74 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION 



as digestion is concerned; for the secretions from the three pairs 

 of glands become mixed in the mouth, and it is their combined 

 effect which, in any particular case, is observed. Saliva con- 

 tains in 1,000 parts about 994 of water, the remaining six parts 

 being organic and inorganic solids. 



These solids are chiefly mucin, ptyalin, albumin and salts. 

 The salts are mainly the chlorides of sodium and potassium, the 

 sulphates of potassium, the phosphates of potassium, sodium, 

 calcium and magnesium, and sulphocyanide of potassium. The 

 mucin gives the ropy consistence to the fluid and serves a mechan- 

 ical purpose only. The sulphocyanide of potassium is unusual 

 in the body secretions and its presence here is interesting. It 

 may represent an end product of proteid metabolism. The true 

 digestive value of saliva is due to ptyalin, an amylolytic enzyme. 



Were it not for the presence of epithelial cells in suspension, 

 saliva would be clear and transparent. Its reaction is alkaline, 

 its specific gravity is about 1004 to 1008, and the average amount of 

 daily secretion is about 2} pounds. 



The parotid saliva is much more watery and mixes much more 

 readily with the food than the submaxillary and sublingual, 

 which latter is mucilaginous and gives to the bolus a glairy 

 coating. The sublingual saliva is thicker and more viscid than 

 the submaxillary. 



Nerve Supply. The connection of the nervous system with 

 salivary secretion deserves particular attention, since the phe- 

 nomena presented under its influence are typical, and, if not 

 explanatory of occurrences elsewhere in the body, are at least 

 very suggestive. 



Each one of the three glands is supplied with both cerebro- 

 spinal and sympathetic fibers. Each one of them has three 

 kinds of nerve fibers, secretory, vaso-dilator and vaso-constrictor. 

 The secretory and vaso-dilator reach the gland in the cerebro- 

 spinal trunks; the vaso-constrictor in the sympathetic. The 

 vaso-constrictors and vaso-dilators are distributed to the walls 



