entire laboratory, furnished with a chemist's outfit of knives, mortars,
baths, chemicals, filters, etc. The food is (1) chewed, mixed with the
saliva in the mouth, and swallowed; (2) it is acted upon by the gastric
juice in the stomach; (3) it is passed into the intestines, where it
receives the bile, pancreatic juice, and other liquids which completely
dissolve it; [Footnote: Digestion, says Berzelius, is a process of
rinsing. The digestive apparatus secretes, and again absorbs with the food
which it has dissolved, not less than three gallons of liquid per day.--
BARNARD, BIDDER, SCHMIDT, and others.] (4) the nourishing part is absorbed
in the stomach and intestines, and thence thrown into the blood vessels,
whence it is whirled through the body by the torrent of the circulation.
These processes take place within the _alimentary canal_, a narrow
tortuous tube which commences at the mouth, and is about thirty feet long.
[Footnote: The digestive apparatus is lined with mucous membrane that
possesses functions similar to those of the outer skin. It absorbs certain
substances and rejects waste matter. On account of this close connection
between the inner and the outer skin, it is not surprising to find that in
the lowest animals digestion is performed by means of the external skin.
The amba, which is merely a gelatinous mass, when it takes its food,
extemporizes a stomach for the occasion. It simply wraps itself around the
morsel, and, like an animated apple dumpling with the apple for food and
the crust for animal, goes on with the process until the operation is
completed, when it unrolls itself again and lets the indigestible residue
escape. The common hydra of our brooks can live when turned inside out,
like a glove; either side serving for skin or stomach, as necessity
requires.]
FIG. 46.
[Illustration: _The Parotid--one of the salivary glands._]
I. MASTICATION AND INSALIVATION.--l. _The Saliva_.--The food while
being cut and ground by the teeth is mixed with the saliva. This is a
thin, colorless, frothy, slightly alkaline liquid, secreted [Footnote: By
secretion is meant merely a separation or picking out from the blood.] by
the mucous membrane lining the mouth, and by three pairs of salivary
glands (parotid, submaxillary, and sublingual) opening into the mouth
through ducts, or tubes. The amount varies, but on the average is about
three pounds per day, and in health is always sufficient to keep the mouth
moist. [Footnote: The presence and often the thought of food will "make
one's mouth water." Fear checks the flow of saliva, and hence the East
Indians sometimes attempt to detect theft by making those who are
suspected chew rice. The person from whom it comes out driest is adjudged
the thief.] It softens and dissolves the food, and thus enables us to get
the flavor or taste of what we eat. It contains a peculiar organic
principle called _ptyalin_, [Footnote: One part of ptyalin will
convert eight thousand parts of starch into sugar.--MIALEE.
The saliva has no chemical action on the fats or the albuminous bodies.
Its frothiness enables it to carry oxygen into the stomach, and this is
thought to be of service. The action of the ptyalin commences with great
promptness, and sugar has been detected, it is said, within half a minute
after the starch was placed in the mouth. The process, however, is not
finished there, but continues after reaching the stomach.--VALENTIN. The
saliva thus prepares a small portion of food for absorption at once, and
so insures at the very beginning of the operation of digestion a supply of
force-producing material for the immediate use of the system.] which,
acting upon the starch of the food, changes it into glucose or grape
sugar.
2. _The Process of Swallowing._--The food thus finely pulverized,
softened, and so lubricated by the viscid saliva as to prevent friction as
it passes over the delicate membranes, is conveyed by the tongue and cheek
to the back of the mouth. The soft palate lifts to close the nasal
opening; the epiglottis shuts down, and along this bridge the food is
borne, without danger of falling into the windpipe or escaping into the
nose. The muscular bands of the throat now seize it and take it beyond our
control. The fibers of the sophagus contract above, while they are lax
below, and convey the food by a worm-like motion into the stomach.
[Footnote: We can observe the peculiar motion of the sophagus by watching
a horse's neck when he is drinking.]
II. GASTRIC DIGESTION.--1. _The Stomach_ is an irregular expansion of
the digestive tube. Its shape has been compared to that of a bagpipe. It
holds about three pints, though it is susceptible of some distension. It
is composed of an inner, mucous membrane, which secretes the digestive
fluids; an outer, smooth, well-lubricated serous one, which prevents
friction, and between them a stout, muscular coat. The last consists of
two principal layers of longitudinal and circular fibers. When these
contract, they produce a peculiar churning motion, called the
_peristaltic_ (_peri_, round; _stallein_, to arrange) movement, which
thoroughly mixes the contents of the stomach. At the farther end, the
muscular fibers contract and form a gateway, the _pylorus_ (a gate),
as it is called, which carefully guards the exit, and allows no food
to pass from the stomach until properly prepared. [Footnote: With a
wise discretion, however, it opens for buttons, coins, etc., swallowed
by accident; and when we overload the stomach, it seems to become weary of
constantly denying egress, and, finally, giving up in despair, lets
everything through.]
FIG. 47.
[Illustration: _Diagram of the Digestion of the Food. Notice how the
food is submitted to the action of alkaline, acid, and then alkaline
fluids. (See note, p._ 165.)]
2. _The Gastric Juice_.--The lining of the stomach is soft, velvety,
and of a pinkish hue; but, as soon as food is admitted, the blood vessels
fill, the surface becomes of a bright red, and soon there exudes from the
gastric glands a thin, colorless fluid--the gastric juice. (See p. 319.)
This is secreted to the amount of twelve pounds per day. [Footnote: The
amount secreted by a healthy adult is variously estimated from five to
thirty-seven pounds. As it is reabsorbed by the blood, there is no loss.]
Its acidity is probably due to muriatic or lactic acid--the acid of sour
milk. It contains a peculiar organic principle called _pepsin_
[Footnote: Pepsin is prepared and sold as an article of commerce. The best
is said to be made from the stomachs of young, healthy pigs, which, just
before being killed, are excited with savory food that they are not
allowed to eat. One grain is sufficient to dissolve eight hundred grains
of coagulated white of egg. A temperature of 130° renders pepsin inert.]
(_peptein_, to digest), which acts as a ferment to produce changes in
the food, without being itself modified.
The flow of gastric juice is influenced by various circumstances. Cold
water checks it for a time, and ice for a longer period. Anger, fatigue,
and anxiety delay and even suspend the secretion. The gastric juice has no
effect on the fats or the sugars of the food; its influence being mainly
confined to the albuminous bodies, which it so changes that they become
soluble in water. [Footnote: The question is often asked why the stomach
itself is not digested by the gastric juice, since it belongs to the
albuminous substances. Some have assigned as the probable reason that life
protects that organ, and assert that living tissues can not be digested;
but the fallacy of this has been clearly shown by experiments that have
been made with living tissues in the course of scientific research. The
latest opinion is that the blood which circulates so freely through the
vessels of the lining of the stomach, being alkaline, protects the tissue
against the acidity of the gastric juice.]
The food, reduced by the action of the gastric juice to a grayish, soupy
mass, called _chyme_ (kime), escapes through that jealously guarded
door, the pylorus.
Fig. 48.
[Illustration: _A vertical Section of the Duodenum, highly
magnified._ 1, _a fold-like villus;_ 2, epithelium, or cuticle;_
3, _orifices of intestinal glands;_ 5, _orifice of duodenal glands;_
4, 7, _more highly magnified sections of the cells of a duodenal gland._]
III. INTESTINAL DIGESTION--The structure of the intestines is like that
of the stomach. There is the same outer, smooth, serous membrane
(peritoneum) to prevent friction, the lining of mucous membrane to secrete
the digestive fluids, and the muscular coating to push the food forward.
The intestines are divided into the _small_ and the _large_. The
first part of the former opens out of the stomach, and is called the
_du-o-de'-num_, as its length is equal to the breadth of twelve
fingers. Here the chyme is acted upon by the _bile_, and the
_pancreatic juice_.
FIG. 49.
[Illustration: _The Mucous Membrane of the Ilium, highly magnified._
1, _cellular structure of the epithelium, or outer layer;_ 2, _a
vein;_ 3, _fibrous layer;_ 4, _villi covered with epithelium;_ 5,
_a villus in section, showing its lining of epithelium, with its
blood vessels and lymphatics;_ 6, _a villus partially uncovered;_
7, _a villus stripped of its epithelium;_ 8, _lymphatics or lacteals;_
9, _orifices of the glands opening between the villi;_ 10, 11, 12,
_glands;_ 13, _capillaries surrounding the orifices of the gland._]
1. _The Bile_ is secreted by the liver. This gland weighs about four
pounds, and is the largest in the body. It is located on the right side,
below the diaphragm. The bile is of a dark, golden color, and bitter
taste. About three pounds are secreted per day. When not needed for
digestion, it is stored in the gall cyst. [Footnote: A gall bladder can be
obtained from a butcher, and the contents kept in a bottle for
examination.] Its action on the food, though not fully understood, is
necessary to life. [Footnote: The bile is produced, unlike all the other
animal secretions, from venous blood; that is, the already contaminated
blood of the portal vein. Its complete suppression produces symptoms of
poisoning analogous to those which follow the stoppage of respiration, and
the patient dies, usually in a comatose condition, at the end of ten or
twelve days.--DALTON. The alkaline bile neutralizes the acid contents of
the stomach as they flow into the duodenum, and thus prepares the way for
the pancreatic juice. It has also a slight emulsifying power (note, p.
167).]
2. _The Pancreatic Juice_ is a secretion of the pancreas, or
"sweetbread"--a gland nearly as large as the hand, lying behind the
stomach. It is alkaline, and contains a ferment called _trypsin_.
This juice has the power of changing starch to sugar. Its main work,
however, is in breaking up the globules of fat into myriads of minute
particles, that mix freely with water, and remain suspended in it like
butter in new milk. The whole mass now assumes a milky look, whence it is
termed _chyle_ (kile) and passes on to the small intestine.
[Footnote: It is curious to observe that while the gastric juice is
decidedly acid, the fluids with which the food next comes into contact are
alkaline. It is thus submitted to the operation alternately of alkaline,
acid, and again of alkaline secretions. In the herbivora there is also a
second acid juice. The reason of these alternations is not known, but it
can hardly be doubted that they serve to make the digestion of the food
more perfect. And although the solvent power of the gastric juice is
placed in abeyance when its acidity is neutralized by the alkaline fluids,
yet it appears to be the case here, as in respect to the saliva, that
effects are produced by the mixture of the various secretions which are
poured together into the digestive tube, that would not result from either
alone.--HINTON.]
3. _The Small Intestine_ is an intricately folded tube, about twenty
feet long, and from an inch to an inch and one half in diameter. As the
chyle passes through this tortuous channel, it receives along the entire
route secretions which seem to combine the action of all the previous
ones--starch, fat, and albumen being equally affected.
IV. ABSORPTION is performed in two ways, by the _veins_, and the
_lacteals_. (1.) The veins in the stomach [Footnote: The veins and
the lacteals are separated from the food by a thin, moist membrane,
through the pores of which the fluid food rapidly passes, in accordance
with a beautiful law ("Popular Physics," p. 53) called the _Osmose_
of liquids. If two liquids of different densities are separated by an
animal membrane, they will mix with considerable force. There is a similar
law regulating the interchange of gases through a porous partition, in
obedience to which the carbonic acid of the blood, and the oxygen of the
lungs, are exchanged through the thin membrane of the air cells.]
immediately begin to take up the water, salt, grape sugar, and other
substances that need no special preparation. The starch and the albuminous
bodies are also absorbed as they are properly digested, and this process
continues along the whole length of the alimentary canal. In the small
intestine, there is a multitude of tiny projections (_villi_) from
the folds of the mucous membrane, more than seven thousand to the square
inch, giving it a soft, velvety look. These little rootlets, reaching out
into the milky fluid, drink into their minute blood vessels the nutritious
part of every sort of food. (2.)The lacteals [Footnote: From _lac_,
milk, because of the milky look given to their contents by the chyle.] (p.
126), a set of vessels starting in the villi side by side with the veins,
absorb the principal part of the fat. They convey the chyle through the
lymphatics and the thoracic duct (Fig. 43) to the veins, and so within the
sweep of the circulation.
The Portal Vein [Footnote: So named because it enters the liver by a sort
of gateway.] carries to the liver the food absorbed by the veins of the
stomach and the villi of the intestines. On the way, it is greatly
modified by the action of the blood itself. In the cells of the liver, it
undergoes as mysterious a process as that performed by the lymphatic
glands, and is then cast into the circulation. [Footnote: In these cells,
the sugar is changed into a kind of starch called _glycogen_. This is
insoluble, and so is stored up in the liver, and even in the substance of
the muscles, until it is needed by the body, when it is once more
converted into soluble sugar and taken up by the circulation. The liver
also changes the waste and surplus albuminous matter into bile, and into
urea and uric acid--the forms in which nitrogenized waste is excreted by
the kidneys.] The food, potent with force, is now buried in that river of
life from which the body springs momentarily afresh.
THE COMPLEXITY of the process of digestion, as compared with the
simplicity of respiration and circulation, is very marked. The mechanical
operation of mastication; the lubrication of the food by mucus; the
provision for the security of the respiratory organs; the grasping by the
muscles of the throat; the churning movement of the stomach; the
guardianship of the pylorus; the timely introduction by safe and protected
channels of the saliva, the gastric juice, the bile, the pancreatic juice,
and the intestinal fluids, each with its special adaptation; the curious
peristaltic motion of the intestines; the twofold absorption by the veins
and the lacteals; the final transformation in the lymphatics, the portal
vein, and the liver,--all these present a complexity of detail, the
necessity of which can be explained only when we reflect upon the variety
of the substances we use for food, and the importance of its thorough
preparation before it is allowed to enter the blood.
THE LENGTH OF TIME REQUIRED for digesting a full meal is from two to four
hours. It varies with the kind of food, state of the system, perfection of
mastication, etc. In the celebrated observations made upon Alexis St.
Martin [Footnote: In 1822, Alexis St. Martin, a Canadian in the employ of
the American Fur Company, was accidentally shot in the left side. Two
years after, the wound was entirely healed, leaving, however, an opening
about two and a half inches in circumference into the stomach. Through
this the mucous membrane protruded, forming a kind of valve which
prevented the discharge of food, but could be readily depressed by the
finger, thus exposing the interior. For several years he was under the
care of Dr. Beaumont, a skillful physician, who experimented upon him by
giving various kinds of food, and watching their digestion through this
opening. By means of these observations, and others performed on Katherine
Kutt, a woman who had a similar aperture in the stomach, we have very
important information as to the digestibility of different kinds of food.]
by Dr. Beaumont, his stomach was found empty in two and a half hours after
a meal of roast turkey, potatoes, and bread. Pigs' feet and boiled rice
were disposed of in an hour. Fresh, sweet apples took one and a half
hours; boiled milk, two hours; and unboiled, a quarter of an hour longer.
In eggs, which occupied the same time, the case was reversed,--raw ones
being digested sooner than cooked. Roast beef and mutton required three
and three and a quarter hours respectively; veal, salt beef, and broiled
chicken remained for four hours; and roast pork enjoyed the bad
preeminence of needing five and a quarter hours.
VALUE OF THE DIFFERENT KINDS OF FOOD.--_Beef_ and _Mutton_
possess the greatest nutritive value of any of the meats. _Lamb_ is
less strengthening, but more delicate. Like the young of all animals, it
should be thoroughly cooked, and at a high temperature, properly to
develop its delicious flavor. _Pork_ has much carbon. It sometimes
contains a parasite called trichina, which may be transferred to the human
system, producing disease and often death. The only preventive is thorough
cooking. _Fish_ is more watery than flesh, and many find it difficult
of digestion. Like meat, it loses its mineral constituents and natural
juices when salted, and is much less nourishing. Oysters are highly
nutritious, but are more easily assimilated when raw than when cooked.
_Milk_ is a model food, as it contains albumen, starch, fat, and
mineral matter. No other single substance can sustain life for so long a
time. _Cheese_ is very nourishing--one pound being equal in value to
two of meat, but it is not adapted to a weak stomach. (See p. 322.)
_Eggs_ are most easily digested when the white is barely coagulated
and the yolk is unchanged. _Bread_ [Footnote: Very fresh bread, warm
biscuit, etc., are condensed by mastication into a pasty mass that is not
easily penetrated by the gastric juice, and hence they are not healthful.
In Germany bread is not allowed to be sold at the baker's till it is
twenty-four hours old--a wise provision for those who have not strength to
resist temptation. This rule of eating may well be adopted by every one
who cares more for his health than for a gratification of his appetite.]
should be made of unbolted flour. The bran of wheat furnishes the mineral
matter we need in our bones and teeth, gives the bulk so essential to the
proper distension of the organs, and by its roughness gently stimulates
them to action. _Corn_ is rich in fat. It contains, however, more
indigestible matter than any other grain, except oats, and is less
nutritious than wheat. [Footnote: Persons unaccustomed to the use of corn
find it liable to produce derangement of the digestive organs. This was
made fearfully apparent in the prisons of Andersonville during the late
civil war. The vegetable food of the Federal prisoners had hitherto been
chiefly wheat bread and potatoes--the corn bread so extensively used at
the South being quite new to most of them as a constant article of diet.
It soon became not only loathsome, but productive of serious diseases. On
the other hand, it was the principal article in the rations of the
Confederate soldiers, to whom habit made it a nutritious and wholesome
form of food, as was shown by their endurance.--FLINT, _Physiology of
Man_, Vol. II, page 41.] The _Potato_ is two thirds water,--the
rest being mainly starch. _Ripe Fruits_, and those vegetables usually
eaten raw, dilute the more concentrated food, and also supply the blood
with acids, which are cooling in summer, and useful, perhaps, in
assimilation.
THE STIMULANTS.--_Coffee_ is about half nitrogen, and the rest fatty,
saccharine, and mineral substances. It is, therefore, of much nutritive
value, especially when taken with milk and sugar. Its peculiar stimulating
property is due to a principle called _caffeine_. Its aroma is
developed by browning, but destroyed by burning. No other substance so
soon relieves the sense of fatigue. [Footnote: In the late civil war, the
first desire of the soldiers upon halting after a wearisome march, was to
make a cup of coffee. This was taken without milk, and often without
sugar, yet was always welcome.] Taken in moderation, it clears the
intellect, tranquilizes the nerves, and usually leaves no unpleasant
reaction. It serves also as a kind of negative food, since it retards the
process of waste.
In some cases, however, it produces a rush of blood to the head, and
should be at once discarded. At the close of a full meal it hinders
digestion, and at night produces wakefulness. In youth, when the vital
powers are strong, and the functions of nature prompt in rallying from
fatigue, it is not needed, and may be injurious in stimulating a sensitive
organization.
_Tea_ possesses an active principle called _theine_. When used
moderately, its effects are similar to those of coffee, except that it
exerts an astringent action. It contains tannin, which, if the tea is
strong, coagulates the albumen of the food--_tans_ it--and thus
delays digestion. In excess, tea causes nervous tremor, disturbed sleep,
palpitation of the heart, and indigestion. [Footnote: Tea and coffee
should be made with, boiling water, but should not be boiled afterward.
During the "steeping" process, so customary in this country, the volatile
aroma is lost and a bitter principle extracted. In both England and China
it is usual to infuse tea directly in the urn from which it is to be
drawn. The tannin in tea is shown when a drop falls on a knife blade. The
black spot is a tannate of iron--a compound of the acid in the tea and the
metal.] (See p. 322.)
_Chocolate_ contains much fat, and also nitrogenous matter resembling
albumen. Its active principle, _theobromine_, [Footnote: It is said
that Linnæus, the great botanist, was so fond of chocolate that he named
the cocoa tree "Theobroma," the food of the gods.] has some of the
properties of caffeine and theine.
THE COOKING OF FOOD breaks the little cells, and softens the fibers of
which it is composed. In broiling or roasting meat, it should be exposed
to a strong heat at once, in order to coagulate the albumen upon the
outside, and thus prevent the escape of the nutritious juices. The cooking
may then be finished at a lower temperature. The same principle applies to
boiling meat. In making soups, on the contrary, the heat should be applied
slowly, and should reach the boiling point for only a few moments at the
close. This prevents the coagulation of the albumen. Frying is an
unhealthful mode of cooking food, as thereby the fat becomes partially
disorganized.
RAPID EATING produces many evil results. 1. There is not enough saliva
mixed with the food; 2. The coarse pieces resist the action of the
digestive fluids; 3. The food is washed down with drinks that dilute the
gastric juice, and hinder its work; 4. We do not appreciate the quantity
we eat until the stomach is overloaded; 5. Failing to get the taste of our
food, we think it insipid, and hence use condiments that overstimulate the
digestive organs. In these various ways the appetite becomes depraved, the
stomach vexed, the system overworked, and the foundation of dyspepsia is
laid. [Footnote: When one is compelled to eat in a hurry, as at a railway
station, he would do well to confine himself principally to meat; and to
dilute this concentrated food with fruit, crackers, etc., taken afterward
more leisurely.] (See p. 324.)
THE QUANTITY AND QUALITY OF FOOD required vary with the age and habits of
each individual. The diet of a child [Footnote: In youth, repair exceeds
waste; hence the body grows rapidly, and the form is plump. In middle
life, repair and waste equal each other, and growth ceases. In old age,
waste exceeds repair; hence the powers are enfeebled and the skin lies in
wrinkles on the shrunken form.] should be largely vegetable, and more
abundant than that of an aged person. A sedentary occupation necessitates
less food than an outdoor life. One accustomed to manual labor, on
entering school, should practice self-denial until his system becomes
fitted to the new order of things. He should not, however, fall into the
opposite error. We read of great men who have lived on bread and water,
and the conscientious student sometimes thinks that, to be great, he, too,
must starve himself. [Footnote: As Dr. Holland well remarks, the
dispensation of sawdust has passed away. If we desire a horse to win the
race, we must give him plenty of oats.] On the contrary, many of the
greatest workers are the greatest eaters. A powerful engine needs a
corresponding furnace. Only, we should be careful not to use more fuel
than is needed to run the machine. (See p. 325.)