far as to make two or three sections. But animals that chew the cud (ruminants) have the most complex stomach. It is divided into four peculiar chambers: First, the paunch (rumen), the largest of all, receives the half-masticated food when it is first swallowed. The inner surface is covered with papillæ, except in the camel, which has large cells for storing up water. From this, the food passes into the honeycomb stomach (reticulum), so named from its structure. Liquids swallowed usually go directly to this cavity, without passing through the paunch, and hence it is sometimes called the water bag. Here the food is made into little balls,

FIG. 254.-Complex Stomach of a Ruminant: a, gullet; b, rumen, or paunch; c, reticulum; d, psalterium, or manyplies; e, abomasus; f, pylorus leading to duodenum. and returned to the mouth to undergo a thorough mastication. When finally swallowed, it is directed, by a groove from the esophagus, to the third, and smallest, cavity, the manyplies (psalterium), named from its numerous folds, which form a strainer to keep back any undivided food; and thence it passes into the true stomach (abomasus), from which, in the calf, the rennet is procured for curdling milk in the manufacture of cheese. This fourth cavity is like the human stomach in form and function, and is the only part which secretes gastric juice. The rumen and reticulum are rather dilatations of the esophagus than parts of the stomach itself; while the latter is divided by constriction into two chambers, the psalterium and abomasus, as in many other animals.

In structure the stomach resembles the esophagus. The smooth outside coat (peritoneum) is a reflection of the membrane which lines the whole abdomen. The middle, or muscular, coat consists of three layers of fibers, running lengthwise, around and obliquely. The successive contraction and relaxing of these fibers produce the wormlike motion of the stomach, called peristaltic. The innermost, or mucous, membrane, is soft, velvety, of a reddish gray color in man, and filled with multitudes of glands, which secrete the gastric juice. The human stomach, when distended, will hold about five pints; that of the kangaroo is as long as its body.

FIG. 255.

m.m.

The intestinal canal in mammals begins at the pyloric end of the stomach, where there is a kind of valve or circular muscle. Like the stomach, it varies greatly, according to the nature of the food. It is generally follicles; m, gastric tubuli, longest in the vegetable feeders, and or follicles; mm, dense con- shortest in the flesh feeders. The nective tissue; sm, sub-mucous tissue; cm, transverse greater length in the former is due muscular fiber; lm, longitudinal muscular fibers; to the fact that vegetable food res, fibrous, or serous, coat. quires a longer time for digestion, and that a greater bulk of such food is required to obtain a given quantity of nutriment. The intestines measure 150 feet in a full-grown ox, while they are but three times the length of the body in the lion, and six times in man. Save in some lower forms, as the whales, there are two main divisions, the "small" and "large" intestines, at the junction of which is a

valve. The former is the longer of the two, and in it digestion is completed, and from it the most of absorption takes place. The large intestine is mainly a temporary lodging place for the useless part of the food, until it is expelled from the body. The beginning of the small intestine is called the duodenum, into which the ducts from the liver and pancreas open. The intestinal canal has the same structure as the stomach, and by a peristaltic motion its contents are propelled downward. The inside of the small intestine is covered with a host of threadlike processes (villi), resembling the pile of velvet.

In taking this general survey of the succession of forms which the digestive apparatus presents among the principal groups of animals, we cannot fail to trace a gradual specialization. First, a simple sac, one orifice serving as inlet for food and outlet for indigestible matter; next, a short tube, with walls of its own suspended in the body cavity; then a canal passing through the body, and, therefore, having both mouth and vent; next, an apparatus for mastication, and a swelling of the central part of the canal into a stomach, having the special endowment of secreting gastric juice; then a distinction between the small and large intestine, the former thickly set with villi, and receiving the secretions of large glands. We also notice that food, the means of obtaining it, the instruments for mastication, and the size and complexity of the alimentary canal, are closely related.

CHAPTER XIII*

HOW ANIMALS DIGEST

The Object of the Digestive Process is the reduction of food into such a state that it can be absorbed into the system. For this purpose, if solid, it is dissolved; for fluidity is a primary condition, but not the only one. Many soluble substances have to undergo a chemical change before they can form parts of the living body. If albumen or sugar be injected into the veins, it will not be assimilated, but be cast out unaltered.

To produce these two essential changes, solution and transmutation, two agencies are used-one mechanical, the other chemical. The former is not always needed, for many animals find their food already dissolved, as the butterfly; but solid substances, to facilitate their solution, are ground or torn into pieces by teeth, as in man; by jaws, as in the lobster; or by a gizzard, as in the turkey.

The chemical preparation of food is indispensable.101 It is accomplished by one or more solvent fluids secreted in the alimentary canal. The most important, and one always present, is the gastric juice, the secretion of which is restricted to the stomach, when that cavity exists. In the higher animals, numerous glands pour additional fluids into the digestive tube, as saliva into the upper part or mouth, and bile and pancreatic juice into the upper part of the intestine. In fact, the mucous

* See Appendix.

membrane, which lines the alimentary canal throughout, abounds with secreting glands or cells.

The Digestive Process is substantially the same in all animals, but it is carried farther in the more highly developed forms. In the Infusoria, the food is acted upon by some secretion from the protoplasm of the body, the exact nature of which is unknown. In the starfish and sea urchin, we find two solvents — a gastric juice, and another resembling pancreatic juice; but the two appear to mingle in the stomach. Mollusks and arthropods

show a clear distinction between the stomach and intestine, and the contents of the pancreas are poured into the latter. There are, therefore, two stages in the digestive act: first, the food is dissolved by the gastric juice in the stomach, forming chyme; secondly, the chyme, upon entering the intestine, is changed into chyle by the action of the pancreatic secretion, and is then ready to be absorbed into the system.

In vertebrates, a third solvent is added, the bile, which aids the pancreatic juice in completing digestion. But mammals and insects have a still more perfect and elaborate process; for in them the saliva of the mouth acts chemically upon the food; while the saliva in many other animals has no other office, so far as we know, than to moisten the food for swallowing.

Taking man as an example, let us note the main facts in the process. During mastication, by which the relative surface is increased, the food is mixed with saliva, which moistens it,102 and turns a small part of the starch into grape sugar. Passed into the stomach, the food meets the gastric juice. This is acid, and, first, stops the action of the saliva; secondly, by means of the pepsin which it contains, and the acid, it dissolves the albumen, fibrin, and other such constituents of the food. This solution of albuminoids is called a peptone,