500 



SALIYA 



[CH. XXXII. 



ally, but it has never been satisfactorily separated chemically from 

 ptyalin. 



After secretion, due to the administration of food or of such a 

 drug as pilocarpine, the cells shrink, they stain more readily, their 



FIG. 351. Alveoli of parotid gland. A, before secretion ; B, in the first stage of secretion ; C, after 

 prolonged secretion. (Langley.) 



nuclei become more conspicuous, and the outer part of each cell becomes 

 clear and free from granules (fig. 351). 



The Secretion of Saliva. 



The process of secretion consists of a number of events which 

 may be divided into two categories : 



1. The transference of water and certain substances dissolved 



in the water from the blood of the surrounding capillaries 

 to the lumen of the acinus. 



2. The modification of the chemical composition of this solu- 



tion either by the addition to it of substances manu- 

 factured by the gland-cells, or by the prevention of 

 substances in the lymph from traversing the gland-cell 

 and reaching the lumen. 



In regard to the first of these, we must regard each secreting cell 

 as an organ which pumps the water through its own substance from 

 the lymph space to the lumen. The arrows in the accompanying 

 figure (fig. 352) show the direction in which water passes. 



The result of this pumping action is fco increase the pressure in 

 the lumen, and diminish that in the lymph space. The reduction of 

 pressure in the lymph space causes water to flow into it through the 

 capillary wall from the blood, and this filtration is further helped by 

 a rise of pressure within the capillary, for during secretion the 

 muscular fibres encircling the arteriole wall relax, and thus the 

 arterial pressure is more fully communicated to the capillary. 



The extent to which the pressure in the lumen rises depends 

 upon the ease with which the secreted fluid can pass away along 

 the duct, and if the saliva is made to run up a vertical tube inserted 



