SALIVA. 259 



while on- the outside there is a plexus of capillary blood- 

 vessels. The accompanying diagram is intended to show 

 the typical structure of such glands as the salivary (fig. 67). 



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 I'OOS, 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 colourless, or with a pale 

 bluish-grey t'nt. In reaction, the saliva, when first secreted, 

 appears to be always alkaline ; and . that from, the parotid 

 gland is said to be more strongly alkaline than that from 

 the other salivary glands. This alkaline condition is most 

 evident when digestion is going on, and according to 

 Dr. Wright, the degree of alkalinity of the saliva bears a 

 direct proportion to the acidity of the gastric fluid secreted 

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

 secreted alkaline, shortly becomes neutral ; and it does so 

 especially 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 : 



s 2 



