TISSUES. 



55 



Since the blood supply never penetrates epithelial 

 layers, it is evident that the superficial layers of cells 

 receive less nourishment and ultimately die, which, 

 perhaps, accounts for the constant exfoliation. It is 

 also evident that anything that will increase the 

 blood supply will increase the nourishment, as fric- 

 tion, massage, and hot applications. The nour- 

 ishment, at best, is not very good, which explains the 



Epithelium of cornea. 



Substantia propria of 

 cornea. 



Fig. 23. Section showing corneal epithelium of the eye of pig. 



ease with which skin grafts are made. Epithelial 

 cells will live for twenty-four hours or more in 

 normal salt solutions, and will even multiply in 

 favorable culture media. 



The nerve termination among the epithelial cells 

 is an important relation which, in large part, controls 

 their metabolism. A disturbed nervous system may 

 impair or even cause a destruction of epithelial cells. 



