ROW ANIMALS EAT. 



225 



purpose, the manner is exceedingly various and interesting. The mas- 

 tication of food is usually accomplished by a mill in the mouth. And 

 absence of a masticatory apparatus at the entrance of the food-canal is 

 usually compensated by a mill within the body, as the gizzard of a 

 bird, or by strong chemical solvents as shown in the snake. 



Fig. 2. Section of Tooth of Cat, in situ: 1, enamel; 2, dentine, or ivory; 3, cement, or bone; 

 4, periosteum ; 5, bone of lower jaw; 6, pulp-cavity. 



To soften the food and so assist in its trituration, nearly all ani- 

 mals are provided with saliva. But, as this fluid serves to lubricate the 

 food for swallowing, and also has a chemical power, it may be consid- 

 ered in connection with digestion. 



Those animals which subsist wholly on liquids or on minute par- 

 ticles require, of course, no masticating organs. Of the first class, are 

 most parasites, the butterflies and some humming-birds ; and, of the 

 second, bivalve mollusks and the whalebone whale. The little sea-ur- 



VOL. XYII. 15 



