74 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION 



Composition and Properties of Saliva. While it is possi- 

 ble to draw certain distinctions between the saliva from the 

 different glands, these distinctions are comparatively unim- 

 portant, so far 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 contains 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, so- 

 dium, calcium and magnesium, and sulphocyanide of potas- 

 sium. The mucin gives the ropy consistence to the fluid and 

 serves a mechanical purpose only. The sulphocyanide of po- 

 tassium is unusual in the body secretions and its presence 

 here is interesting. It may represent an end product of pro- 

 teid metabolism. The true digestive value of saliva is due to 

 ptyalin, an amylolytic enzeme. 



Were it not for the presence of epithelial cells in suspen- 

 sion, 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 2^4 pounds. 



The parotid saliva is much more watery and mixes much 

 more readily with the food than the submaxillary and sub- 

 lingual, 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 

 phenomena 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 cere- 

 bro-spinal and sympathetic fibers. Each one of them has 

 three kinds of nerve fibers, secretory, vaso-dilator and vaso- 



