DESQUAMATION. 389 



chemical 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 epi- 

 thelial 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 secre- 

 tions 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. 



