128 THE VITAL FUNCTIONS. 



tain proportion of fluid. Hence, all animals that masticate 

 their food are provided with organs which secrete a fluid, 

 called the Saliva, and which pour this fluid into the mouth 

 as near as possible to the grinding surfaces of the teeth. 

 These organs are glands, placed in such a situation as to be 

 compressed by the action of the muscles which move the 

 jaw, and to pour out the fluid they secrete in greatest quan- 

 tity, just at the time when the food is undergoing masti- 

 cation. Saliva contains a large quantity of water, together 

 with some salts and a little animal matter. Its use is not 

 only to soften the food, but also to lubricate the passage 

 through which it is to be conveyed into the stomach; and 

 the quantity secreted has always a relation to the nature of 

 the food, the degree of mastication it requires, and the mode 

 in which it is swallowed. In animals which subsist on ve- 

 getable materials, requiring more complete maceration than 

 those which feed on flesh, the salivary glands are of large 

 size: they are particularly large in the Rodentia, which feed 

 on the hardest materials, requiring the most complete tritu- 

 ration; and in these animals we find that the largest quantity 

 of saliva is poured out opposite to the incisor teeth, which 

 are those principally employed in this kind of mastication. 

 In Birds and Reptiles, which can hardly be said to masti- 

 cate their food, the salivary glands are comparatively of 

 small size; the exceptions to this rule occurring chiefly in 

 those tribes which feed on vegetables, for in these the glands 

 are more considerable.* In Fishes there is no structure of 

 this kind provided, there being no mastication performed: 

 and the same observation applies to the Cetacea. In the 

 cephalopodous and gasteropodous Mollusca, we find a sali- 

 vary apparatus of considerable size: Insects, and the *flnne- 

 lida$ also, generally present us with organs which appear 

 to perform a similar office. 



* The large salivary gland in the woodpecker, is seen at s, Fig. 271, page 

 99. 



f The bunch of filaments, seen at s, Fig. 260 (p. 78) are the salivary or- 

 gans of the leech. 



