SyVLIVAUV APPARATUS. 175 



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 sur- 

 faces 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 quantity, just at the time when the food 

 is undergoing mastication. 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 pas- 

 sage 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 

 vegetable materials, requiring more complete 

 maceration than those which feed on flesh, the 

 salivary glands are of large size : they are parti- 

 cularly large in the Rodentia, which feed on the 

 hardest materials, requiring the most complete 

 trituration ; 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 mas- 

 ticate their food, the salivary glands are compa- 

 ratively of small size ; the exceptions to this rule 

 occurring chiefly in those tribes which feed on 



