CHAPTER XXI . 



SECRETION. 



1. Secretion is one of the most obscure and mysterious 

 functions in the animal economy. /To secrete means to sepa- 

 rate ;ybut most of the fluids formed by this process, did not 

 previously exist in the blood, but only the elements out of 

 which they are made. It is purely a vital and not a mechan- 

 ical process, like straining through a seive, to which some 

 have compared it : and the vessels by which it is accom- 

 plished may well be called the architects and chemists of the 

 system ; for out of the same material, the blood, they con. 

 struct a variety of wonderful fabrids, and chemical com- 

 pounds. 



2. We see the same wonderful power possessed also by 

 vegetables ffor, out of the same materials, the olive pre- 

 pares its oil ; the cocoa-nut its milk ; the cane its sugar ; the 

 poppy its narcotic juice ; the henbane its poison ; the oak 

 its green pulpy leaves, its light pith, and its dense woody fi- 

 brej all composed of the same, few simple elements, only ar- 

 ranged in different order and proportions. 



3. fin like manner, we find the vessels in animal bodies, 

 capable of forming all the various textures and substances 

 which make up the frame ; the cellular tissue ; the mem- 

 branes ; the ligaments ; the cartilages ; the bones ; the mar- 

 row ; the muscles, with their tendons ; the lubricating fluid 

 of the joints ; the pulp of the brain ; the transparent jelly of 

 the eye ; in short, all the textures of the various organs of 

 which the body is composed ; and still all are made out of 

 the same blood ; and consist of the same ultimate elements.^ 



4. (The most simple form of secretion, however, seems to 

 be, the mere separation of some principle, which previously 

 existed in the blood \ as senim is deposited in most of the ca- 

 vities, by a kind of exhalation. Some other principles also 



