HOW ANIMALS DIGEST. 601 



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, how- 

 ever, 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." 



Deglutition. 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 oesophagus 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 stom- 

 ach 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 mus- 

 cular layers, longitudinal and circular, which act in accord. Immedi- 

 ately 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 ac- 

 tion 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 gul- 

 let, 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 



