350 



COMPARATIVE PHYSIOLOGY. 



When we consider that the blood tends to maintain an equi- 

 librium, it must be evident that the removal of substances from 

 the alimentary canal, unless there is to be excessive activity of 



Fig. 287. — A. Section of villus of rat Icilled during fat absorption (SchSfer). ep, epi- 

 thelium; etr, striated border; c, lymph-celle; c', lymph-ceils in epithelium: I, cen- 

 tral lacteal containing disintegrating corpuscles. B. Mucous membrane of frog's 

 intestine during fat absorption (Schafer). ep, epithelium; sir, striated border; C, 

 lymph-corpuscles; I, lacteal. 



the excretory organs and waste of energy both by them and 

 the digestive tract, must in some degree depend on the demand 

 for the products of digestion by the tissues. That there is to 

 some extent a corrective action of the excretory organs always 

 going on is no doubt true, and that it may in cases of emergency 

 be great is also true ; but that this is minimized in ways too 

 complex for us to follow in every detail is equally true. Diges- 

 tion waits on appetite, and the latter is an expression of the 

 needs of the tissues. We believe it is literally true that in a 

 healthy organism the rate and character of digestion and of 

 the removal of prepared products are largely dependent on the 

 condition of the tissues of the body. 



Why is digestion more perfect in overfed animals after 

 a short fast ? The whole matter is very complex, but we think 



