by the muscles moving the jaw. When the mouth is closed, the fangs lie on the gums; but in opening the mouth they are brought into a striking position. The muscles also press at the same time upon the glands which secrete the venom, and force the latter through a deep channel or canal in the fang.
Some birds of ancient times had true teeth placed in sockets in the jaws; but all modern birds depend wholly on the gizzard for mastication. This is literally a mill. It is formed of powerful muscles, has a horny lining, and pulverizes hard grain and indeed almost any substance by rubbing between the tough walls. To assist the grinding, the grain-eaters swallow gravel, bits of broken glass and crockery, metals, etc., which are pulverized in turn. One would suppose such
Fig. 8.—Dinotherium Giganteum.
diet would injure the organ; but the tough lining yields without being cut. In birds the gizzard follows the stomach; that is to say, the food is not ground until after it is subjected to the gastric juice. This seems a reversal of the proper order, but the hard grain is more easily pulverized by rubbing after being softened by the solvent fluid. In other animals the gizzard, if a distinct cavity, usually precedes the true stomach. The power of this mill is proportional to the resistance of the food. Thus in flesh-eating birds the gizzard is weak.
Mastication is best exhibited in mammals and is almost entirely by means of teeth. Mammalian teeth are of three kinds: incisors or front cutting teeth; canines, which characterize flesh-eaters; and molars or masticators. They are placed in sockets in the bones, but always in a single row on the outer edge of the jaws, and are never renewed more than once.
The extreme numerical variation of mammalian teeth is found rather strangely in the same group, the order of whales. A river-dolphin of South America has the greatest number, two hundred and twenty-two; while the whalebone whale has no developed teeth, they being replaced with baleen-plates. The narwhal, or sea-unicorn, has, of two
