PROTECTIVE AND EXCRETOEY FUNCTIONS 

 OP THE SKIN. 



As has been intimated from time to time, thus far, as a 

 result of the metabolism of the tissues, certain products require 

 constant removal from the blood to prevent poisonous effects. 

 These substances are in all probability much more numerous 

 than physiological chemistry has as yet distinctly recognized 

 or, at all events, isolated. Quantitatively considered, the most 

 important are carbonic anhydride, water, urea, and, of less im- 

 portance, perhaps, certain salts. 



In many invertebrates and in all vertebrates several organs 

 take part in this work of elimination of waste products or puri- 

 fication of the blood, one set of which — the respiratory — we 

 have just studied ; and we now continue the consideration of 

 the subject of excretion, this term being reserved for the pro- 

 cess of separating harmful products from the blood and dis- 

 charging them from the body. 



We strongly recommend the student to make the study of 

 excretion comparative in the sense of noting how one organ 

 engaged in the process supplements another. A clear under- 

 standing of this relation even to details makes the practice of 

 medicine more scientific and practically effective, and gives 

 physiology greater breadth. 



The skin has a triple function : it iii^ protective, excretory, 

 sensory, and, we may add, nutritive (absorptive) and respirar 

 tory, especially in some groups of animals. 



As a sensory organ, the skin will receive attention later. 



Protective Fnnction of the Skin. — Comparative. — Among 

 many groups of invertebrates the principal use of the exterior 

 covering of the body is manifestly J)rotection. Among these 

 forms, an internal skeleton being absent, the exo-skeleton is 

 developed externally, and serves not only for protection, but 

 for the attachment of muscles, as seen in crustaceans and in- 



