purpose, the manner is exceedingly various and interesting. The mastication 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 animals are provided with saliva. But, as this fluid serves to lubricate the food for swallowing, and also has a chemical power, it may be considered in connection with digestion.
Those animals which subsist wholly on liquids or on minute particles require, of course, no masticating organs. Of the first class, are most parasites, the butterflies and some hummingbirds; and, of the second, bivalve mollusks and the whalebone whale. The little sea-ur-
