262 DIGESTIOK 



of viscid saliva, the subm axillary glands are remarkably 

 developed, while the parotids are not of unusual size. 



Beyond these, its mechanical purposes, saliva performs 

 (4) a chemical part in the digestion of the food. When 

 saliva, or a portion of a salivary gland, or even a portion 

 of dried ptyalin, is added to starch paste, the starch is very 

 rapidly transformed into dextrin and grape-sugar; and 

 when common raw starch is masticated and mingled with 

 saliva, and kept with it at a temperature of 90 or IOO, 

 the starch-grains are cracked or eroded, and their contents 

 are transformed in the same manner as the starch-paste. 

 Changes similar to these are effected on the starch of fari- 

 naceous food (especially after cooking) in the stomach ; and 

 it is reasonable to refer them to the action of the saliva, be- 

 cause the acid of the gastric fluid tends to retard or prevent, 

 rather than favour the transformation of the starch. It 

 may therefore be held, that one purpose served by the 

 saliva in the digestive process is that of assisting in the 

 transformation of the starch, which enters so largely 

 into the composition of most articles of vegetable food, 

 and which (being naturally insoluble) is converted into 

 soluble dextrin and grape-sugar, and made fit for ab- 

 sorption. 



Besides saliva, many azotized substances, especially if in 

 a state of incipient decomposition, may excite the trans- 

 formation of starch, such as pieces of the mucous mem- 

 brane of the mouth, bladder, rectum, and other parts, 

 various animal and vegetable tissues, and even morbid 

 products ; but the gastric fluid will not produce the same 

 effect. The transformation in question is effected much 

 more rapidly by saliva, however, than by any of the other 

 fluids or substances experimented with, except the pan- 

 creatic secretion, which, as will be presently shown, is very 

 analogous to saliva. The actual process by which these 

 changes are effected is still obscure. Probably the azotized 

 substance, ptyalin, acts as a kind of ferment, like diastase 



