THE DIGESTIVE JUICES 



69 



has no action on cellulose ; hence it is inoperative on uncooked 

 starch grains, for in these the cellulose layers are intact. 



Fw. 15. Alveoli of serous gland ; A, loaded befoi'e secretion ; B, after a short period of active 

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



FIG. 16. Mucous cells from a fresh submaxillary gland of dog : a, loaded with mucinogen granules 

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

 the cell ; a' and 6' 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.) 



FK;. 17. Section of part of the human submaxillary gland. (Heidenhain.) To the right is a group 

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



Ptyalin acts best about the temperature of the body (35-40), 

 and in a neutral medium ; a small amount of alkali makes but 



