344 COMPARATIVE PHYSIOLOGY OF DIGESTION. 



into the third stomach, the manyplies (psalterium) , which is absent in camels,. 

 and thence to the true, fourth stomach, the rennet-stomach (abomasum). In 

 the two first stomachs starch and cellulose are digested, the sugar formed in part 

 passing over into lactic acid. The third stomach performs chiefly mechanical 

 work, while the fourth really digests albumin. In the small intestine proteids 

 and carbohydrates are further digested. 



The intestine is divided into the small and the large intestine. It is short 

 in carnivora, and considerably longer in herbivora. The cecum, which in her- 

 bivora attains considerable size as the most important organ of digestion, and in 

 some rodents is even multiple, represents in human beings an insignificant, 

 typical remnant, and is wholly absent in carnivora. In birds the esophagus, 

 especially in birds of prey and granivora, often possesses a diverticular appendix, 

 the crop, for the maceration of the food. In the crop of pigeons there occurs, 

 at the breeding-season, the secretion of crop-milk, the product of a special gland r 

 which is also used as food for the young. The stomach consists of the proven- 

 triculus well supplied with glands, and the thick-walled muscle-stomach, which, 

 with the aid of the inner horny plates, effects the crushing especially of grain. In 

 the intestine, at the junction with the short large intestine, there is almost con- 

 stantly present a pair of ceca shaped like a glove-finger. The intestinal mucous 

 membrane exhibits principally longitudinal folds. The alimentary canal of fish 

 is usually simple. The stomach frequently represents only a dilatation. Less 

 commonly the pylorus possesses one, more frequently a large number of divertic- 

 ular appendices, containing a large number of glands (appendices pyloricae, as, 

 for example, in the salmon). The mucous membrane of the usually short intes- 

 tine exhibits longitudinal plication, as a rule, or the so-called spiral valve, as in 

 the sturgeon, resulting from a spiral arrangement. The alimentary canal of fish, 

 from the esophagus to the rectum, possesses peptonizing power. The short 

 rectum is provided, in sharks and rays, with a diverticular appendage (bursa 

 entiana). 



In amphibia and reptiles the stomach is generally a simple dilatation. The 

 intestine is longer in herbivora than in carnivora. Especially interesting in 

 this connection is the fact that the vegetable-eating frog-larvas acquire a 

 much shorter intestine with the metamorphosis that makes them carnivorous, 

 terrestrial animals. The intestinal mucous membrane of reptiles exhibits 

 numerous plications. The liver is not wanting in any vertebrate, and is 

 especially large in fish. The amphioxus has only a diverticulum indicative of 

 the liver. The gall-bladder is wanting occasionally in all classes, in accord with 

 which is the experimental observation that extirpation of the gall-bladder is 

 unattended with appreciable influence on digestion and absorption. The pan- 

 creas is wanting only in some fish. One opening (in the amphioxus) or two open- 

 ings (in the shark, the ray, the sturgeon, the eel and the salmon) lead from with- 

 out freely into the abdominal cavity; the same conditions prevail also in crocodiles. 



Among the molluscs, snails and cephalopods only have true organs of 

 mastication. Some herbivorous land-snails have a movable, horny grinding 

 plate situated in the upper pharyngeal wall. Horizontal maxillary plates, with 

 hard edges working one upon the other, are present particularly in carnivorous 

 snails with uncovered gills. A horny grinding plate, placed like a tongue, whose 

 peculiar form serves for the systematic differentiation of various snails, is fre- 

 quently present in others. Cephalopods possess a strong biting apparatus in 

 the form of a large, horny pair of jaws, resembling a parrot's beak in shape. They 

 also have a grinding plate upon a tongue-like prominence, studded with spines. 

 The alimentary canal is divided into esophagus, stomach and intestine, at times 

 provided with diverticula. In many mussels the rectum pierces the heart and 

 the pericardium. In snails the anus is usually in the vicinity of the respiratory 

 organs. The liver is, as a rule, large. The vineyard-snail has a cellulose- 

 splitting ferment in the secretion of the liver. In the cephalopods the ink-bag 

 opens into the rectum or near the anus. 



Among vertebrates crustaceans have a masticating apparatus transformed 

 from feet; in some, true masticating feet are still present; in parasitic crabs there 

 are also sucking mouth-organs. Among arachnids the mites have sucking mouth- 

 organs; in true spiders, there are, in addition to the sucking mouth-organs, 

 horizontally acting clutching jaws, in part connected with poison-glands. Centi- 

 pedes possess a strong pair of jaws, acting horizontally. Of insects, those provided 

 with masticating mouth-organs possess, between the upper and lower lips, two 

 pairs of jaws, acting horizontally against each other, of which the upper (man- 



