AND ON THE LAWS OF ITS FUNCTION. 299 



the secretion is retained till the quantity has reached its proper limit, and till the 

 period has arrived for its discharge. 



Each primary secreting cell is endowed with its own peculiar property, ac- 

 cording to the organ in which it is situated. In the liver it secretes bile in the 

 mamma, milk, &c. 



The primary secreting cells of some glands have merely to separate from the 

 nutritive medium a greater or less number of matters already existing in it. Other 

 primary secreting cells are endowed with the more exalted property of elaborating 

 from the nutritive medium matters which do not exist in it. 



The discovery of the secreting agency of the primitive cell does not remove 

 the principal mystery in which this function has always been involved. One cell 

 secretes bile, another milk ; yet the one cell does not differ more in structure from 

 the other than the lining membrane of the duct of one gland from the lining mem- 

 brane of the duct of another. The general fact, however, that the primitive cell 

 is the ultimate secreting structure, is of great value in physiological science, inas- 

 much as it connects secretion with growth, as phenomena regulated by the same 

 laws. The force, of whatever kind it may be, which enables one primary formative 

 cell to produce nerve and another muscle, by an elaboration within itself of the 

 common materials of nutrition, is identical with that force which enables one pri- 

 mary secreting cell to elaborate bile and another milk. 



Instead of growth being a species of imbibing force, and secretion on the con- 

 trary a repulsive, the one centripetal, the other centrifugal, they are both centri- 

 petal. Even in their later stages the two processes, growth and secretion, do not 

 differ. The primary formative cell, after becoming distended with its peculiar 

 elaborated nutritive matter, in some instances changes its forms and arrangements 

 according to certain laws, and then, after a longer or shorter period, dissolves and 

 disappears in the inter-cellular space in which it is situated, its materials being 

 taken up by the circulating system, if it be an internal, and being merely thrown 

 off if it be an external cell. The primary secreting cell, again, after distention 

 with its secretion, does not change its form so much as certain of the formative 

 cells, but the. subsequent stages are identical with those of the latter. It bursts 

 or dissolves, and throws out its contents either into ducts or gland cavities (both of 

 which, as I shall afterwards shew, are inter-cellular spaces), or from the free sur- 

 face of the body. 



The general fact of every secretion being formed within cells, explains a diffi- 

 culty which has hitherto puzzled physiologists, viz., why a secretion should only 

 be poured out on the free surface of a gland-duct or secreting membrane. 



" Why," says Professor MULLEK, " does not the mucus collect as readily be- 

 tween the coats of the intestine as exude from the inner surface ? Why does not 

 the bile permeate the walls of the biliary ducts, and escape on the surface of the 

 liver, as readily as it forces its way outwards in the course of the ducts ? Why 



VOL. XV. PART II. 4 M 



