DIGESTION.
_The Principal Processes of Nutrition--The General Plan of
Digestion--Mastication--The Teeth--Preservation of the
Teeth--Insalivation--The Stomach and the Gastric Juice--The Movements of
the Stomach--Gastric Digestion--The Intestines--The Bile and Pancreatic
Juice--Intestinal Digestion--Absorption by means of Blood-vessels and
Lacteals--The Lymphatic or Absorbent System--The Lymph--Conditions which
affect Digestion--The Quality, Quantity, and Temperature of the Food--The
Influence of Exercise and Sleep._
[Sidenote: 1. Design of food? How accomplished?]
1. NUTRITION.--The great design of food is to give _nutriment_ or
nourishment to the body. But this is not accomplished directly, as the food
must first pass through certain preparatory changes, as follows: (1),
_Digestion_, by which the food is reduced to a soluble condition; (2),
_Absorption_, by which, when digested, it is imbibed into the blood; (3),
_Circulation_, which carries the enriched blood to the various parts of the
system; and (4), _Assimilation_, by which each tissue derives from the
blood the materials necessary for its support.
[Sidenote: 2. Sustaining power of food? Simile of the engine? Operation in
the human body?]
2. By these means the sustaining power of food is gradually developed and
employed, and the vital machinery kept in working order, somewhat after the
manner of the steam-engine. To operate the latter, the force imprisoned
within the coal and water is set free and converted into motion by the
combustion of the fuel and the vaporization of the water. It will be seen,
however, when we come to study these operations in the human body, that
they are conducted silently and harmoniously, with marvellous delicacy and
completeness, and without that friction, and {81} consequent loss of power,
which attend the working of the most perfect machinery of man's invention.
[Sidenote: 3. Change of food in digestion? Process of digestion? Describe
the alimentary canal.]
3. GENERAL PLAN OF DIGESTION.--The great change which food undergoes in
digestion is essentially a reforming process, reducing articles of diet,
which are at first more or less solid, crude, and coarse, to a liquid and
finely comminuted condition, suitable for absorption into the blood. The
entire process of digestion takes place in what is called the alimentary
canal, a narrow, tortuous tube, about thirty feet in its entire length.
This canal begins in the mouth, extends thence downward through the gullet
to the stomach (a receptacle in which the principal work of digestion is
performed), and thence onward through the small and large intestines.
[Illustration: FIG. 16.--SECTION OF THE TRUNK SHOWING THE CAVITIES OF THE
CHEST AND ABDOMEN.
A, Cavity of Chest; B, Diaphragm; C, Abdomen; D, E, Spinal Column.]
[Sidenote: 4. Situation of the stomach and intestines? Action of the food?
Mechanical action? Chemical?]
4. The stomach and intestines are situated in the cavity of the abdomen
(Fig. 16, C, and Fig. 22), and occupy about two-thirds of its space. The
action to which the food is subjected in these organs is of two
kinds--mechanical and chemical. By the former it is crushed, agitated, and
carried onward from one point to another; by the latter it is changed in
form through the solvent power of the various digestive juices. {82}
[Sidenote: 5. Describe the process of mastication? How many and what
movements?]
5. MASTICATION.--As soon as solid food is taken into the mouth, it
undergoes mastication, or chewing. It is caught between the opposite
surfaces of the teeth, and by them is cut and crushed into very small
fragments. In the movements of chewing, the lower jaw plays the chief part;
the upper jaw, having almost no motion, acts simply as a point of
resistance, to meet the action of the former. These movements of the lower
jaw are of three sorts: a vertical or cutting, a lateral or grinding, and a
_to-and-fro_ or gnawing motion.
[Illustration: FIG. 17.--SECTION OF A TOOTH.
_a_, Enamel; _b_, Cavity; _c c_, Roots; _d_, Body of the Tooth.]
[Sidenote: 6. Composition of the teeth? Enamel of the teeth? Interior of
teeth?]
6. The teeth are composed of a bone-like material, and are held in place by
roots running deeply into the jaw. The exposed portion, or "crown," is
protected by a thin layer of enamel (Fig. 17, _a_), the hardest substance
in the body, and, like flint, is capable of striking fire with steel. In
the interior of each tooth is a cavity, containing blood-vessels and a
nerve, which enter it through a minute opening at the point of the root
(Fig. 19).
[Sidenote: 7. The milk teeth? The permanent teeth?]
7. There are two sets of teeth; first, those belonging to the earlier years
of childhood, called the milk teeth, which are twenty in number and small.
At six or eight years of age, when the jaw expands, and when the growing
body requires a more powerful and numerous set, the roots of {83} the milk
teeth are absorbed, and the latter are "shed," or fall out, one after
another (Fig. 18), to make room for the permanent set.
[Illustration: FIG. 18.--SECTION OF THE JAWS.
1' 2' 3' 4' 5', The Milk Teeth; 1" to 8", The Germs of the Permanent Set.]
[Sidenote: 8, 9. Number of teeth? How distributed?]
8. There are thirty-two teeth in the permanent set, as many being in one
jaw as the other. Each half-jaw has eight teeth, similarly shaped and
arranged in the same order: thus, two incisors, one canine, two bicuspids,
and three molars. The front teeth are small, sharp, and chisel-edged, and
are well adapted for cutting purposes; hence their name incisors. The
canines stand next, one on each side of the jaw; these receive their name
from their resemblance to the long, pointed tusks of the dog (Fig. 19).
{84}
[Illustration: FIG. 19.--SECTION OF THE JAWS--RIGHT SIDE.
V, A, N, Veins, Arteries, and Nerves of the Teeth. The root of one tooth in
each jaw is cut vertically to show the cavity and the blood-vessels, etc.,
within it. 1 to 8, Permanent Teeth.]
9. The bicuspids, next in order, are larger and have a broader crown than
the former; while behind them are the molars, the largest and most powerful
of the entire set. These large back teeth, or "grinders," present a broad,
rough surface, suitable for holding and crushing the food. The third molar,
or "wisdom tooth," is the last to be cut, and does not appear until about
the twenty-first year. {85} The order of arrangement of the teeth is
indicated by the following dental formula:--
[Illustration]
[Sidenote: 10. Different forms of teeth? Human teeth? The inference?]
10. It is interesting, at this point, to notice the different forms of
teeth in different animals, and observe how admirably their teeth are
suited to the respective kinds of food upon which they subsist. In the
_carnivora_, or flesh-feeders, the teeth are sharp and pointed, enabling
them both to seize their prey, and tear it in pieces; while the
_herbivora_, or vegetable-feeders, have broad, blunt teeth, with rough
crowns, suitable for grinding the tough grasses and grains upon which they
feed. Human teeth partake of both forms; some of them are sharp, and others
are blunt; they are therefore well adapted for the mastication of both
flesh and vegetables. Hence we argue that, although man may live
exclusively upon either vegetable or animal food, he should, when possible,
choose a diet made up of both varieties.
[Sidenote: 11. Cleaning of teeth? Effects of not cleaning?]
11. PRESERVATION OF THE TEETH.--In order that the teeth shall remain in a
sound and serviceable condition, some care is of course requisite. In the
first place, they require frequent cleansing; for every time we take food,
some particles of it remain in the mouth; and these, on account of the heat
and moisture present, soon begin to putrefy. This not only renders the
breath very offensive, but promotes decay of the teeth. {86}
[Sidenote: 12. Effects upon the saliva? Formation of tartar? How prevented?
How destroyed?]
12. The saliva, or moisture of the mouth, undergoes a putrefactive change,
and becomes the fertile soil in which a certain minute fungus has its
growth. This fluid, too, if allowed to dry in the mouth, collects upon the
teeth in the form of an unsightly, yellow concretion, called tartar. To
prevent this formation, and to remove other offensive substances, the teeth
should be frequently cleaned with water, applied by means of a soft
tooth-brush. The destruction of the tartar fungus is best effected by the
use of a weak solution of carbolic acid.
[Sidenote: 13. Destruction of the enamel? How guarded against?]
13. Again, it should be borne in mind that the enamel, Nature's protection
for the teeth, when once destroyed, is not formed anew; and the body of the
tooth thus exposed, is liable to rapid decay. On this account, certain
articles are to be guarded against; such as sharply acid substances that
corrode the enamel, and hard substances that break or scratch it--as gritty
tooth powders, metal tooth picks, and the shells of hard nuts. Sudden
alternations from heat to cold, when eating or drinking, also tend to crack
the enamel.
[Sidenote: 14. Mixing of food with the saliva? What is the saliva? How
secreted? The salivary glands?]
14. INSALIVATION.--When the morsel of food is cut and ground by the teeth,
it is at the same time also intimately mixed with the saliva, or fluids of
the mouth. This constitutes the second step of digestion, and is called
insalivation. The saliva, the first of the digestive solvents, is a
colorless, watery, and frothy fluid. It is secreted (_i. e._ separated from
the blood) partly by the mucous membrane which lines the mouth; but chiefly
by the salivary glands, of which there are three pairs situated near the
mouth.
[Sidenote: 15. The flow of saliva? The thought of food? Anxiety and grief?
Animals fed upon dry and coarse food?]
{87} 15. These glands consist of clusters of very small pouches, around
which a delicate network of blood-vessels is arranged: they empty into the
mouth by means of little tubes, or ducts. The flow from these glands is at
all times sufficient to maintain a soft and moist condition of the tongue
and mouth; but when they are excited by the presence and taste of food,
they pour forth the saliva more freely. Even the mere thought of food will
at times cause the saliva to flow, as when the appetite is stimulated by
the sight or smell of some savory article; so that the common expression is
correct that "the mouth waters" for the favorite articles of food. Anxiety
and grief prevent its flow, and cause "the tongue to cleave to the roof of
the mouth." In the horse and other animals, that feed upon dry and coarse
fodder, and require an abundant supply of saliva, we find large salivary
glands, as well as powerful muscles of mastication.
[Illustration: FIG. 20.--STRUCTURE OF A SALIVARY GLAND.]
[Illustration: FIG. 21.--THE HEAD OF A HORSE, showing the large salivary
gland (_a_), its duct (_b_), the muscles of mastication (_c_, _d_, _e_,
_f_, and _g_).]
[Sidenote: 16. Importance of the process? The first place? The second? The
third?]
16. The mingling of the saliva with the food seems a simple process, but it
is one that plays an important part {88} in digestion. In the first place,
it facilitates the motions of mastication, by moistening the food and
lubricating the various organs of the mouth. Secondly, it prepares the way
for other digestive acts: by the action of the teeth, the saliva is forced
into the solid food, softens the harder substances, and assists in
converting the whole morsel into a semi-solid, pulpy mass, that can be
easily swallowed, and readily permeated by other digestive fluids. The
saliva also, by dissolving certain substances, as sugar and salt, develops
the peculiar taste of each; whereas, if the tongue be dry and coated, they
are tasteless. Hence, if substances are insoluble, they are devoid of
taste.
[Sidenote: 17. Its final importance? Starch? How effected? Ptyalin?]
17. Finally, the saliva has the property of acting chemically upon the
food. As we have before stated (Chap. IV.), starch, as starch, cannot enter
the tissues of the body; but, in order to become nutriment, must first be
changed to grape sugar. This change is, in part, effected by the saliva,
and takes place almost instantly, whenever it comes in contact with cooked
starch. This important function is due to an organic ingredient of the
saliva called _ptyalin_. This substance has been extracted from the saliva
by the chemist, and has been found, by experiment, to convert into sugar
two thousand times its own weight of starch.
[Sidenote: 18. Each of the processes? Why is a knowledge of the digestive
functions important? How shown?]
18. IMPORTANCE OF MASTICATION AND INSALIVATION.--Each of these processes
complements the other, and makes the entire work available; for, by their
joint action, they prepare the food in the best possible manner for further
digestive changes. The knowledge of these preliminary functions will appear
the more important, when we reflect that they are the only ones which we
can regulate by the will. For, as soon as the act of swallowing begins, the
food not only passes out of sight, but beyond {89} control; and the
subsequent acts of digestion are consequently involuntary and unconsciously
performed.
[Sidenote: 19. Rapid eating? Describe the process and effects.]
19. It is generally known that rapid eating interferes with digestion. How
does this occur? In the first place, in rapid eating, the flow of the
saliva is insufficient to moisten the solid parts of the food, so that they
remain too hard and dry to be easily swallowed. This leads to the free and
frequent use of water, or some other beverage, at meals, to "wash down" the
food,--a most pernicious practice. For these fluids, not only cannot take
the place of the natural digestive juices, but, on the contrary, dilute and
weaken them.
[Sidenote: 20. Loss of taste? Another effect of rapid eating? Mistakes?]
20. Secondly, the saliva being largely the medium of the sense of taste,
the natural flavors of the food are not developed, and consequently it
appears comparatively insipid. Hence the desire for high-seasoned food, and
pungent sauces, that both deprave the taste and over excite the digestive
organs. Rapid eating also permits the entrance of injurious substances
which may escape detection by the taste, and be unconsciously received into
the system. In some instances, the most acrid and poisonous substances have
frequently been swallowed "by mistake," before the sense of taste could
act, and demand their rejection.
[Sidenote: 21. Effect of imperfectly broken food in the stomach? Dyspepsia?
Overeating?]
21. Thirdly, the food, being imperfectly broken up by the teeth, is hurried
onward to the stomach, to be by it more thoroughly divided. But the task
thus imposed upon the stomach, it is not at all adapted to perform; so that
the crude masses of food remain a heavy burden within the stomach, and a
source of distress to that organ, retarding the performance of its
legitimate duty. Hence persons who habitually eat too rapidly, frequently
fall victims to dyspepsia. Rapid eating also conduces to overeating. The
food is introduced so rapidly, that the system has not {90} time to
recognize that its real wants are met, and that its losses have been made
good; and hence the appetite continues, although more nutriment has been
swallowed than the system requires, or can healthfully appropriate.
[Illustration: FIG. 22.--SECTION OF CHEST AND ABDOMEN.
A, Heart. B, The Lungs. C, Stomach. D, The Liver. E, Large Intestine.
G, Small Intestine.]
[Sidenote: 22. Gullet? Describe the stomach and its location. Effects of
gormandizing?]
22. THE STOMACH.--As soon as each separate portion of food is masticated
and insalivated, it is swallowed; that is, it is propelled downward to the
stomach, through a narrow muscular tube about nine inches in length, called
the _oesophagus_, or gullet (Fig. 23). The stomach is the only large
expansion of the digestive canal, and is the most important organ of
digestion. It is a hollow, pear-shaped pouch, having a capacity of three
pints, in the adult. Its walls are thin and yielding, and may become
unnaturally distended, as in the case of those who subsist on a bulky,
innutritious diet, and of those who habitually gormandize.
[Sidenote: 23. Heart-orifice? Gatekeeper? Coins, etc.? Indication of the
soft and yielding texture of the stomach?]
23. The stomach has also two openings; that by which food enters, being
situated near the heart, is called the _cardiac_, or heart orifice; the
other is the _pylorus_, or "gatekeeper," which guards the entrance to the
intestines, {91} and, under ordinary circumstances, permits only such
matters to pass it as have first been properly acted upon in the stomach.
Coins, buttons, and the like are, however, readily allowed to pass, because
they can be of no use if retained. The soft and yielding texture of this
organ--the stomach--indicates that it is not designed to crush and
comminute solid articles of food.
[Illustration: FIG. 23.--THE ORGANS OF DIGESTION.
O, Oesophagus. S, Stomach. L, Liver. M, Pylorus. C, Large Intestines.
P, Pancreas. I, Small Intestines. N, Spleen. G, Gall-bladder. ]
[Sidenote: 24. What is meant by the gastric juice?]
24. THE GASTRIC JUICE.--We have seen how the presence of food in the mouth
excites the salivary glands, causing the saliva quickly to flow. In the
same manner, when food reaches the stomach, its inner lining, the mucous
membrane, is at once excited to activity. (At first its surface, which
while the stomach is empty presents a pale pink hue, turns to a bright red
color, for the minute blood-vessels which course through it, are filled
with blood. Presently a clear, colorless, and acid fluid exudes, drop by
drop, from its numerous minute glands or "tubules," until finally the
surface is moistened in every part, and the fluid begins to mingle with the
food. This fluid is termed the gastric juice. {92}
[Sidenote: 25. What is the office of the gastric juice? Acidity of the
gastric juice? Quantity of gastric juice used? What becomes of it?]
25. The gastric juice is the proper solvent of certain articles of food,
especially those belonging to the albuminoid class. This solvent power is
due to its peculiar ingredient, _pepsin_; in digestion, this substance acts
like a ferment; that is, it induces changes in the food simply by its
presence, but does not itself undergo change. The acidity of the gastric
juice, which is due to _lactic acid_, is not accidental; for we find that
the pepsin cannot act in an alkaline solution--that is, one which is not
acid or neutral. The quantity of gastric juice secreted daily is very
large, probably not less than three or four pints at each meal. Though this
fluid is at once used in the reduction of the food, it is not lost; since
it is soon re-absorbed by the stomach, together with those parts of the
food which it has digested and holds in solution.
[Sidenote: 26. Muscular coat of the stomach? Expansion and contraction of
its fibres? Action of the fibres?]
26. MOVEMENTS OF THE STOMACH.--The inner coating of the stomach is the
mucous membrane, which, as we have seen, furnishes the gastric juice. Next
to this coating lies another, called the muscular coat, composed of
involuntary muscular fibres, some of which run circularly, and others in a
longitudinal direction. These expand to accommodate the food as it is
introduced, and contract as it passes out. In addition, these fibres are in
continual motion while food remains in the stomach, and they act in such
manner that the contents are gently turned round from side to side, or from
one end of it to the other.
[Sidenote: 27. Peristaltic movements? What is said of our consciousness of
and power over these movements? Describe the movements of the pylorus.]
27. By these incessant movements of the stomach, called the _peristaltic_
movements, the gastric juice comes in contact with all parts of the food.
We are, however, not conscious that these movements take place, nor have we
the power to control them. When such portions of the food as are
sufficiently digested approach the pylorus, it {93} expands to allow them
to pass out, and it closes again to confine the residue for further
preparation.
[Sidenote: 28. How has the knowledge and the workings of the stomach been
ascertained? St. Martin? How else?]
28. The knowledge of these and other interesting and instructive facts has
been obtained by actual observation; the workings of the stomach of a
living human being have been laid open to view and examined--the result of
a remarkable accident. Alexis St. Martin, a Canadian _voyageur_, received a
gun-shot wound which laid open his stomach, and which, in healing, left a
permanent orifice nearly an inch in diameter. Through this opening the
observer could watch the progress of digestion, and experiment with
different articles of food. Since that occurrence, artificial openings into
the stomach of the inferior animals have been repeatedly made, so that the
facts of stomach-digestion are very well ascertained and verified.
[Sidenote: 29. What was formerly thought? What do we now know? What else do
we now know? Water, salt, and sugar? Absorption?]
29. GASTRIC DIGESTION.--What portions of the food are digested in the
stomach? It was formerly thought that all the great changes of digestion
were wrought here, but later investigation has taught us better. We now
know that the first change in digestion takes place in the mouth, in the
partial conversion of starch into sugar. We also know that, of the three
organic food principles (considered in Chapter IV.) two--the fats and the
sugars--are but slightly affected by the stomach; but that its action is
confined to that third and very important class, from which the tissues are
renewed, the albuminoids. A few articles need no preparation before
entering the system, as water, salt, and grape-sugar. These are rapidly
taken up by the blood-vessels of the stomach, which everywhere underlie its
mucous membrane in an intricate and most delicate network. In this way the
function of absorption begins.
[Sidenote: 30. Albuminose? The process? Chyme?]
30. The albuminoid substances are speedily attacked and {94} digested by
the gastric juice. From whatever source they are derived, vegetable or
animal, they are all transformed into the same digestive product, called
_albuminose_. This is very soluble in water, and is readily absorbed by the
blood-vessels of the stomach. After a longer or shorter time, varying from
one to five hours, according to the individual and the quantity and quality
of his food, the stomach will be found empty. Not only has the digested
food passed out, but also those substances which the stomach could not
digest or absorb have passed little by little through the pylorus, to
undergo further action in the intestines. At the time of its exit, the
digested food is of a pulpy consistence, and dark color, and is then known
as the _chyme_.
[Sidenote: 31. What are the intestines? The small intestines? The large
intestines? Their structure?]
31. THE INTESTINES.--The intestines, or "bowels," are continuous with the
stomach, and consist of a fleshy tube, or canal, twenty-five feet in
length. The small intestine, whose diameter is about one inch and a half,
is twenty feet long and very tortuous. The large intestine is much wider
than the former, and five feet long (Fig. 23). The general structure of
these organs resembles that of the stomach. Like it, they are provided with
a mucous membrane, or inner lining, whence flow their digestive juices;
and, just outside of this, a muscular coat, which propels the food onward
from one point to another.
[Sidenote: 32. Peritoneum? The work of digestion?]
32. Moreover, both the intestines and stomach are enveloped in the folds of
the same outer tunic or membrane, called the _peritoneum_. This is so
smooth and so well lubricated, that the intestines have the utmost freedom
of motion. In the small intestines, the work of digestion is completed, the
large intestine receiving from them the indigestible residue of the food,
and in time expelling it from the body.
[Sidenote: 33. The presence of food in the intestines? Bile?]
33. INTESTINAL DIGESTION.--As soon as the food passes the pylorus and
begins to accumulate in the upper {95} part of the intestines, it excites
the flow of a new digestive fluid, which enters through a small tube, or
_duct_, about three inches below the stomach. It is formed by the union of
two distinct fluids--the _bile_ and the _pancreatic_ juice. The bile is
secreted by the liver, which is the largest gland of the body, and is
situated on the right side and upper part of the abdomen (Fig. 22). The
bile is constantly formed, but it flows most rapidly during digestion.
During the intervals of digestion it is stored in the _gall-bladder_, a
small membranous bag attached to the under side of the liver. This fluid is
of a greenish-yellow color, having a peculiar smell, and a very bitter
taste.
[Sidenote: 34. The pancreatic juice? The joint action of these fluids?]
34. The pancreatic juice is the product of a gland called the _pancreas_,
situated behind the stomach. This fluid is colorless, viscid, and without
odor. Like the digestive juices previously described, it owes its solvent
power to its peculiar ferment principle, called _pancreatin_. By the joint
action of these fluids, the fatty parts of the food are prepared for
absorption. By previous steps of digestion the fats are merely separated
from the other components of the food; but here, within the intestines,
they are reduced to a state of minute division, or _emulsion_, resembling
the condition of butter in milk, before it has been churned. There results
from this action a white and milky fluid, termed the _chyle_, which holds
in solution the digestible portions of the food, and is spread over the
extensive absorbent surface of the small intestines.
[Sidenote: 35. The mucous membrane? Experiments on inferior animals?]
35. The mucous membrane of the intestines, also, secretes or produces, a
digestive fluid by means of numerous "follicles," or minute glands; this is
called the intestinal juice. From experiments on the inferior animals, it
has been ascertained that this fluid exerts a solvent influence over each
of the three organic food principles, and in this way may supplement and
complete the action of the {96} fluids previously mentioned, viz.:--of the
saliva in converting starch into sugar, of the gastric juice in digesting
the albuminoids, and of the pancreatic juice and bile in emulsifying the
fats.
[Sidenote: 36, 37. How much thus far has been done with the food? The next
process? Give the first way.]
36. ABSORPTION.--With the elaboration of the chyle, the work of digestion
is completed; but, in a certain sense, the food is yet outside of the body;
that is, the blood is not yet enriched by it, and it is not in a position
to nourish the tissues. The process by which the liquefied food passes out
of the alimentary canal into the blood is called absorption. This is
accomplished in two ways; first, by the _blood-vessels_. We have seen how
the inner membrane of the stomach is underlaid by a tracery of minute and
numerous vessels, and how some portions of the food are by them absorbed.
The supply of blood-vessels to the intestines is even greater; particularly
to the small intestines, where the work of absorption is more actively
carried on.
37. The absorbing surface of the small intestines, if considered as a plane
surface, amounts to not less than half a square yard. Besides, the mucous
membrane is formed in folds with an immense number of thread-like
prolongations, called _villi_, which indefinitely multiply its absorbing
capacity. These minute processes, the villi, give the surface the
appearance and smoothness of velvet; and during digestion, they dip into
the canal, and, by means of their blood-vessels, absorb its fluid contents,
just as the _spongioles_ which terminate the rootlets of plants, imbibe
moisture from the surrounding soil.
[Sidenote: 38. How is absorption effected in another way? Describe it. Name
of the lacteals? Thoracic duct?]
38. Secondly, absorption is also effected by the _lacteals_, a set of
vessels peculiar to the small intestines. These have their beginnings in
the little villi just mentioned, side by side with the blood-vessels. These
two sets of absorbents run in different courses, but their destination is
the same, {97} which is the right side of the heart. The lacteals receive
their name from their milky-white appearance. After a meal containing a
portion of fat, they are then distended with chyle, which they are
specially adapted to receive: at other times they are hardly discernible.
The lacteals all unite to form one tube, the _thoracic duct_, which passes
upward through the _thorax_, or chest, and empties into a large vein,
situated just beneath the left collar-bone.
[Illustration: FIG. 24.--THE LACTEALS.
A, Small Intestine. B, Lacteals. C, Thoracic Duct. D, Absorbents.
E, Blood-vessel. ]
[Sidenote: 39. The absorbents? Lymph? What further of the lymph?]
39. THE ABSORBENTS.--The lacteals belong to a class of vessels known as
absorbents, or lymphatics, which ramify in nearly all parts of the body,
except the brain and spinal cord. The fluid which circulates through the
lymphatics of the limbs, and all the organs not concerned in digestion, is
called _lymph_. This fluid is clear and colorless, like water, and thus
differs from the milky chyle which the lacteals carry after digestion: it
consists chiefly of the watery part of the blood, which was not required by
the tissues, and is returned to the blood by the absorbents or lymphatics.
[Sidenote: 40. What can you state as to the time required for digestion?]
40. CIRCUMSTANCES AFFECTING DIGESTION.--What length of time is required for
the digestion of food? From observations made, in the case of St. Martin,
the Canadian {98} already referred to, it has been ascertained that, at the
end of two hours after a meal, the stomach is ordinarily empty. How much
time is needed to complete the digestion of food, within the small
intestines, is not certain; but, from what we have learned respecting their
functions, it must be evident that it largely depends upon the amount of
starch and fat which the food contains.
[Sidenote: 41. Circumstances affecting duration of digestion? Fresh food?]
41. In addition to the preparations which the food undergoes in cooking,
which we have already considered, many circumstances affect the duration of
digestion; such as the quality, quantity, and temperature of the food; the
condition of the mind and body; sleep, exercise, and habit. Fresh food,
except new bread and the flesh of animals recently slain, is more rapidly
digested than that which is stale; and animal food more rapidly than that
from the vegetable kingdom.
[Sidenote: 42. Food in concentrated form? A large quantity of food?
Experiment on the dog? Ice-water? Variety of articles?]
42. Food should not be taken in too concentrated a form, the action of the
stomach being favored when it is somewhat bulky; but a large quantity in
the stomach often retards digestion. If the white of one egg be given to a
dog, it will be digested in an hour, but if the white of eight eggs be
given it will not disappear in four hours. A wineglassful of ice-water
causes the temperature of the stomach to fall thirty degrees; and it
requires a half-hour before it will recover its natural warmth--about a
hundred degrees--at which the operations of digestion are best conducted. A
variety of articles, if not too large in amount, is more easily disposed of
than a meal made of a single article; although a single indigestible
article may interfere with the reduction of articles that are easily
digested.
[Sidenote: 43. Strong emotion? The tongue of the patient?]
43. Strong emotion, whether of excitement or depression, checks digestion,
as do also a bad temper, anxiety, long fasting, and bodily fatigue. The
majority of these {99} conditions make the mouth dry, that is, they
restrain the flow of the saliva; and without doubt they render the stomach
dry also, by preventing the flow of the gastric juice. And, as a general
rule, we may decide, from a parched and coated tongue, that the condition
of the stomach is not very dissimilar, and that it is unfit for the
performance of digestive labor. This is one of the points which the
physician bears in mind when he examines the tongue of his patient.
[Sidenote: 44. Eating between meals? Severe exercise? Sleep after meals?]
44. The practice of eating at short intervals, or "between meals," as it is
called, has its disadvantage, as well as rapid eating and over-eating,
since it robs the stomach of its needed period of entire rest, and thus
overtasks its power. With the exception of infants and the sick, no persons
require food more frequently than once in four hours. Severe exercise
either directly before or directly after eating retards digestion; a period
of repose is most favorable to the proper action of the stomach. The
natural inclination to rest after a hearty meal may be indulged, but should
not be carried to the extent of sleeping; since in that state the stomach,
as well as the brain and the muscles, seeks release from labor.
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QUESTIONS FOR TOPICAL REVIEW.
PAGE
1. What do you understand by nutrition? 80
2. How is the process of nutrition carried on? 80
3. What further can you state on the subject? 80, 81
4. Describe the general plan of digestion. 81
5. How is the process of mastication carried on? 80, 82
6. State what you can in relation to the formation of the
teeth. 82, 86
7. What, in relation to their arrangement? 83, 84
8. What, in relation to the process of "shedding?" 82, 83, 84
9. In relation to the different forms of teeth in different
animals? 85
10. What causes operate to injure or destroy the teeth? 85, 86
11. What suggestions and directions are given for the
preservation of the teeth? 85, 86
12. What do you understand by insalivation? 80, 86
13. How is the process of insalivation carried on? 86, 87, 88
14. Of what importance is the saliva to the process? 87, 88
15. Of what importance are mastication and insalivation? 88, 89
16. Describe the consequences of rapid eating. 89, 90
17. What becomes of the food directly after it has undergone
mastication and insalivation? 90
18. Describe the location and formation of the stomach. 90, 91, 92
19. Describe the process by which the gastric juice is formed. 91
20. What are the properties and uses of the gastric juice? 92
21. What are the movements of the stomach, and what their
uses? 92, 93
22. What further can you state on the subject? 93
23. What portions of the food are digested in the stomach? 93, 94
24. What are the first changes of digestion? 93
25. Describe the location and formation of the stomach. 94
26. What further can you state in relation to the stomach? 94
27. Describe the process of intestinal digestion. 94, 95, 96
28. What do you understand by absorption? 80, 96
29. How is the process of absorption effected? 96, 97
30. What are the lacteals and of what use are they? 96, 97
31. What length of time is required for the digestion of
food? 97, 98
32. What circumstances, of food, affect digestion? 98
33. What circumstances, of emotion, affect digestion? 98, 99
34. What suggestions and directions are given upon the subject of
eating and drinking? 98, 99
* * * * *
[Illustration: CIRCULATION OF THE BLOOD.
[Heart, Lungs, Arteries & Veins.]
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