212 DIGESTION. 



mucoid secretion of the snbmaxillary, palatine, and toiisillitic 

 glands is spread over the surface of the softened mass, to enable 

 it to slide more easily through the fauces and oesophagus. 

 This view obtains confirmation from the interesting fact pointed 

 out by Professor Owen, that in the great ant-eater, whose enor- 

 mously elongated tongue is kept moist by a large quantity of 

 viscid saliva, the submaxillary glands are remarkably devel- 

 oped, 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 100, 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 gastic fluid tends to retard 

 or prevent, rather than favor 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 transforma- 

 tion 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 transformation 

 of starch, such as pieces of the mucous membrane 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 

 pancreatic 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 in the starch which 

 result in its transformation, first into dextrin and then into 

 sugar. 



The majority of observers agree that the transformation of 

 starch into sugar ceases on the entrance of the food into the 

 stomach, or on the addition of gastric fluid to it in a test-tube : 

 while others maintain that it still goes on. Probably all are 



