DIGESTION. 



27 



matter is formed in the cells during the rest, and discharged into the ducts 

 during the activity of the gland. 



In the mucous glands, e.g., sub-maxillary and sub-lingual, the changes 

 that occur in the cells are somewhat different (Fig. 2). During the inter- 

 vals of digestion, the cells lining the gland are large, clear and highly 

 refractive, and contain a large quantity of mucigen. After secretion has 

 taken place, the cells exhibit a marked change. The mucigen cells have 

 disappeared, and in their place are cells which are small, dark and com- 

 posed of protoplasm. It would appear that the cells, during rest, elabor- 

 ate the mucigen which is discharged into the tubules during secretory 

 activity, to become part of the secretion. 



FIG. 2. 



SECTION OF A " MUCOUS " GLAND. 



A. In a state of rest. B. After it has been for some time actively secreting. After 



LM vdowsky . 



Saliva is an opalescent, slightly viscid, alkaline fluid, having a specific 

 gravity of 1.005. Microscopical examination reveals the presence of 

 salivary corpuscles and epithelial cells. Chemically it is composed of 

 water, proteid matter, a ferment (ptyalin) and inorganic salts. The amount 

 secreted in 24 hours is about 2^ Ibs. Its function is twofold : 



1. Physical. Softens and moistens the food, glues it together, and 

 facilitates swallowing. 



2. Chemical. Converts starch into grape stigar. This action is due 

 to the presence of the organic ferment, ptyalin. Ptyalin is an amorphous 

 nitrogenized substance, which can be precipitated from the saliva by calcium 

 phosphate. Its power of converting starch into grape sugar is manifested 

 most decidedly at the temperature of the living body and in a slightly 



