7. The stomach is the largest organ of digestion. It lies immediately under the false ribs, on the left side, below the midriff, and is shaped somewhat like a bag-pipe. It has two openings, the upper one, which admits the food, is called car. diac, and the lower one, the pyloric orifice. In a middle sized man, this organ is about ten inches long, and three or four in diameter, and holds from three to six pints.

8. The stomach has four distinct coats; the inner one is called mucous or villous; being thin, soft, and spongy; the second vascular; as it is made up chiefly of blood vessels; the third muscular; composed of muscular fibres, and very strong; the fourth serous; as it secretes a serous or watery fluid. The stomach is freely supplied with blood vessels and nerves; the latter being furnished partly from the spinal marrow, and partly from the brain.

The human stomach: a, the esophagus or gullet; b, the cardiac portion; c, the left extremity; d, the small extremity; e, the pylorus tied ; gg, the omentum or caul, which is attached to the outside of the stomach, and falls over the intestines like a curtain.

9. The intestines in man, are from five to six times the length of the body; and are divided into large and small intestines; the latter making about four fifths of the whole. The small intestines are divided into duodenum, jejunum, and ileon. The duodenum is so called because it is about twelve finger's-breadth in length; the jejunum from its generally being found empty; the ducts or canals from the liver and pancreas enter the duodenum near the middle, and it is abundantly supplied with lacteals. The different portions of the large intestine are called coecum, colon, and rectum.

10. The liver is the largest gland in the body; and it lies directly under the ribs, on the right side, and reaching below them. In a grown person, it is about ten inches in diameter, and weighs not far from four pounds. It secretes the bile, which is poured out into a bag called gall-bladder, connected with it; from thence it is carried into the duodenum. The liver is found in all the vertebrated animals and in the mollusca; in birds, reptiles and fishes, its size is greater in proportion to that of the body than in the human species. It is attached to the diaphragm, which lies above it, by a fold of the peritoneum, called the suspensory ligament of the liver.

11. The pancreas is a gland about five inches in length, of a whitish colour, lying immediately behind the stomach. It is supplied, like the stomach, with numerous blood-vessels and nerves. It secretes a fluid, called the pancreatic fluid, which is white, viscid, inodorous, slightly saline, and contains a large proportion of albumen. This gland is found in all the mammalia, in birds, and in amphibious animals, or such as live both in air and water, like the frog; but it is much larger in animals that live on vegetables, than in those that feed on flesh. Hence, the fluid which it secretes, is supposed to aid in difficult digestion. (See Fig. 3.)

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Fig. 3.

The above cut represents the whole tract of the intestinal canal, not exactly in its natural position, but spread out so as to show the relative proportions; f, the esophagus; g, cardia; h, pylorus; i, i, the duodenum, about twelve fingers' breadth in length; k, hepatic duct; 1, gall blad

der; m, cystic duct; n, ductus communis, formed by the union of both; o, the opening of this duct into the duodenum; p, pancreatic duct; q, its opening into the duodenum; r, jejunum; s, ilium; these constitute the small intestines, and are about twenty-six feet in length, or five times the length of the body; t, termination of ilium in the coecum; u, superior fold of valve of colon; v, inferior do.; w, coecum; x, vermiform process; y, y, colon; z, rectum. The coecum, colon, and rectum, form the large intestines, and are about six feet in length; the coecum being about 4 inches long, and the same in diameter. The arrows show the direction which the food takes in digestion.

12. Hunger and thirst are sensations designed to teach us the necessity of supplying those losses which the system is constantly undergoing by the different secretions and excretions, amounting to several pounds in the course of twentyfour hours. The blood first feels the loss, and then the solids, whose particles are continually taken up by the absorbents, and carried into the blood, and thus ejected from the system; and were not these losses supplied by the timely introduction of food, the body would rapidly emaciate, till death closed the scene. Hunger is supposed to be owing to a peculiar affection of the nerves of the stomach; for when the nerve which goes from the brain to the stomach (the parvagum,) is divided, the sensation of hunger is lost, or at least the appetite for food is destroyed.

13. While the food is undergoing the process of mastication, that is, of being divided and ground down by the teet it is thoroughly mixed with a quantity of saliva, amounting it is supposed to between eight and ten ounces. The food is thus brought into a condition to be easily swallowed, and readily dissolved by the action of the stomach. It is very important, therefore, that the food should be slowly chewed, and reduced to as fine a state as possible in the mouth, in order that digestion may be easy. Too great rapidity in eating, probably lays the foundation of many cases of indigestion.

14. After the food has been sufficiently masticated, it is carried down the esophagus into the stomach, first by the

contraction of the muscles of the pharynx, and then by a successive contraction of the circular fibres of the gullet itself, from above downwards. That the morsel is not carried down merely by its own weight, is proved by the fact, that a man can swallow with his head downwards.

15. In the stomach, the food is converted into a soft, grey, pulpy mass, called chyme. This process has been called chymosis. It is produced by the motions of the stomach, together with the agency of the secretions, which are thrown out by the gastric vessels. These motions have been called vermicular or worm-like; and undulatory, or like wave succeeding wave. The crawling of a worm furnishes a very good illustration of the successive contractions of the muscular fibres of the stomach, commencing, as they do, at the æso. phagus, and proceeding onwards to the pylorus, and so back again. These motions of course bring every portion of the contents of the stomach in contact with all parts of its surface; and so they become intimately mixed with the gastric fluid.

16. The food is thus carried round the interior of the stomach, from one extremity to the other; and from one to three minutes are employed in each revolution. In the mean time, both orifices are closed; so that the contents cannot escape. During this process, the gastric fluid is secreted in large quantities, and becomes mixed with the food as it passes round. In a short time, the taste, smell, and other sensible properties of the food, are entirely changed. This is produced by the agency of the gastric juice, which subverts the chemical affinities of the food; and with its ele ments forms new combinations.

17. The gastric fluid is a clear, transparent fluid, produced by arterial exhalation; acid to the taste, slightly saline, and free from odour. It possesses the singular property of coagulating albumen; resists the putrefaction of animal matter; and dissolves nearly every kind of alimentary substance. Its acid properties are owing to the muriatic and acetic acids