AN INTRODUCTION TO ANIMAL BIOLOGY 



49 



fluids (called secretions), which are poured into the stomach 

 and intestine. These, coming into contact with foods, change 

 them so that they are dissolved in water taken with the food. 

 We say that the secretions of the gastric and intestinal glands 

 and of the liver and pancreas digest the various kinds of foods 

 which the frog eats ; by this we simply mean that the foods 

 are dissolved and prepared so that they can be absorbed by 

 a process of " soaking through " the lining of the stomach and 



FIG. 19. Part of a cross section of small intestine of frog, pe, peritoneum 

 surrounding the intestine; m.l, outer muscle layer (longitudinal); m.c, 

 inner, circular muscle layer; bl, blood-vessels; e.s, connective tissue layer 

 (submucosa) ; ep, epithelial lining; eg, one-cell glands. (After Howes.) 



intestine into the blood-tubes (Fig. 19). The nature of these 

 digestive secretions and of their action on the various kinds 

 of foods will be taken up more carefully in close connection 

 with similar processes which occur in the human body (Chap- 

 ter XVII) . For our present purpose it is sufficient to point out 

 that digestion is preparing foods for absorption into the cells. 

 45. Distribution of Digested Foods. We have noted 

 reasons why digested foods must be distributed to all parts 

 of the body (See 43). We shall now consider the means by 

 which this is accomplished. 



