386 MANUAL OF PHYSIOLOGY. 



the term, but which is certainly elaborated as the result of the nu- 

 tritive changes going on in the protoplasm of the cell during its 

 life-history, just as we know that many other substances are pro- 

 duced as the result of the working of gland cells. 



The work of the epidermal cells supplies not a peculiar chemi- 

 cal reagent, as do some of the gland cells of the digestive tract, 

 nor yet a nutrient fluid like milk but the exterior of the body 

 with an insoluble, impervious, tough coating, which though thin 

 and elastic, is very strong and resisting. 



The nearest analogy to the secretion of the keratin in the epi- 

 dermal cells, is the production of mucin in the cells of the epithe- 

 lial lining of the mucous membranes. Both substances may be 

 looked upon as excretions, as they never reenter the system and 

 are cast off, but each of them performs a definite function, and is 

 produced by special protoplasmic elements, like the secretions 

 more generally recognized as such. 



The amount of nitrogenous substances thus excreted cannot 

 well be reckoned, but, having regard to the great extent of 

 surface from which they are derived, it must be considerable. 



