ALIMENTATION 103 



vessels leading to the bile duct. The gall bladder is a dilatation 

 of the bile duct in which bile is stored. Bile consists of water, 

 salts, pigment, bile acids, lipoidal substances such as cholesterin 

 and lecithin, neutral fats and soaps, a nucleo-albumin, which 

 gives bile a mucilaginous consistency, and traces of urea. It is 

 partly excretory in nature but is also important in the digestion 

 and absorption of fats. It has an alkaline reaction and is 

 secreted continuously, and in those forms in which a gall bladder 

 is present it is discharged intermittently into the duodenum. 

 The liver, as already noted in an earlier chapter, in addition to 

 secreting bile, performs other important functions in metabolism, 

 such as the storage of sugar and the storage of fats, the produc- 

 tion of urea and the formation of fibrinogen. In its develop- 

 ment the liver is an outgrowth of the endoderm, forming the 

 lining of the embryonic intestine. The liver is formed of the 

 distal part of this outgrowth and the bile duct represents 

 the proximal part. 



Pancreas. — The pancreas is also a digestive gland. It con- 

 tains two sorts of secretory cells: (1) alveolar cells, whose secretion 

 is removed by ducts, and (2) islet cells having no connection 

 with ducts and whose secretion is removed by the blood stream. 

 The alveoli are spherical or tubular arrangements of cells sur- 

 rounding a lumen, drained by the ducts. The epithelium of the 

 ducts is simply the continuation of the glandular epithelium. 

 The islet cells are isolated groups of cells scattered between 

 alveoli at irregular intervals. The secretion produced by the 

 alveolar cells is conveyed by ducts to one or more large trunks 

 terminating in the duodenum at or near the mouth of the bile 

 duct. In the frog the pancreatic ducts all open into the bile 

 duct before the latter reaches the duodenum. In man the 

 pancreatic duct joins the bile duct near the intestine, thus 

 forming a short common hepatopancreatic duct, or there may be 

 in a certain percentage of cases an additional pancreatic outlet 

 into the duodenum. The pancreatic fluid produced by the 

 alveolar cells of the pancreas contains important digestive agents, 

 or inactive forms of these agents, that act upon proteins, fats, 

 and carbohydrates in the alimentary canal (Fig. 70). 



Digestion. — The absorption of nutritive substances from the 

 alimentary canal is preceded by a process of digestion which 

 renders food soluble, since only liquids can pass through the 



