CH. XXXII.] 



THE SECRETION OF SALIVA 



501 



into the duct, it may reach a height which is greater than the 

 pressure of the arterial blood ; this is an experiment which disposes 

 of the idea that the sole source of the salivary flow is the blood- 

 pressure, for there is in addition to this the pumping action of the 

 secretory cells. Although to aid the imagination we roughly compare 

 the cell to a purnp, we have no real knowledge of its mechanism. 



-- Arteriole. 

 '"Circular Muscle Fibres. 



Capillary. 



rV- Lymph Space. 



I 



if Gland Cells. 



"Lumen. 



-Venule 



FIG. 352. Diagram of a secreting acinus. 



Whatever may be the actual mechanism, we are nevertheless quite 

 sure that the cells really do active work, and the proofs of this are 

 the following : 



1. Increase of work finds its expression in increase of combustion. 

 By examining the gases of the blood leaving the gland, it is found 

 that the amounts of oxygen used and carbonic acid produced are 

 increased at least fourfold when saliva is being actively produced ; 

 for instance, when the submaxillary gland is thrown into action by 

 stimulation of its secretory nerve (chorda tympani). 



2. It may be proved mathematically that, whenever any solution 

 of electrolytes is divided into two solutions each differing from the 

 original, positive work must be done. Salivary secretion is a case in 

 point ; for the arterial blood is changed into venous blood and saliva, 

 neither of which has the same composition as the arterial blood. 



