216 DIGESTIVE SYSTEM. 



are combined in a more complex manner than in any olher 

 animal. 



Beside the action of the jaws in tearing, cutting, and 

 crushing the food, there is also an action of the tongue, lips, 

 and cheeks, which aid mastication by pushing the food be- 

 tween the teeth, and keeping it in place. 



Mastication is a voluntary act, and yet it may be said to 

 belong, in some respects, to the class of reflex actions : thus 

 mastication becomes slow, difficult, and even impossible, 

 when there is an insufficiency of saliva, or when the want of 

 food is not felt. There must, then, be here as everywhere, a 

 special peripheral impression, which, being reflected in the 

 nervous centres (the bulb, in mastication), causes the phe- 

 nomenon of reflex action. Mastication, like walking and 

 many other movements which are, apparently, quite volun- 

 taiy, is performed, in a great measure, and during most of 

 the time, by means of the mechanism of reflex actions. (See 

 page 45, Physiology of the nervous centres : bulb.) 



B. Salivation. 



The organs of salivation are not only the salivary glands 

 properly so called, but the whole glandular system spread 

 throughout the cavity of the mouth: such as the molar 

 glands, or glands of the cheeks, the glands of the lips, 

 those of the under surface of the tongue, those of the roof of 

 the mouth, and those of the velum of the palate, which are 

 improperly called mucous glands. All these glands are 

 formed by masses of globules arranged in ramified tubes, open- 

 ing, sometimes, singly to the outside, and, at others, uniting 

 in a single excretory tube, Steno's duct (parotid), Whartoris 

 duct (sub-maxillary). The saliva is a deliquium, produced 

 by the fusion of the globules of these glands as they fall into 

 decay. 



The salivary juice is found to differ slightly in the different 

 glands, but it has one general feature, that of being very 

 watery, and, in this respect, differs greatly from the mucus; 

 it is water, containing scarcely from one to two per cent of 

 solid matter; its reaction is alltaline: when taken from a 

 person in a fasting condition, it is sometimes found to be 

 (slightly acid, but this acidity is simply owing to decomposi- 

 tion of the food remaining between the teeth. 



The saliva contains an organic nitrogenized (azotic) sub- 

 stance (discovered by Leuchs, 1831) ; it is not well-defined, 

 but is a peculiar form of albuminous substance called ptya- 



