ALIMENTARY TRACT AS AN ABSORPTIVE SYSTEM. 261 



intestinal mucosa and pass into the blood and lymph streams. 

 The process of diffusion within the animal organism does not 

 go on as simply antics uninterruptedly, however, as in a 

 vessel containing a ^soTvent ig^ a soluble substance. The 

 rate of entrance of even the simplest .chemical substances 

 into the epithelial cells of the intestikal mucosa is fSr" dif- 

 ferent from the rates of diffusion of these same substances 

 outside of the body. As will become apparent later an 

 important reason for this is to be found in the constitution of 

 protoplasm itself, and of the membranes surrounding this- 

 protoplasm. 





