34 SECRETION. 



tain of tlia liquid and saline constituents of the blood can 

 escape by exosmosis through the homogeneous walls of the 

 capillaries, 1 but the more complex fluids require for their 

 formation a different kind of action ; although, in the act 

 of secretion, there is considerable transudation of liquid and 

 saline matters, which take up in their course the peculiar 

 principles formed by the cells. 



Though it is somewhat difficult to draw a line between 

 transudation and the simplest forms of secretion, it may be 

 assumed, in general terms, that fluids which are exhaled 

 directly from the blood-vessels, without the intervention of 

 glandular apparatus or of a secreting membrane, are transu- 

 dations ; while all fluids produced by simple membranes, by 

 follicles, or discharged from the ducts of glands, are secre- 

 tions. This division places the intermuscular fluid and the 

 fluid found in all soft tissues among the transudations, and 

 the serous and synovial fluids among the secretions. 



The serous and synovial membranes present the simplest 

 form of a secreting apparatus. Blood is supplied to them 

 in small quantity, and on their free surfaces are arranged 

 one or two layers of epithelial cells which effect the slight 

 changes that take place in the transuded fluids. In some 

 of the serous membranes, as the pleura and peritoneum, the 

 amount of secretion is very small, being hardly more than a 

 vaporous exhalation ; but others, like the serous pericardium 

 and the synovial membranes, secrete a considerable quantity 

 of fluid. The action of all of these membranes may become 

 exaggerated, as a pathological condition, and the amount of 

 their secretions is then very large. 



Anatomists have now a pretty clear idea of the structure 

 of what are called the glandular organs ; and it will be seen 

 that they simply present an arrangement by which the se- 

 creting surface is increased, and at the same time compressed, 

 as it were, into a comparatively small space. The mucous 

 follicles, for example, are simple inversions of a portion of 



1 See vol. ii., Absorption, p. 505. 



