SEROUS MEMBRANES. 395 



however, the only particulars in which secretions and 

 excretions can be distinguished ; for, in general, the struc- 

 ture of the parts engaged in eliminating excretions, e.g., 

 the kidneys, is as complex, as that of the parts concerned 

 in the formation of secretions. And since the differences 

 of the two processes of separation, 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 mem- 

 brane, named the primary or basement-membrane ; certain 

 cells ; and blood-vessels. These three structural elements 

 are arranged together in various ways ; but all the varieties 

 may be classed under one or other of two principal divi- 

 sions, namely, membranes and glands. 



SECRETING MEMBRANES. 



The principal secreting membranes are the serous and 

 synovial membranes, the mucous membranes, and the 

 skin.* 



Fig. 104-t 



The serous membranes are formed of nbro-cellular tissue, 

 interwoven so as to constitute a membrane, the free surface 

 of which is covered with a single layer of flattened cells, 

 forming, in most instances, a simple tesselated epithelium. 

 Between the epithelium and the subjacent layer of nbro- 

 cellular tissue, is situated the primary or basement mem- 

 brane (Bowman). 



* The skin will be described in a subsequent chapter. 



t Fig. 104. Plan of a secreting membrane : a, membrana propria, or 

 basement-membrane ; b, epithelium composed of secreting nucleated 

 cells ; c, layer of capillary blood-vessels (after Sharpey). 



