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THE POPULAR SCIENCE MONTHLY. 



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 mastica- 

 tion. This is literally a mill. It is formed of powerful muscles, has 

 a horny lining, and pulverizes hard grain and indeed almost any sub- 

 stance 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. Dinothekium 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 rath- 

 er 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 



