a honey-combed or reticulated appearance. Here the food, already

Fig. 17.

COMPOUND STOMACH OF OX.-a. Esophagus. b. Rumen, or first stomach. c. Reticulum, or second. d. Omasus, or third. e. Obomasus, or fourth. f. Duodenum.

triturated in the mouth, and mixed with the saliva, is further macerated in the fluids swallowed by the animal, which always accumulate in considerable quantity in the reticulum. The next cavity is the omasus, or "psalterium" (d), in which the mucous membrane is arranged in longitudinal folds, alternately broad and narrow, lying parallel with each other, like the leaves of a book, so that the extent of mucous surface, brought in contact with the food, is very much increased. The exit from this cavity leads directly into the abomasus, or "rennet" (e), which is the true

digestive stomach, in which the mucous membrane is softer, thicker, and more glandular than elsewhere, and in which an acid and highly solvent fluid is secreted. Then follows the intestinal canal with its various divisions and variations.

In the carnivora, on the other hand, the alimentary canal is shorter and narrower than in the preceding, and presents fewer complexities. The food, upon which these animals subsist, is softer than that of the herbivora, and less encumbered with indigestible matter; so that the process of its solution requires a less extensive apparatus.

In the human species, the food is naturally of a mixed character, containing both animal and vegetable substances. But the digestive apparatus in man resembles almost exactly that of the carnivora. For the vegetable matters which we take as food are, in the first place, artificially separated, to a great extent, from indigestible impurities; and secondly, they are so softened by the process of cooking as to become nearly or quite as easily digestible as animal substances.

In the human species, however, the process of digestion, though simpler than in the herbivora, is still complicated. The alimentary canal is here, also, divided into different compartments or cavities, which communicate with each other by narrow orifices. At its

commencement (Fig. 18), we find the cavity of the mouth, which is guarded at its posterior extremity by the muscular valve of the

isthmus of the fauces.

Through the pharynx and

Fig. 18.

a

œsophagus (a), it communicates with the second compartment, or the stomach (b), a flask-shaped dilatation, which is guarded at the cardiac and pyloric orifices by circular bands of muscular fibres. Then comes the small intestine (e), different parts of which, owing to the varying structure of their mucous membranes, have received the different names of duodenum, jejunum, and ileum. In the duodenum, we have the orifices of the biliary and pancreatic ducts (f, g). Finally, we have the large intestine (h, i, j, k), separated from the smaller by the ileo-cæcal valve, and terminating, at its lower extremity, by the anus, at which is situated a double sphincter, for the purpose of guarding its orifice. Everywhere the alimentary canal is composed of a mucous membrane and a muscular coat, with a layer of submucous areolar tissue between the two. The muscular coat is everywhere composed of a double layer of longitudinal and transverse fibres, by the alternate contraction and relaxation of which the food is carried through the canal from above downward. The mucous

7:

HUMAN ALIMENTARY CANAL.-a. Esophagus. b. Stomach. c. Cardiac orifice. d. Pylorus. e. Small intestine. f. Biliary duct. g. Pancreatic duct. h. Ascending colon. i. Transverse colon. 3. Descending colon. k. Rectum.

membrane presents, also, a different structure, and has different properties in different parts. In the mouth and oesophagus, it is smooth, with a hard, whitish, and tessellated epithelium. This kind. of epithelium terminates abruptly at the cardiac orifice of the stomach. The mucous membrane of the gastric cavity is soft and glandular, covered with a transparent, columnar epithelium, and thrown into minute folds or projections on its free surface, which are sometimes reticulated with each other. In the small intestine, we find large transverse folds of mucous membrane, the valvula conniventes, the minute villosities which cover its surface, and the peculiar glandular structures which it contains. Finally, in the large intestine, the mucous membrane is again different. It is here smooth and shining, free from villosities, and provided with a different glandular apparatus.

Furthermore, the digestive secretions, also, vary in these different regions. In its passage from above downwards, the food meets with no less than five different digestive fluids. First it meets with the saliva in the cavity of the mouth; second, with the gastric juice, in the stomach; third, with the bile; fourth, with the pancreatic fluid; and fifth, with the intestinal juice. It is the most important characteristic of the process of digestion, as established by modern researches, that different elements of the food are digested in different parts of the alimentary canal by the agency of different digestive fluids. By their action, the various ingredients of the alimentary mass are successively reduced to a fluid condition, and are taken up by the vessels of the intestinal mucous membrane.

The action which is exerted upon the food by the digestive fluids is not that of a simple chemical solution. It is a transformation, by which the ingredients of the food are altered in character at the same time that they undergo the process of liquefaction. The active agent in producing this change is in every instance an organic matter, which enters as an ingredient into the digestive fluid; and which, by coming in contact with the food, exerts upon it a catalytic action, and transforms its ingredients into other substances. It is these newly formed substances which are finally absorbed by the vessels, and mingled with the general current of the circulation.

In our study of the process of digestion, the different digestive fluids will be examined separately, and their action on the alimentary substances in the different regions of the digestive apparatus successively investigated.

MASTICATION.

In the first division of the alimentary canal, viz., the mouth, the food undergoes simultaneously two different operations, viz., mastication and insalivation. Mastication consists in the cutting and trituration of the food by the teeth, by the action of which it is reduced to a state of minute subdivision. This process is entirely a mechanical one. It is necessary, in order to prepare the food for the subsequent action of the digestive fluids. As this action is chemical in its nature, it will be exerted more promptly and efficiently if the food be finely divided than if it be brought in contact with the digestive fluids in a solid mass. This is always the case when a solid body is subjected to the chemical action of a solvent fluid; since, by being broken up into minute particles, it offers a larger surface to the contact of the fluid, and is more readily attacked and dissolved or decomposed by it.

Fig. 19.

In the structure of the teeth, and their physiological action, there are certain marked differences, corresponding with the habits of the animal, and the kind of food upon which it subsists. In fish and serpents, in which the food is swallowed entire, and in which the process of digestion, accordingly, is comparatively slow, the teeth are simply organs of prehension. They have generally the form of sharp, curved spines, with their points set backward (Fig. 19), and arranged in a double or triple row about the edges of the jaws, and sometimes covering the mucous surfaces of the mouth, tongue, and palate. They serve merely to retain the prey, and prevent its escape, after it has been seized by the animal. In the carnivorous quadrupeds, as those of the dog and cat kind, and other similar families, there are three different kinds of teeth adapted to different mechanical purposes. (Fig. 20.) First, the incisors, twelve in number, situated at the anterior part of the jaw, six in the superior, and six in the inferior maxilla, of flattened form, and placed with their thin edges running from side to side. The incisors, as their name indicates, are adapted for dividing the food by a cutting motion, like that of a pair of shears. Behind them come the canine teeth, or tusks, one on each side of the upper and under jaw. These are long, curved, conical, and pointed; and are used as

SKULL OF RATTLESNAKE. (After Achille-Richard.)

weapons of offence, and for laying hold of and retaining the prey. Lastly, the molars, eight or more in number on each side, are

Fig. 20.

larger and broader than the incisors, and provided with serrated edges, each presenting several sharp points, arranged generally in a direction parallel with the line of the jaw. In these animals, mastication is very imperfect, since the food is not ground up, but only pierced and mangled by the action of the teeth before being swallowed into Anterior view; the stomach. In the herbivora, on the other hand, the incisors are pre

sent only in the lower jaw in the ruminating animals, though in the horse they are found in both the upper and lower maxilla (Fig.

Fig. 21.

SKULL OF THE HORSE.

Fig. 22.

21). They are used merely for cutting off the bundles of grass or herbage, on which the animal feeds. The canines are either absent or only slightly developed, and the real process of mastication is performed altogether by the molars. These are large and thick (Fig. 22), and present a broad, flat surface, diversified by variously folded and projecting ridges of enamel, with shallow grooves, intervening between them. By the lateral rubbing motion of the roughened surfaces against each other, the food is effectually comminuted and reduced to a pulpy mass.

MOLAR TOOTH OF THE HORSE. face.

Grinding sur

In the human subject, the teeth combine the