SECRETION. 83 



the urine, &c. The first are regarded more peculiarly as secretions; 

 the latter, as excretions. 



Of the reason why one organ should separate bile, another milk, 

 and another sperm, no other account can be given than that which 

 refers them to the special endowments of the cells, the real instru- 

 ments in the process. That the particular modification of structure 

 which the organ may present, has no essential connexion with the 

 character of the secretion is evident, from the fact that almost every 

 gland may be found under a variety of forms in different parts of 

 the animal series ; and for every gland there is a part of the animal 

 scale below which it does not exist, and when it makes its first 

 appearance it almost always presents a character nearly as simple, 

 as that of the least complex glandular structures in the higher or- 

 ders. 



The simplest fortn of secreting organ is that of the simple animal 

 m.embrane, well supplied with blood-vessels and covered with an epi- 

 thelium ; of such a membrane we have an example in the serous and 

 synovial membranes. The next is that of the follicle, a depression, 

 or inversion of the animal membrane, lined with epithelium cells, 

 and abundantly supplied with blood-vessels from which are elaborated 

 their peculiar secretions. (Figure 13, page 56, already referred 

 to.) The third and last form of secreting organs is the gland, which 

 is nothing but an aggregation of follicles, closely 

 packed together, so as to present a large secreting Fig. 17. 



surface in as small a bulk as possible. In some 

 glands the sacs or follicles, are prolonged into caeca, 

 or blind tubes, as in the kidney arid testis ; these are 

 called tubular glands; or else they are very greatly 

 multiplied, and clustered together (like currants on 

 a stalk) upon efferent ducts common to several of 

 them. (Fig. 17.) 



In all secreting organs the important agents are 

 the cells which are developed upon the lining mem- 

 brane of the follicles and tubes, and which select and elaborate the 

 materials from the blood and discharge their contents into the 

 excretory duct. These cells are being constantly cast off and re- 

 placed by a new growth, having their origin in the basement mem- 

 brane of the mucous membrane which lines the ducts or follicles. 



The simplest condition of a secreting cell in the animal body is 

 that of the adipose tissue, every cell of which has the power of 

 selecting its materials from the blood. The contents of these, how- 

 ever, are not discharged, but remain stored as a reservoir in time of 

 need. The adipose tissue has already been described. 



^ There is a difference between the processes of secretion and exhala- 

 tion; the former is a vital process, the latter a physical. Wherever 

 a fluid requires to be elaborated, it is done by a process of secretion, 

 and the agents are cells ; but where no such process is necessary. 



