements, such as turning to face the sun, reaching over
toward the light, and so on. But animals, taking this power at
second-hand from plants by eating their leaves or fruits, can use it not
merely to grow with, but also to run, to fight, to climb, to cry out,
and to carry out all those movements and processes which we call life.
Plants, on the other hand, are quite independent of animals; for they
can take up, or drink, this sun-power directly, with the addition of
water from the soil sucked up through their roots, and certain salts[1]
melted in it. Plants can live, as we say, upon non-living foods. But
animals must take their supply of sun-power at second-hand by eating the
leaves and the fruits and the seeds of plants; or at third-hand by
eating other animals.
[Illustration: WHERE SUN-POWER IS MADE INTO FOOD FOR US]
All living things, including ourselves, are simply bundles of sunlight,
done up in the form of cabbages, cows, and kings; and so it is quite
right to say that a healthy, happy child has a "sunny" disposition.
Plants and Animals Differ in their Way of Taking Food. As plants take
in their sun-food and their air directly through their leaves, and their
drink of salty water through their roots, they need no special opening
for the purpose of eating and drinking, like a mouth; or place for
storing food, like a stomach. They have mouths and stomachs all over
them, in the form of tiny pores on their leaves, and hair-like tubes
sticking out from their roots. They can eat with every inch of their
growing surface.
But animals, that have to take their sun-food or nourishment at
second-hand, in the form of solid pieces of seeds, fruits, or leaves of
plants, and must take their drink in gulps, instead of soaking it up all
over their surface, must have some sort of intake opening, or mouth,
somewhere on the surface; and some sort of pouch, or stomach, inside the
body, in which their food can be stored and digested, or melted down. By
this means they also get rid of the necessity of staying rooted in one
place, to suck up moisture and food from the soil. One of the chief and
most striking differences between plants and animals is that animals
have mouths and stomachs, while plants have not.
THE DIGESTIVE SYSTEM
How the Food Reaches the Stomach. Our body, then, has an opening,
which we call the _mouth_, through which our food-fuel can be taken in.
A straight delivery tube, called the _gullet_, or _esophagus_, runs down
from the mouth to a bag, or pouch, called the _stomach_, in which the
food is stored until it can be used to give energy to the body, just as
the gasoline is stored in the automobile tank until it can be burned.
The mouth opening is furnished with _lips_ to open and close it and
assist in picking up our food and in sucking up our drink; and, as much
of our food is in solid form, and as the stomach can take care only of
fluid and pulpy materials, nature has provided a mill in the mouth in
the form of two arches, of semicircles, of _teeth_, which grind against
each other and crush the food into a pulp.
[Illustration: THE FOOD ROUTE IN THE DIGESTIVE SYSTEM
In this diagram the entire alimentary canal is shown enlarged, and the
small intestine greatly shortened, in order to show distinctly the
course of the food in the process of digestion.]
In the bottom or floor of the mouth, there has grown up a movable bundle
of muscles, called the _tongue_, which acts as a sort of waiter, handing
the food about the mouth, pushing it between the teeth, licking it out
of the pouches of the cheeks to bring it back into the teeth-mill again,
and finally, after it has been reduced to a pulp, gathering it up into a
little ball, or _bolus_, and shooting it back down the throat, through
the gullet, into the stomach.
The Intestines. When the food has been sufficiently melted and
partially digested in the stomach, it is pushed on into a long tube
called the _intestine_, or _bowel_. During its passage through this part
of the food tube, it is taken up into the veins, and carried to the
heart. From here it is pumped all over the body to feed and nourish the
millions of little cells of which the body is built. This bowel tube, or
intestine, which, on account of its length, is arranged in coils,
finally delivers the undigested remains of the food into a somewhat
larger tube called the _large intestine_, in the lower and back part of
the body, where its remaining moisture is sucked out of it, and its
solid waste material passed out of the body through the _rectum_ in the
form of the _feces_.
THE JOURNEY DOWN THE FOOD TUBE
The Flow of Saliva and "Appetite Juice." We are now ready to start
some food-fuel, say a piece of bread, on its journey down our food tube,
or _alimentary canal_. One would naturally suppose that the process of
digestion would not begin until the food got well between our teeth;
but, as a matter of fact, it begins before it enters our lips, or even
before it leaves the table. If bread be toasted or freshly baked, the
mere smell of it will start our mouths to watering; nay, even the mere
sight of food, as in a pastry cook's window, with the glass between us
and it, will start up this preparation for the feast.
This flow of saliva in the mouth is of great assistance in moistening
the bread while we are chewing it; but it goes farther than this. Some
of the saliva is swallowed before we begin to eat; and this goes down
into the stomach and brings word to the juices there to be ready, for
something is coming. As the food approaches the mouth, a message also is
telegraphed down the nerves to the stomach, which at once actively sets
to work pouring out a digestive juice in readiness, called the "appetite
juice." This shows how important are, not merely a good appetite, but
also attractive appearance and flavor in our food; for if this appetite
juice is not secreted, the food may lie in the stomach for hours before
the proper process of digestion, or melting, begins.
The Salivary Glands. Now, where does this saliva in the mouth come
from? It is poured out from the pouches of the cheeks, and from under
the tongue, by some little living sponges, or juice factories, known as
_salivary glands_.[2]
[Illustration: THE SALIVARY GLANDS
In this diagram are shown the three glands (_G_) of the left side. The
duct (_D_) from the parotid gland empties through the lining of the
cheek; those from the lower glands empty at the front of the mouth under
the tongue (_T_). _N_, nerve; _A_, artery; _V_, vein.]
All the juices poured out by these glands, indeed nearly all the fluids
or juices in our bodies, are either _acid_ or _alkaline_. By acid we
mean sour, or sharp, like vinegar, lemon juice, vitriol (_sulphuric
acid_), and _carbonic acid_ (which forms the bubbles in and gives the
sharp taste to plain soda-water). By alkaline we mean "soap-like" or
flat, like soda, lye, lime, and soaps of all sorts. If you pour an acid
and an alkali together--like vinegar and soda--they will "fizz" or
effervesce, and at the same time _neutralize_ or "kill" each other.
The Use of the Saliva. As the chief purpose of digestion is to prepare
the food so that it will dissolve in water, and then be taken up by the
cells lining the food-tube, the saliva, like the rest of the body
juices, consists chiefly of water. Nothing is more disagreeable than to
try to chew some dry food--like a large, crisp soda cracker, for
instance--which takes more moisture than the salivary glands are able to
pour out on such short notice. You soon begin to feel as if you would
choke unless you could get a drink of water. But it is not altogether
advisable to take this short cut to relief, because the salivary juice
contains what the drink of water does not--a _ferment_, or digestive
substance (_ptyalin_), which possesses the power of turning the _starch_
in our food into _sugar_. As starch is only very slowly soluble, or
"meltable," in water, while sugar is very readily so, the saliva is of
great assistance in the process of melting, known as _digestion_. The
changing of the starch to sugar is the reason why bread or cracker,
after it has been well chewed, begins to taste sweetish.
This change in the mouth, however, is not of such great importance as
we at one time thought, because even with careful mastication, a certain
amount of starch will be swallowed unchanged. Nature has provided for
this by causing another gland farther down the canal, just beyond the
stomach, called the _pancreas_, to pour into the food tube a juice which
is far stronger in sugar-making power than the saliva, and this will
readily deal with any starch which may have escaped this change in the
mouth. Moreover, this "sugaring" of starch goes on in the stomach for
twenty to forty minutes after the food has been swallowed.
Starchy foods, like bread, biscuit, crackers, cake, and pastry, are
really the only ones which require such thorough and elaborate chewing
as we sometimes hear urged. Other kinds of food, like meat and
eggs--which contain no _starch_ and consequently are not acted upon by
the saliva--need be chewed only sufficiently long and thoroughly to
break them up and reduce them to a coarse pulp, so that they can be
readily acted upon by the acid juice of the stomach.
Down the Gullet. When the food has been thoroughly moistened and
crushed in the mouth and rolled into a lump, or bolus, at the back of
the tongue, it is started down the elevator shaft which we call the
gullet, or esophagus. It does not fall of its own weight, like coal down
a chute, but each separate swallow is carried down the whole nine inches
of the gullet by a wave of muscular action. So powerful and closely
applied is this muscular pressure that jugglers can train themselves,
with practice, to swallow standing on their heads and even to drink a
glass of water in that position; while a horse or a cow always drinks
"up-hill." This driving power of the food tube extends throughout its
entire length; it is carried out by a series of circular rings of
muscles, which are bound together by other threads of muscle running
lengthwise, together forming the so-called _muscular coat_ of the tube.
By contracting, or squeezing down in rapid succession, one after
another, they move the food along through the tube. The failure of these
little muscles to act properly is one of the causes of constipation and
biliousness. Sometimes the action of the muscles is reversed, and then
we get a gush of acid, or bitter, half-digested food up into the mouth,
which we call "heart-burn" or "water-brash."
The Stomach--its Shape, Position, and Size. By means of muscular
contraction, then, the gullet-elevator carries the food into the
stomach. This is a comparatively simple affair, merely a ballooning out,
or swelling, of the food tube, like the bulb of a syringe, making a
pouch, where the food can be stored between meals, and where it can
undergo a certain kind of melting or dissolving. This pouch is about the
shape of a pear, with its larger end upward and pointing to the left,
and its smaller end tapering down into the intestine, or bowel, on the
right, just under the liver. The middle part of the stomach lies almost
directly under what we call the "pit of the stomach," though far the
larger part of it lies above and to the left of this point, going right
up under the ribs until it almost touches the heart, the diaphragm only
coming between.[3] This is one of the reasons why, when we have an
attack of indigestion, and the stomach is distended with gas, we are
quite likely to have palpitation and shortness of breath as well,
because the gas-swollen left end of the stomach is pressing upward
against the diaphragm and thus upon the heart and the lungs. Most cases
of imagined heart trouble are really due to indigestion.
The Lining Surface of the Stomach. Now let us look more carefully at
the lining surface of the stomach, for it is very wonderful. Like all
other living surfaces, it consists of tiny, living units, or "body
bricks" called _cells_, packed closely side by side like bricks in a
pavement. We speak of the _mucous membrane_, or lining, of our food
tube, as if it were one continuous sheet, like a piece of calico or
silk; but we must never forget that it is made up of living ranks of
millions of tiny cells standing shoulder to shoulder.
These cells are always actively at work picking out the substances they
need, and manufacturing out of them the ferments and acids, or alkalies,
needed for acting upon the food in their particular part of the tube,
whether it be the mouth, the stomach, or the small intestine.
[Illustration: A SECTION OF THE LINING SURFACE OF THE STOMACH
(Greatly magnified)
Showing the mouths of the stomach glands, and the furrows, or folds, of
the lining.]
The Peptic Juice. The cells of the stomach glands manufacture and pour
out a slightly sour, or acid, juice containing a ferment called
_pepsin_. The acid, which is known as _hydrochloric acid_, and the
pepsin together are able to melt down pieces of meat, egg, or curds of
milk, and dissolve them into a clear, jelly-like fluid, or thin soup,
which can readily be absorbed by the cells lining the intestine.[4]
You can see now why you shouldn't take large doses of soda or other
alkalies, just because you feel a little uncomfortable after eating.
They will make your stomach less acid and perhaps relieve the
discomfort, but they stop or slow down digestion. Neither is it well to
swallow large quantities of ice-water, or other very cold drinks, at
meal times, or during the process of digestion. As digestion is largely
getting the food dissolved in water, the drinking of moderate quantities
of water, or other fluids, at meals is not only no hindrance, but rather
a help in the process. The danger comes only when the drink is taken so
cold as to check digestion, or when it is used to wash down the food in
chunks, before it has been properly ground by the teeth.
[Illustration: A LONGITUDINAL SECTION OF STOMACH, OR PEPTIC, GLANDS
(Greatly magnified)
The long duct of each gland is but a deep fold of the stomach lining
(see note, p. 11). Into this duct the ranks of cells around it pour out
the peptic juice.]
Digestion in the Stomach. Although usually a single, pear-shaped
pouch, the stomach, during digestion, is practically divided into two
parts by the shortening, or closing down, of a ring of circular muscle
fibres about four inches from the lower end, throwing it into a large,
rounded pouch on the left, and a small, cone-shaped one on the right.
The gullet, of course, opens into the large left-hand pouch; and here
the food is stored as it is swallowed until it has become sufficiently
melted and acidified (mixed with acid juice) to be ready to pass on into
the smaller pouch. Here more acid juice is poured out into it, and it is
churned by the muscles in the walls of the stomach until it is changed
to a jelly-like substance.
Digestion in the Small Intestine. The food-pulp now passes on into the
_small intestine_, where it is acted upon by two other digestive
juices--the _bile_, which comes from the _liver_, and the _pancreatic
juice_, which is secreted by the pancreas.
The liver and the pancreas are a pair of large glands which have budded
out, one on each side of the food tube, about six inches below where the
food enters the small intestine from the stomach. The liver[5] weighs
nearly three pounds, and the pancreas about a quarter of a pound.
Of these two glands, the pancreas, though the smaller, is far more
important in digestion. In fact, it is the most powerful digestive gland
in the body. Its juice, the pancreatic juice, can do everything that any
other digestive juice can, and do it better. It contains a ferment for
turning starch into sugar, which is far more powerful than that of the
saliva; also another (_trypsin_), which will dissolve meat-stuffs nearly
twice as fast as the pepsin of the stomach can; and still another, not
possessed by either mouth or stomach glands, which will melt fat, so
that it can be sucked up by the lining cells of the intestine.
What does this great combination of powers in the pancreas mean? It
means that we have now reached the real centre and chief seat of
digestion, namely, the small intestine, or upper bowel. This is where
the food is really absorbed, taken up into the blood, and distributed to
the body. All changes before this have been merely preparatory; all
after it are simply a picking up of the pieces that remain.
In general appearance, this division of the food tube is very
simple--merely a tube about twenty feet long and an inch in diameter,
thrown into coils, so as to pack into small space, and slung up to the
backbone by broad loops of a delicate tissue (_mesentery_). It looks not
unlike twenty feet of pink garden hose.
The intestine also is provided with glands that pour out a juice known
as the _intestinal juice_, which, although not very active in digestion,
helps to melt down still further some of the sugars, and helps to
prevent putrefaction, or decay, of the food from the bacteria[6] which
swarm in this part of the tube.
By the time the food has gone a third of the way down the small
intestine, a good share of the starches in it have been turned into
sugar and absorbed by the blood vessels in its wall; and the meats,
milk, eggs, and similar foods have been digested in the same way.
There still remains the bulk of the fats to be disposed of. These fats
are attacked by the pancreatic juice and the bile, and made ready for
digestion. Like other foods, they are then eaten by the cells of the
intestinal wall; but instead of going directly into the blood vessels,
as the sugars and other food substances do, they are passed on into
another set of little tubes or vessels, called the _lymphatics_. In
these they are carried through the _lymph glands_ of the abdomen into
the great _lymph duct_, which finally pours them into one of the great
veins not far from the heart. Tiny, branching lymphatic tubes are found
all over the body, picking up what the cells leave of the fluid which
has seeped out of the arteries for their use and returning it to the
veins through the great lymph duct.
All these different food substances, in the process of digestion, do not
simply soak through the lining cells of the food tube, as through a
blotting paper or straining cloth, but are actually eaten by the cells
and very much changed in the process, and are then passed through the
other side of the cells, either into the blood vessels of the wall of
the intestine or into the lymph vessels, practically ready for use by
the living tissues of the body. It is in the cells then that our food is
turned into blood, and it is there that what we have eaten becomes
really a part of us. It may even be said that we are living upon the
leavings of the little cell citizens that line our food tube; but they
are wonderfully decent, devoted little comrades of the rest of our body
cells, and generous in the amount of food they pass on to the blood
vessels.
As the food-pulp is squeezed on from one coil to another through the
intestine, it naturally has more and more of its nourishing matter
sucked out of it; until, by the time it reaches the last loop of the
twenty feet of the small intestine, it has lost over two-thirds of its
food value.
The Final Stage--the Journey through the Large Intestine. From the
small intestine what remains of the food-pulp is poured into the last
section of the food tube, which enlarges to from two to three inches in
diameter. It is known as the large intestine, or large bowel. This
section is only about five feet long. The first three-fourths of it is
called the _colon_; the last or lowest quarter, the rectum, the
discharge-pipe of the food tube. The principal use of the colon is to
suck out the remaining traces of nourishing matter from the food and the
water in which it is dissolved, thus gradually drying the food-pulp down
to a solid or pasty form, in which condition it collects in a large "S"
shaped loop of the bowel just above the rectum, until discharged.
The Waste Materials. By the time that the remains of the food-pulp
have reached the middle of the large intestine, they have lost all their
nutritive value and most of their water. All the way down from the upper
part of the small intestine they have been receiving solid waste
substances poured out by the glands of the intestines; indeed, the bulk
of the feces is made up of these intestinal secretions, not, as is
generally supposed, of the undigested remains of the food. Ninety-five
per cent of our food is absorbed; the body-engine burns up its fuel very
clean. The next largest part of the feces is bacteria, or germs; and the
third and smallest, the indigestible fragments and remainders of food,
such as vegetable fibres, bran, fruit skins, pits, seeds, etc. Hence the
feces are not only worthless from a food point of view, but full of all
sorts of possibilities for harm; and the principal interest of the body
lies in getting rid of them as promptly and regularly as possible.
It can easily be seen how important it is that a habit should be formed,
which nothing should be allowed to break, of promptly and regularly
getting rid of these waste materials. For most persons, once in
twenty-four hours is normal; for some, twice or even three times in the
day. Whatever interval is natural, it should be attended to, beginning
at a fixed hour every morning.
Constipation, and how to Prevent It. Constipation should not be
treated by the all too common method of swallowing salts, which will
cause a flood of watery matters to be poured through the food tube and
sluice it clean of both poisons and melting food at the same time,
leaving it in an exhausted and disturbed condition afterwards; nor by
taking some irritating vegetable cathartic, generally in the form of
pills, which sets up a violent action of the muscles of the food tube,
driving its contents through at headlong speed; nor by washing out the
lower two or three feet of the bowel with injections of water; although
any or all of these may be resorted to occasionally for temporary
relief. A very large portion of the food eaten is sucked out of the food
tube into the blood vessels, passes through a large area of the body,
and is poured out again as waste through the glands of the lining of the
lower third of the bowel. Constipation, therefore, is caused by
disturbances which interfere with these processes _all over the body_,
not only in the stomach and bowels. Its only real and permanent cure is
through exercise in the open air, sleep, and proper ventilation of
bedrooms, with abundance of nourishing food, including plenty of green
vegetables and fresh fruits.
The Appendix and Appendicitis. The beginning of the large bowel, where
the small bowel empties into it, is the largest part of it, and forms a
curious pouch called the _cecum_, or "blind" pouch. From one side of
this projects a little wormlike tube, twisted and coiled upon itself,
from three to six inches long and of about the size of a slate pencil.
This is the famous _appendix vermiformis_ (meaning, "wormlike tag"),
which is such a frequent source of trouble. It is the shrunken and
shriveled remains of a large pouch of the intestine which once opened
into the cecum, and was used originally as a sort of second stomach for
delaying and digesting the remains of the food. The reason why it gives
rise to so much trouble is that it is so small--scarcely larger than
will admit a knitting-needle--and so twisted upon itself that germs or
other poisonous substances swallowed with the food may get into it,
start a swelling or inflammation, get trapped in there by the closing of
the narrow mouth of the tube, and form an abscess, which leaks through,
or bursts into, the cavity of the body, called the _peritoneum_. This
causes a very serious and often fatal blood poisoning.
Fortunately, _appendicitis_, or inflammation of the appendix, is not a
very common disease, causing only one in one hundred of all deaths that
occur; and these are mostly cases that were not treated promptly. Yet,
if you have a severe, constant pain, rather low down in the right-hand
corner of your abdomen, and if, when you press your hand firmly down in
that corner, it hurts, or you feel a lump, it is decidedly safest to
call a doctor and let him see what the condition really is, and advise
you what to do.
FOOTNOTES:
[1] The term _salts_ includes, as will be explained later, a large
number of substances, like ordinary table salt, baking soda, and the
laxative salts.
[2] There are three pairs of these: one just below the ears and behind
the angles of the jaw, known as the _parotid_; one under the middle of
the lower jaw known as the _submaxillary_; and a small pair just under
the tip of the tongue, called the _sublingual_. These glands have grown
up from the very simplest of beginnings. At first there was just a
little pocketing or pouching down of the mucous lining, like the finger
of a glove; then a couple of smaller hollow fingers budded off from the
bottom of the first finger; then four smaller fingers from the bottom of
these; and so on, until a regular little hollow tree or shrub of these
tiny tubes was built up, all discharging through the original hollow
stem, which has now become what we call the _duct_ of the gland. Every
secreting gland in the body--the stomach (or peptic) glands, the
salivary glands, the liver, the pancreas--is built up upon this simple
plan. The saliva and the juice of the pancreas and that of the liver
(bile) are alkaline, as are also the blood and most juices of the body.
The stomach juice is acid, as also are the urine and the perspiration.
[3] It is wonderfully elastic and constantly changing in size,
contracting till it will scarcely hold a quart when empty, and
expanding, as food or drink is put into it, until it will easily hold
two quarts, or even a gallon or more when greatly distended, as by gas.
[4] If you take some pepsin which has been extracted from the stomach of
a pig or a calf, melt it in water in a glass tube, then drop one or two
little pieces of meat or hard-boiled white of egg into it, you can see
them slowly melt away like sugar in a cup of coffee. If you add a few
drops of hydrochloric acid, the melting will go on much faster; and if
you warm up the tube to about the heat of the body, it will proceed
faster still. So nature knew just what she was doing when she provided
pepsin and acid and warmth in the stomach.
[5] The liver and the bile are more fully described in chapter XVII.
[6] Tiny plant cells, known also as _germs_, which cause fermentation,
decay, and many diseases.