viii NUTRITION 135 



passes into the hepatic ducts, thence making its way 

 either directly into the intestine or into the gall-bladder 

 (p. 70) . The whole liver, which is the largest gland in the 

 body, is traversed by a complex network of capillaries 

 (bl), arising partly from the hepatic artery, partly 

 from the hepatic portal vein ; and from the blood 

 thus supplied, the liver-cells obtain the materials neces- 

 sary to enable them to discharge their function of 

 secreting the bile. 



The liver-cells have, however, other functions, one of 

 which is to manufacture a substance called glycogen or 

 animal-starch. This is stored up in the cells in the 

 form of minute insoluble granules, which, being after- 

 wards transformed into soluble sugar, pass into the 

 blood and so to the tissues. 



Connection of the foregoing facts with the Physiology 

 of Nutrition. You will now be able to understand more 

 clearly the various processes connected with the nutrition 

 of the frog, hitherto studied without the aid of histology. 



When the food enters the enteric canal the various 

 gland-cells are stimulated into activity, and the gastric 

 juice, bile, and pancreatic juice are poured out and 

 mingled with the food, which is digested in the manner 

 already described. The soluble products of digestion 

 peptones, sugar, salts, fatty acids, and glycerine diffuse 

 through the epithelium of the enteric canal into the blood- 

 capillaries of the underlying mucous membrane, and the 

 blood, now loaded with nutriment, is carried by the 

 portal vein to the liver and thence by the hepatic and 

 postcaval veins to the heart (see Fig. 23). At the same 

 time the fats make their way into the lymph-capillaries 

 and are finally pumped, by the lymph-hearts, into the 

 veins. Thus the products of digestion all find their way 

 ultimately into the blood, and are distributed, through 

 the circulatory mechanism, to all parts of the body. 



