314 SECKETION. 



for their separation are termed secretions; in the latter, they 

 are named excretions. 



Most of the secretions consist of substances which, probably, 

 do not pre-exist in the same form in the blood, but require 

 special organs and a process of elaboration for their formation, 

 e. g., the liver for the formation of bile, the mammary gland 

 for the formation of milk. The excretions, on the other hand, 

 commonly or chiefly consist of substances which, as urea, car- 

 bonic acid, and probably uric acid, exist ready-formed in the 

 blood, and are merely abstracted therefrom. If from any 

 cause, such as extensive disease or extirpation of an excretory 

 organ, the separation of an excretion is prevented, and an ac- 

 cumulation of it in the blood ensues, it frequently escapes 

 through other organs, and may be detected in various fluids of 

 the body. But this is never the case with secretions ; at least 

 with those that are most elaborated ; for after the removal of the 

 special organs by which any of them is elaborated, it is no longer 

 formed. Cases sometimes occur in which the secretion con- 

 tinues to be formed by the natural organ, but not being able to 

 escape towards the exterior, on account of some obstruction, is 

 reabsorbed into the blood, and afterwards discharged from it 

 by exudation in other ways ; but these are not instances of true 

 vicarious secretion, and must not be thus regarded. 



These circumstances, and their final destination, are, how- 

 ever, the only particulars in which secretions and excretions 

 can be distinguished ; for, in general, the structure of the parts 

 engaged in eliminating excretions, e. g., the kidneys, is as com- 

 plex as that of the parts concerned in the formation of secre- 

 tions. And since the diiferences of the two processes of sepa- 

 ration, corresponding with those in the several purposes and 

 destinations of the fluids, are not yet ascertained, it will be 

 sufficient to speak in general terms of the process of separation 

 or secretion. 



Every secreting apparatus possesses, as essential parts of its 

 structure, a simple and apparently textureless membrane, 

 named the primary or basement-membrane ; certain cells ; and 

 bloodvessels. These three structural elements are arranged 

 together in various ways ; but all the varieties may be classed 

 under one or other of two principal divisions, namely, mem- 

 branes and glands. 



SECRETING MEMBRANES. 



The principal secreting membranes are the serous and syno- 

 vial membranes, the mucous membranes, and the skin. 1 



1 The skin will be described in a subsequent chapter. 



