OF SECRETION IN GENERAL. 465 



CHAPTER XL 



OF SECRETION AND EXCRETION. 



1. Of Secretion in General. 



381. THE literal meaning of the term Secretion is separation; and this is 

 nearly its true acceptation in Physiology. But the ordinary processes of 

 Nutrition involve a separation of certain of the components of the Blood, 

 which are withdrawn from it by the appropriating power of the solid tex- 

 tures ; and every such removal may be considered in the light of an act of 

 excretion, so far as the blood and the rest of the organism are concerned 

 ( 218). Moreover, the separation of certain matters from the blood in a 

 fluid state, either for the purpose of being cast forth from the body, or of 

 being employed for some special purpose within it, which constitutes what 

 is ordinarily known as Secretion, is effected by an instrumentality of the 

 same nature with that whose operation constitutes an essential part of the 

 nutritive process, namely, the production and subsequent agency of cells. 

 Hence there is no other fundamental difference between the two processes, 

 than such as arises out of the diverse destinations of the separated matters, 

 and from the anatomical arrangements which respectively minister to these. 

 For the products of the Secreting action are all poured forth, either upon 

 the external surface of the body, or upon the lining of some of the cavities 

 which communicate with it; and the cells by which they are separated from 

 the blood, usually stand in the relation of epithelium-cells to those prolonga- 

 tions of the skin or of mucous membranes, that form the follicles or extended 

 tubuli of which the Glandular organs are for the most part composed (Figs. 

 165, 172). The act of Secretion appears to consist, in some cases, in the 

 successive production and exuviation of the cells which minister to it, these 

 cells giving up, by rupture or deliquescence, the substances which they have 

 eliminated from the blood ; such, for example, appears to be the mode of 

 separation of the Sebaceous secretion of the skin, of the Mucous secretion of 

 mucous membranes, of the secretion of Milk, and perhaps also of the Biliary 

 secretion. On the other hand, there can be little question that those more 

 liquid secretions, in which there is either very little solid matter (as is the 

 case with the Cutaneous transpiration and the Lachrymal fluid), or in which 

 the solids, though in larger amount, are in a state of such perfect solution 

 as to be capable of easy transudation (as is the case with the Urine), are not 

 formed in this mode ; since neither are exuviated cells normally found in the 

 secreted fluids, nor do the epithelial cells lining the glandular tubes or 

 follicles present indications of being in a state of continual change. Still, 

 even in these cases, it seems fair to conclude that the selective powers of the 

 gland-cells are employed in drawing from the blood, on one side, the special 

 products which are to be set free by transudatiou on the other. Each group 

 of cells is thus adapted to separate a product of some particular kind, which 

 constitutes its special pabulum ; and the rate of its production seems to de- 

 pend, cceteris paribus, upon the amount of that pabulum supplied by the 

 circulating fluid. The substances at the expense of which the secreting cells 

 grow, however, may not be precisely those which are subsequently cast forth ; 

 for it is very probable that some of them at least undergo a certain degree 



