SECRETION 121 



is examined after a period of rest the cells are found to be 

 filled with granules. After secretion these granules are 

 much reduced in size and in number, those which are 

 present occupying only the part of the cell nearest the 

 lumen. The cell itself, instead of being distended as in 

 the resting phase, has undergone shrinkage. Secretion, 

 then, consists histologically of a breaking up of granules. 

 Sometimes these are themselves discharged from the 

 gland, but this is not usually the case, the secretion being 

 generally quite clear. The granules are regarded by some 

 authorities as constituting a store-house for the secretion, 

 by others as constituting not only the store-house but also 

 the seat of formation. 



The act of secretion is accompanied by dilatation of 

 blood-vessels. Vaso -dilatation, however, is not the cause 

 of secretion, for at the onset of secretion there is usually 

 a transient diminution in the volume of the gland. More- 

 over, vaso-dilatation may be unaccompanied by secretion, - 

 as when the latter process is abohshed by drugs such as 

 atropine. The dilatation of blood-vessels seems to be due 

 to two factors : a direct effect of the stimulating agent, 

 be it nerve or hormone, upon the blood-vessels, and an 

 indirect effect due to the chemical products of secretory 

 activity. 



In some cases, as in the salivary glands, secretion is 

 brought about by a reflex nervous action, in others, as 

 in the pancreas, by a hormone or chemical substance 

 elaborated elsewhere. 



In the process of secretion there is, besides the formation 

 in the gland of the specific constituent of the fluid secreted, 

 a constant passage of water and other substances, from 

 the blood to the cell and from the cell to the lumen. This 

 cannot be due to filtration, for the secretion pressure in 

 the duct may be greater than the blood-pressure within 

 the gland. Attempts have been made to explain it by 

 osmosis. It is held that the first change in the cell is a 

 breakdown of molecules. This causes a rise in osmotic 



