270 DIGESTION. 



maxillary glands are remarkably developed, while the 

 parotids are not of unusual size. 



Beyond these, its mechanical purposes, saliva performs 

 (4) some 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 

 1 00, 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 farinaceous food (especially after cooking) in 

 the stomach; and it is reasonable to refer them to the 

 action of the saliva, because the acid of the gastric fluid 

 tends to retard or prevent, rather than favour the trans- 

 formation 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 absorption. 



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 

 in the process of malting, and excites molecular changes 



