118 



GENERAL ZOOLOGY 



that in such cases lesions have been found in some or all of the 

 islet tissue of the pancreas, and (2) that the administration of 

 insulin, a preparation containing the active principle of the islet 

 cells, causes a return to normal sugar metabolism. 



Absorption and Metabolism of Fats. — The end products of fat 

 digestion are glycerin and soaps. These are absorbed as such 

 but are apparently at once combined 

 to form fats in the outer zone of the 

 epithelium of the mucosa because fat 

 droplets can be demonstrated in 

 these cells in sections of tissue taken 

 from an animal during absorption 

 (Fig. 72) . In absorption it is believed 

 that the bile salts or other constitu- 

 ents of the bile dissolve the fatty 

 acids or their soaps, thus enabling 



Fig. 72. Fig. 73. 



Fig. 72. — Intestinal epithelium of frog showing spherules of fat (in black) as 

 they appear during absorption. {After Krehl.) 



Fig. 73. — Diagram of the larger veins and lymphatic vessels of intestine. 

 H, hepatic vein; hp, hepatic portal vein; i, intestine; l, liver; m, mesentery; p, 

 postcava; s, superior cava; td, thoracic duct, which conveys lymph to the sub- 

 clavian vein. 



them to pass into the cells. Thus the bile is not only important in 

 the digestion of fats but also in their absorption. Most of the 

 synthesized fat passes from the epithelium of the mucosa into the 

 lacteals, then into the larger intestinal lymphatics, the thoracic 

 lymph duct, and finally into the blood stream at the point where 

 the thoracic duct joins the right subclavian vein near the heart. 

 During absorption the lymphatics of the intestine are distended 

 with fat, which gives them a milky appearance. Fat in the form 

 found in the lymphatics is known as chyle. A smaller quantity 

 of fat enters the blood capillaries of the mucosa and thus passes 

 to the liver through the portal vein, before entering the general 



