166 DIGESTION. [CHAP.XXIII. 



the inner surface of the digestive tube, fitted for comminuting the 

 food previously to its being acted on by the gastric juice. In the 

 higher classes they are of an osseous nature and fixed to bone, though 

 not originally. Among the invertebrata, the echini are remarkable 

 for their calcareous oral teeth, five in number. The mouth of the 

 leech is armed with three serrated teeth, worked by muscles, which 

 saw their way into the skin. In some insects the gizzard is armed 

 with a very complicated system of horny teeth. In the stomach of 

 the Crustacea is a cartilaginous framework, with projecting teeth, 

 moved by muscles, and capable of very powerful masticatory ac- 

 tions. Among the gasteropods, the bullse have three stomach 

 teeth ; and others, as the Aplysia, a great multitude, of large size 

 and of different forms. 



True osseous teeth are found only in three classes of vertebrata, 

 viz. mammalia, reptiles, and fishes. Some of these, how ever, are with- 

 out them, as, among mammalia, the American ant-eater, the manis, 

 the echidna, the proper whales; among reptiles, the tortoises; and 

 the sturgeon among fishes. The teeth of fishes are fixed not merely 

 to the maxillary bones, but also to the palate and other bones, bound- 

 ing the mouth and throat, and they are often extremely numerous. 

 The bills of tortoises and birds perform some of the functions of teeth; 

 but in those of the latter class which live on hard vegetable sub- 

 stances, the muscular gizzard, with its hard cuticle, and by the help 

 of small angular stones which the instinct of the animals teaches 

 them to swallow with the food, performs the functions of a masti- 

 catory apparatus. Among the mammalia, the teeth are few in num- 

 ber, and limited to a single row in each jaw. 



Teeth may be classed according to their shape and the function 

 they have to perform. Thus, the following varieties may be briefly 

 enumerated : The cutting or gnawing teeth of the rabbit or beaver, 

 the front teeth of man. The conical teeth of fishes for seizing 

 and retaining prey the canine teeth of the lion and dog. The 

 hinder teeth of the carnivora, with several sharp elevations for tear- 

 ing. The more complex crushing teeth of the insectivora, and of 

 the frugivorous monkeys. Lastly, the true grinding teeth of gra- 

 nivorous and graminivorous animals. 



Of the Human Teeth. rThe teeth in the adult human subject are 

 thirty-two in number, of which four are incisors, two canines, four 

 bicuspids, and six large molars, in each jaw. Those of the upper 

 jaw form the larger arch, so as to overlap those of the lower jaw in 

 front, and to overhang them somewhat at the sides, when the mouth 

 is closed. Each tooth has a crown or body, projecting above the gum, 



