222 DIGESTIVE SYSTEM. 



velum of the palate, by acting on the corresponding glands, 

 cause secretion of the saliva. 



Beside the chemical effects resulting from the presence of 

 the ptyaline or animal diastase, the principal use of the 

 saliva is found in the mechanical part which it takes in 

 mastication, the solution of sapid substances, the lubrication 

 of the passages which the food must traverse and of the food 

 itself. We shall presently see that the saliva is essential -to 

 deglutition ; for this purpose, it must accompany even those 

 aliments which are not chewed, and upon which it produces 

 no chemical effect. This explains the presence of the salivary 

 glands in the carnivorous animals, in which the saliva is 

 called upon to act on aliments which are essentially nitro- 

 genous. 



Cl. Bernard, perhaps a little exaggerating the mechanical 

 property of the salivary fluid, at the expense of its chemical 

 function, assigns to each saliva a peculiar, corresponding, 

 mechanical function, associating each of them with one of 

 the three physiological phenomena of mastication, deglutition, 

 and gustation. 



The parotid is the masticatory gland : it exists only in 

 animals which have teeth to grind their food ; it is found to 

 be larger as trituration is slower and more difficult; finally, 

 the parotid secretion takes place especially during the move- 

 ments of mastication ; and when the animal chews, first on 

 one side and then on the other, the parotid situated on the 

 masticating side always secretes most abundantly (Colin). 1 



The eub-maxillary secretion belongs only to the phenome- 

 non of gustation : the most certain method of producing this 

 secretion in experiments is to place a sapid substance on the 

 tongue, and thus to excite the reflex action which we have 

 described above ; in comparative anatomy we find that the 

 sub-maxillary gland disappears wherever there is no need of 

 gustation: it is largely developed in the carnivorous ani- 

 mals, while it disappears almost entirely in the granivorous 

 birds. 



Finally, the sub-lingual gland, the secreted product of 

 which is thick, ropy, and similar to that of the different 

 buccal glands, called mucous, in the same way is more par- 

 ticularly connected with deglutition : it serves to unite the 



1 Colin, '* Trait^ de Physiologic comparee des Animaux." 

 2d edition. Paris, 1871, Vol. I. p. 601. 



