210 



DIGESTION. 



Saliva, as it commonly flows from the mouth, is mixed with 

 the secretion of the mucous membrane, and often with air- 

 bubbles, which, being retained by its viscidity, make it frothy. 



When obtained from the parotid ducts, and free from mucus, 

 saliva is a transparent watery fluid, the specific gravity of 

 which varies from 1.004 to 1.008, and in which, when examined 

 with the microscope, are found floating a number of minute 

 particles, derived from the secreting ducts and vesicles of the 

 glands. In the impure or mixed saliva are found, besides 

 these particles, numerous epithelial scales separated from the 

 surface of the mucous membrane of the mouth and tongue, 

 and mucus-corpuscles, discharged for the most part from 

 the tonsils, which, when the saliva is collected in a deep 

 vessel, and left at rest, subside in the form of a white opaque 

 matter, leaving the supernatant salivary fluid transparent and 

 colorless, or with a pale bluish-gray tint. In reaction, the 

 saliva, when first secreted, appears to be always alkaline ; and 

 that from the parotid gland is said to be more strongly alka- 

 line than that from the other salivary glands. This alkaline 

 condition is most evident when digestion is going on, and ac- 

 cording to Dr. Wright, the degree of alkalinity of the saliva 

 bears a direct proportion to the acidity of the gastric fluid se- 

 creted at the same time. During fasting, the saliva, although 

 secreted alkaline, shortly becomes neutral , and it does so es- 

 pecially when secreted slowly and allowed to mix with the 

 acid mucus of the mouth, by which its alkaline reaction is 

 neutralized. 



The following analysis of the saliva is by Frerichs : 



Composition of Saliva. 



Water, 

 Solids, 



Ptyalin, . 



Fat, .... 



Epithelium and Mucus, 

 Salts : 



Sulphocyanide of Potassium, 

 Phosphate of Soda, 

 " Lime, 



" Magnesia, 



Chloride of Sodium, 

 u Potassium, 



994.10 

 5.90 



1.41 

 0.07 

 2.13 



2.29 



5.90 



The rate at which saliva is secreted is subject to considerable 

 variation. When the tongue and muscles concerned in mas- 



