THE ANIMAL ORGANISM 



33 



the duodenum. In the loop between the duodenum and the 

 stomach lies the pancreas. At the top of the loop the duodenum 

 passes over into the ileum, which turns back and follows a coiled 

 path to the large intestine, from which it is marked off by a con- 

 striction. The spleen, a small globular structure, reddish in 

 color, is attached to the mesentery near the upper end of the large 

 intestine. It is really a part of the circulatory system. The 

 liver consists of three main lobes from which bile is collected by a 

 system of hepatic ducts, which unite 

 to form a common bile duct. The 

 latter passes through the tissue of 

 the pancreas and opens into the 

 middle region of the duodenum. 

 Since the bile duct also collects the 

 secretions of the pancreas, the lower 

 part of the duct serves as a hepato- 

 pancreatic duct. The gall bladder is 

 an enlargement on one or more of the 

 hepatic ducts where bile is stored, 

 until liberated from time to time 

 into the bile duct (Fig. 13). 



The lining of the small intestine is duodenum 



Fig. 13. 



— Rana Pipiens. B, 



C, cystic duct; D, 



, G, gall bladder; H, 



folded into longitudinal and trans- hepatic duct; L, liver; P, pan- 



, . , , , creas; S, stomach. 



verse ridges, which increase the 



absorptive surface and delay the passage of food. Digestion of 

 protein started in the stomach by pepsin is continued in the 

 intestine by trypsin, one of the constituents of the pancreatic 

 secretion. Amylopsin or amylase, and lipase, also produced in the 

 pancreas, act on starches and fats, respectively. Amylase con- 

 verts starches into sugars and lipase changes fats into fatty acids 

 and glycerin. The bile has no digestive function but facilitates 

 fat digestion. The liver is not primarily an organ of digestion. 

 It serves to store sugar absorbed from the intestine, in the form of 

 glycogen or animal starch, and also is concerned in the storage 

 and utilization of fat and in the conversion of certain nitrogenous 

 metabolic products of the blood into urea, which is excreted 

 through the kidneys. Fibrinogen, from which the fibrin of clotted 

 blood is formed, is produced in the liver. 



The large intestine opens into a chamber, the cloaca, which in 

 turn opens to the outside. The cloaca also receives the openings 



