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.
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.
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-cæcal 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.