382 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



IX. DIGESTION IN THE SMALL INTESTINE. 



As the aliments pass into the duodenum, after being subjected to 

 the action of gastric digestion, they immediately mingle with the three 

 other digestive secretions, the bile, the pancreatic juice, and the intes- 

 tinal secretion. 



I. BILE. The bile is the secretion of the liver, and, strictly speak- 

 ing, occurs only in vertebrates. In the lowest invertebrate animals a 

 fluid somewhat analogous to the bile is poured directly into the intes- 

 tine, as the result of the secretion of cells attached to the intestinal 

 mucous membrane. In others it is formed by a series of convoluted 

 tubes surrounding the intestine, or, it may be, directly surrounding the 

 stomach. But although this fluid may be yellowish or brown, it is not 

 to be regarded as bile, since in invertebrates it never contains the specific 

 bile constituents, bile coloring-matters and acids, and the glands which 

 form it differ histologically from the liver. In all the invertebrates the 

 so-called bile is directly poured into the intestine ; in many vertebrates, 

 however, the excretory duct is in communication by a side branch 

 directed obliquely backward from the course of the duct with a reser- 

 voir for the bile, termed the gall-bladder. This reservoir is present in 

 all omnivora and carnivora, and in most herbivora, birds, and reptiles. 

 It is absent in certain of the group of herbivora. It is absent in the 

 solipedeSjthe horse, mule, and ass, and among the ruminants in the stag, 

 camel, and dromedary ; among the pachydermata, in the elephant, the 

 rhinoceros, and tapir ; in the wild boar, and in certain cetaceans ; while 

 in birds it is absent in the pigeon, cuckoo, paraquet, and ostrich. It is 

 also absent in the mouse and marmot. In the horse and elephant the 

 gall-duct is dilated to form a sort of pouch. The ultimate gall-ducts 

 all unite to form a single trunk, or ductus communis choledochus, which 

 in many animals, such as sheep and goats, communicates with the excre- 

 tory duct of the pancreas and pierces the wall of the duodenum obliquely 

 from below upward. As the result of this, an increase of pressure on 

 the intestinal contents simply closes the orifice of the duct, and regurgi- 

 tation of intestinal contents into the duct is impossible. 



In the gall-bladder the bile becomes concentrated, and mucin is 

 added to it as a result of the action of the secretion of the mucous 

 membrane of the gall-bladder. Little or no mucin is found in bile com- 

 ing directly from the hepatic cells. The gall-bladder is necessary in 

 animals whose digestion, as in the carnivora, is intermittent. It is less 

 important for the herbivora, where digestion is nearly constant. The 

 liver differs from all other organs in its blood-supply. In proportion 

 to its size, it receives but a small supply of arterial blood, and although 

 an immense amount of blood passes through it, the greater part reaches 



