Page:Popular Science Monthly Volume 17.djvu/619

Rh laboratory, as nourishment must be like the body in composition. Plants have no need of such an organ, because the food is always at hand in proper condition. Consequently, food is not in the animal body proper when in the stomach. It is within the body, not of the body. It is only dead matter, under the control of the organic forces of the body, and preparing to become a substantial part of the living organism.

The several processes of animal digestion, mechanical, physical, and chemical, are simultaneously performed in varying degree throughout the whole length of the digestive tract. For our present purpose, however, it is more practicable to describe them separately. But that the division is arbitrary should constantly be kept in mind. The main part of mechanical digestion has already been described in the article on "How Animals eat."

—In animals which have the stomach some distance from the mouth or oral aperture, the swallowing of food is a distinct act, requiring special organs. The food must be forced through the connecting tube, known as the œsophagus or gullet. We do not find such in the lowest animals. The whole of nutrition is a single process in the tape-worm; with the amoeba, grasping of food is not distinct from digestion; while in the anemone and jelly-fish the mouth opens directly into the stomach cavity.

If the stomach were always beneath the mouth, as in man and birds, food might with some difficulty reach the former by gravity. Birds do help the descent of food by jerking the head, and most birds in drinking lift the head each time the beak is filled. But animals must be able to swallow in spite of gravity, as commonly in eating the stomach is higher than the mouth. This is possible even in man, for the juggler drinks when standing on his head.

Deglutition is accomplished by a peculiar and beautiful involuntary action of the gullet. The walls of this tube are composed of two muscular layers, longitudinal and circular, which act in accord. Immediately in front of the bolus of food the walls are relaxed, while behind and around it the walls contract, thus urging the matter forward; and, "as it travels, the wave of contraction travels with it." This motion of the gullet is well shown in the neck of a horse when drinking. It is a similar action which propels the food through the entire length of the digestive canal—termed in the intestines peristaltic or vermicular motion. The mill-like action of the gizzard and the churning motion of the stomach are only phases of the same thing. By a reversed action of the gullet, the cud of an ox is thrown from the stomach back to the mouth.

To place the bolus of food within reach of the muscles of the gullet, there is in the highest animals a most complex arrangement of parts in the pharynx or back of the mouth. In mammals, the pharynx is a funnel-shaped cavity having seven openings. Here the gullet crosses