46 



ESSENTIALS OF CHEMICAL PHYSIOLOGY 



in the food, and in virtue of its mucin lubricating the bolus of food 

 to facilitate swallowing. 



Via. 12.— Alveoli of serous gland : A, loaded before secretion; B, after a short period of active 

 secretion ; C, after a prolonged period. (Langley.) 



Tia. 13.— Mucous cells from a fresh submaxillary gland of dog : a, loaded with mucinogen granuli^s 

 before secretion : 6, after secretion : the granules are fewer, especially at the attached border ol 

 the cell : a' and b' represent cells in a loaded and discharged condition respectively which have 

 been irrigated with water or dilute acid. The mucous granules are swollen into a transparent 

 mass of mucin traversed by a network of protoplasmic cell-substance. (Foster, after Langley.) 



Fill. 14.- -Section of part of the human submaxillary gland (Heidonhain.) To the right is a grn\]\> 

 of mucous alveoli, to the left a group of serous alveoli. 



The chemical action of saliva is due to its active principle, 

 ptyalin. This substance belongs to the class of unorganised ferments 



