PHYSIOLOGICAL ANATOMY.
THE DIGESTIVE ORGANS.
_Digestion_ signifies the act of separating or distributing, hence its
application to the process by which food is made available for nutritive
purposes. The organs of digestion are the Mouth, Teeth, Tongue, Salivary
Glands, Pharynx, Esophagus, the Stomach and the Intestines, with their
glands, the Liver, Pancreas, Lacteals, and the Thoracic Duct.
[Illustration: Fig. 26.
A view of the lower jaw. _1_. The body.
_2, 2_. Rami, or branches. _3, 3_. Processes of
the lower jaw. _m_. Molar teeth. _b_. Bicuspids,
_c_. Cuspids. _i_. Incisors.]
The _Mouth_ is an irregular cavity, situated between the upper and the
lower jaw, and contains the organs of mastication. It is bounded by the
lips in front, by the cheeks at the sides, by the roof of the mouth and
teeth of the upper jaw above, and behind and beneath by the teeth of the
lower jaw, soft parts, and palate. The soft palate is a sort of pendulum
attached only at one of its extremities, while the other involuntarily
opens and closes the passage from the mouth to the pharynx. The interior
of the mouth, as well as other portions of the alimentary canal, is
lined with a delicate tissue, called _mucous membrane_.
The _Teeth_ are firmly inserted in the alveoli or sockets, of the upper
and the lower jaw. The first set, twenty in number, are temporary, and
appear during infancy. They are replaced by permanent teeth, of which
there are sixteen in each jaw; four incisors, or front teeth, four
cuspids, or eye teeth, four bicuspids, or grinders, and four molars, or
large grinders. Each tooth is divided into the crown, body, and root.
The _crown_ is the grinding surface; the _body_, the part projecting
from the jaw, is the seat of sensation and nutrition; the _root_ is that
portion of the tooth which is inserted in the alveolus. The teeth are
composed of dentine, or ivory, and enamel. The ivory forms the greater
portion of the body and root, while the enamel covers the exposed
surface. The small white cords communicating with the teeth are the
nerves.
The _Tongue_ is a flat oval organ, the base of which is attached to the
os hyoides, while the apex, the most sensitive part of the body, is
free. Its surface is covered with a membrane, which, at the sides and
lower part, is continuous with the lining of the mouth. On the lower
surface of the tongue, this membrane is thin and smooth, but on the
upper side it is covered with numerous papillae, which, in structure,
are similar to the sensitive papillae of the skin.
[Illustration: Fig. 27.
The salivary glands. The largest one, near the ear, is the
parotid gland. The next below it is the submaxillary gland.
The one under the tongue is the sublingual gland.]
The _Salivary Glands_ are six in number, three on each side of the
mouth. Their function is to secrete a fluid called _saliva_, which aids
in mastication. The largest of these glands, the _Parotid_, is situated
in front and below the ear; its structure, like that of all the salivary
glands, is cellular. The _Submaxillary_ gland is circular in form, and
situated midway between the angle of the lower jaw and the middle of the
chin. The _Sublingual_ is a long flattened gland, and, as its name
indicates, is located below the tongue, which when elevated, discloses
the saliva issuing from its porous openings.
The _Pharynx_ is nearly four inches in length, formed of muscular and
membranous cells, and situated between the base of the cranium and the
esophagus, in front of the spinal column. It is narrow at the upper
part, distended in the middle, contracting again at its junction with
the esophagus. The pharynx communicates with the nose, mouth, larynx,
and esophagus.
The _Esophagus_, a cylindrical organ, is a continuation of the pharynx,
and extends through the diaphragm to the stomach. It has three coats:
first, the muscular, consisting of an exterior layer of fibers running
longitudinally, and an interior layer of transverse fibers; second, the
cellular, which is interposed between the muscular and the mucous coat;
third, the mucous membrane, or internal coat, which is continuous with
the mucous lining of the pharynx.
[Illustration: Fig. 28.
A representation of the interior of the stomach.
_1_. The esophagus. _2_. Cardiac orifice opening into
the stomach. _6_. The middle or muscular coat.
_7_. The interior or mucous coat. _10_. The beginning
of the duodenum. _11_. The pyloric orifice.]
The _Stomach_ is a musculo-membranous, conoidal sac, communicating with
the esophagus by means of the cardiac orifice (see Fig. 28). It is
situated obliquely with reference to the body, its base lying at the
left side, while the apex is directed toward the right side. The stomach
is between the liver and spleen, subjacent to the diaphragm, and
communicates with the intestinal canal by the pyloric orifice. It has
three coats. The peritoneal, or external coat is composed of compact,
cellular tissue, woven into a thin, serous membrane, and assists in
keeping the stomach in place. The middle coat is formed of three layers
of muscular fibers: in the first, the fibres run longitudinally; in the
second, in a circular direction; and in the third, they are placed
obliquely to the others. The interior, or mucous coat, lines this organ.
The stomach has a soft, spongy appearance, and, when not distended, lies
in folds. During life, it is ordinarily of a pinkish color. It is
provided with numerous small glands, which secrete the gastric fluid
necessary for the digestion of food. The lining membrane, when divested
of mucus, has a wrinkled appearance. The arteries, veins, and
lymphatics, of the stomach are numerous.
[Illustration: Fig. 29.
Small and large intestines. _1, 1, 2, 2_.
Small intestine. _3_. Its termination in the
large intestine. _4_. Appendix vermiformis.
_5_. Caecum. _6_. Ascending colon.
_7_. Transverse colon. _8_. Descending colon.
_9_. Sigmoid flexure of colon. _10_. Rectum.]
The _Intestines_ are those convoluted portions of the alimentary canal
into which the food is received after being partially digested, and in
which the separation and absorption of the nutritive materials and the
removal of the residue take place. The coats of the intestines are
analogous to those of the stomach, and are, in fact, only extensions of
them. For convenience of description, the intestines may be divided into
the _small_ and the _large_. The small intestine is from twenty to
twenty-five feet in length, and consists of the Duodenum, Jejunum, and
Ileum. The _Duodenum_, so called because its length is equal to the
breadth of twelve fingers, is the first division of the small intestine.
If the mucous membrane of the duodenum be examined, it will be found
thrown into numerous folds, which are called _valvulæ conniventes_, the
chief function of which appears to be to retard the course of the
alimentary matter, and afford a larger surface for the accommodation of
the absorbent vessels. Numerous _villi_, minute thread-like projections,
will be found scattered over the surface of these folds, set side by
side, like the pile of velvet. Each _villus_ contains a net-work of
blood-vessels, and a lacteal tube, into which the ducts from the liver
and pancreas open, and pour their secretions to assist in the conversion
of the chyme into chyle. The _Jejunum_, so named because it is usually
found empty after death, is a continuation of the duodenum, and is that
portion of the alimentary canal in which the absorption of nutritive
matter is chiefly effected. The _Ileum_, which signifies something
rolled up, is the longest division of the small intestine. Although
somewhat thinner in texture than the jejunum, yet the difference is
scarcely perceptible. The large intestine is about five feet in length,
and is divided into the Caecum, Colon, and Rectum. The _Caecum_ is about
three inches in length. Between the large and the small intestine is a
valve, which prevents the return of excrementitious matter that has
passed into the large intestine. There is attached to the cæcum an
appendage about the size of a goose-quill, and three inches in length,
termed the _appendix vermiformis_. The _Colon_ is that part of the large
intestine which extends from the cæcum to the rectum, and which is
divided into three parts, distinguished as the ascending, the
transverse, and the descending.
[Illustration: Fig. 30.
Villi of the small intestine greatly
magnified.]
[Illustration: Fig. 31.
A section of the Ileum, turned inside out,
so as to show the appearance and arrangement
of the villi on an extended surface.]
The _Rectum_ is the terminus of the large intestine. The intestines are
abundantly supplied with blood-vessels. The arteries of the small
intestine are from fifteen to twenty in number. The large intestine is
furnished with three arteries, called the _colic arteries_. The
_ileo-colic artery_ sends branches to the lower part of the ileum, the
head of the colon, and the appendix vermiformis. The _right colic
artery_ forms arches, from which branches are distributed to the
ascending colon. The _colica media_ separates into two branches, one of
which is sent to the right portion of the transverse colon, the other to
the left. In its course, the _superior hemorrhoidal artery_ divides into
two branches, which enter the intestine from behind, and embrace it on
all sides, almost to the anus.
The _Thoracic Duct_ is the principal trunk of the absorbent system, and
the canal through which much of the chyle and lymph is conveyed to the
blood. It begins by a convergence and union of the lymphatics on the
lumbar vertebræ, in front of the spinal column, then passes upward
through the diaphragm to the lower part of the neck, thence curves
forward and downward, opening into the subclavian vein near its junction
with the left jugular vein, which leads to the heart.
[Illustration: Fig. 32.
_c, c_. Right and left subclavian veins. _b_.
Inferior vena cava. _a_. Intestines. _d_. Entrance
of the thoracic duct into the left
subclavian vein. _4_. Mesenteric glands,
through which the lacteals pass to the
thoracic duct.]
[Illustration: Fig. 33.
The inferior surface of the liver. 1. Right lobe.
2. Left lobe. 3. Gall-bladder.]
The _Liver_, which is the largest gland in the body, weighs about four
pounds in the adult, and is located chiefly on the right side,
immediately below the diaphragm. It is a single organ, of a dark red
color, its upper surface being convex, while the lower is concave. It
has two large lobes, the right being nearly four times as large as the
left. The liver has two coats, the _serous_, which is a complete
investment, with the exception of the diaphragmatic border, and the
depression for the gall-bladder, and which helps to suspend and retain
the organ in position; and the _fibrous_, which is the inner coat of the
liver, and forms sheaths for the blood-vessels and excretory ducts. The
liver is abundantly supplied with arteries, veins, nerves, and
lymphatics. Unlike the other glands of the human body, it receives two
kinds of blood; the arterial for its nourishment, and the venous, from
which it secretes the bile. In the lower surface of the liver is lodged
the gall-bladder, a membranous sac, or reservoir, for the bile. This
fluid is not absolutely necessary to the digestion of food, since this
process is effected by other secretions, nor does bile exert any special
action upon, starchy or oleaginous substances, when mixed with them at a
temperature of 100° F. Experiments also show that in some animals there
is a constant flow of bile, even when no food has been taken, and there
is consequently no digestion to be performed. Since the bile is formed
from the venous blood, and taken from the waste and disintegration of
animal tissue, it would appear that it is chiefly an excrementitious
fluid. It does not seem to have accomplished its function when
discharged from the liver and poured into the intestine, for there it
undergoes various alterations previous to re-absorption, produced by its
contact with the intestinal juices. Thus the bile, after being
transformed in the intestines, re-enters the blood under a new form, and
is carried to some other part of the system to perform its mission.
The _Spleen_ is oval, smooth, convex on its external, and irregularly
concave on its internal, surface. It is situated on the left side, in
contact with the diaphragm and stomach. It is of a dark red color,
slightly tinged with blue at its edges. Some physiologists affirm that
no organ receives a greater quantity of blood, according to its size,
than the spleen. The structure of the spleen and that of the mesenteric
glands are similar, although the former is provided with a scanty supply
of lymphatic vessels, and the chyle does not pass through it, as through
the mesenteric glands. The _Pancreas_ lies behind the stomach, and
extends transversely across the spinal column to the right of the
spleen. It is of a pale, pinkish color, and its secretion is analogous
to that of the salivary glands; hence it has been called the _Abdominal
Salivary Gland_.
[Illustration: Fig. 34.
Digestive organs. _3_. The tongue. _7_. Parotid
gland. _8_. Sublingual gland. _5_. Esophagus. _9_.
Stomach. _10_. Liver. _11_. Gall-bladder, _14_. Pancreas.
_13, 13_. The duodenum. The small and large intestines
are represented below the stomach.]
Digestion is effected in those cavities which we have described as parts
of the alimentary canal. The food is first received into the mouth,
where it is masticated by the teeth, and, after being mixed with mucus
and saliva, is reduced to a mere pulp; it is then collected by the
tongue, which, aided by the voluntary muscles of the throat, carries the
food backward into the pharynx, and, by the action of the involuntary
muscles of the pharynx and esophagus, is conveyed to the stomach. Here
the food is subjected to a peculiar, churning movement, by the alternate
relaxation and contraction of the fibers which compose the muscular wall
of the stomach. As soon as the food comes in contact with the stomach,
its pinkish color changes to a bright red; and from the numerous tubes
upon its inner surface is discharged a colorless fluid, called the
_gastric juice_, which mingles with the food and dissolves it. When the
food is reduced to a liquid condition, it accumulates in the pyloric
portion of the stomach. Some distinguished physiologists believe that
the food is kept in a gentle, unceasing, but peculiar motion, called
_peristaltic_, since the stomach contracts in successive circles. In the
stomach the food is arranged in a methodical manner. The undigested
portion is detained in the upper, or cardiac extremity, near the
entrance of the esophagus, by contraction of the circular fibers of the
muscular coat. Here it is gradually dissolved, and then carried into the
pyloric portion of the stomach. From this, then, it appears, that the
dissolved and undissolved portions of food occupy different parts of the
stomach. After the food has been dissolved by the gastric fluid, it is
converted into a homogeneous, semi-fluid mass, called _chyme_. This
substance passes from the stomach through the pyloric orifice into the
duodenum, in which, by mixing with the bile and pancreatic fluid, its
chemical properties are again modified, and it is then termed _chyle_,
which has been found to be composed of three distinct parts, a
reddish-brown sediment at the bottom, a whey-colored fluid in the
middle, and a creamy film at the top. Chyle is different from chyme in
two respects: First, the alkali of the digestive fluids, poured into the
duodenum, or upper part of the small intestine, neutralizes the acid of
the chyme; secondly, both the bile and the pancreatic fluid seem to
exert an influence over the fatty substances contained in the chyme,
which assists the subdivision of these fats into minute particles. While
the chyle is propelled along the small intestine by the peristaltic
action, the matter which it contains in solution is absorbed in the
usual manner into the vessels of the villi by the process called
_osmosis_. The fatty matters being subdivided into very minute
particles, but not dissolved, and consequently incapable of being thus
absorbed by osmosis, pass bodily through the epithelial lining of the
intestine into the commencement of the lacteal tubes in the villi. The
digested substances, as they are thrust along the small intestines,
gradually lose their albuminoid, fatty, and soluble starchy and
saccharine matters, and pass through the ileo-caecal valve into the
cæcum and large intestine. An acid reaction takes place here, and they
acquire the usual fæcal smell and color, which increases as they
approach the rectum. Some physiologists have supposed that a second
digestion takes place in the upper portion of the large intestine. The
lacteals, filled with chyle, pass into the mesenteric glands with which
they freely unite, and afterward enter the _receptaculum chyli_, which
is the commencement of the thoracic duct, a tube of the size of a
goose-quill, which lies in front of the backbone. The lymphatics, the
function of which is to secrete and elaborate lymph, also terminate in
the _receptaculum chyli_, or receptacle for the chyle. From this
reservoir the chyle and lymph flow into the thoracic duct, through which
they are conveyed to the left subclavian vein, there to be mingled with
venous blood. The blood, chyle, and lymph, are then transmitted directly
to the lungs.
The process of nutrition aids in the development and growth of the body;
hence it has been aptly designated a "perpetual reproduction." It is the
process by which every part of the body assimilates portions of the
blood distributed to it. In return, the tissues yield a portion of the
material which was once a component part of their organization. The body
is constantly undergoing waste as well as repair. One of the most
interesting facts in regard to the process of nutrition in animals and
plants is, that all tissues originate in cells. In the higher types of
animals, the blood is the source from which the cells derive their
constituents. Although the alimentary canal is more or less complicated
in different classes of animals, yet there is no species, however low in
the scale of organization, which does not possess it in some form.[2]
The little polyp has only one digestive cavity, which is a pouch in the
interior of the body. In some animals circulation is not distinct from
digestion, in others respiration and digestion are performed by the same
organs; but as we rise in the scale of animal life, digestion and
circulation are accomplished in separate cavities, and the functions of
nutrition become more complex and distinct.
* * * * *