CHAP. XV.] 



ALIMENTATION. 



179 



m 



Beginning in the middle line of each jaw and counting from 

 before backwards, there are four incisors, two canines, four 

 bicuspids, and six molars in the upper and in the lower jaw. 

 The incisors have wide sharp edges, and are specially adapted 

 for cutting the food ; the canines, or eye teeth, have a sharp 

 pointed edge, are longer than the incisors, and are specially 

 iiseful for tearing food asunder, 

 or, as in dogs and other car- 

 nivora, for holding prey. The 

 bicuspids, or false grinders, are 

 broader, with two points or cusps 

 on each crown : these teeth have 

 only one fang, the fang, however, 

 being more or less completely 

 divided into two. The molars, 

 or true grinders, have broad 

 crowns with small pointed pro- 

 jections, which make them well 

 fitted for crushing and bruising 

 the food : they each have two or 

 three fangs. The twelve molars 

 do not replace the milk teeth, but 

 are gradually added with the 

 extension of the jaws, the last or 

 hindermost molars not appearing 

 until twenty-one years of age : 

 they are often on this account 

 called " wisdom teeth." 



The teeth are composed of 

 three bone-like tissues, enamel, 

 dentine, and cement; these sub- 

 stances are all harder than bone, 

 enamel being the hardest tissue 

 found in the body. In the inte- 

 rior of each tooth is a cavity, the pulp-cavity, which is filled 

 with a highly vascular and nervous tissue called the dental pulp. 

 The teeth are developed from epithelium in much the same way 

 as the hairs ; for description of which see page 192. 



The pharynx. The pharynx or throat cavity is a musculo- 

 membranous bag, shaped somewhat like a cone, with its broad 



FIG. 113. THE MOUTH, NOSE, AND 

 PHARYNX, WITH THE LABYNX AND 

 COMMENCEMENT OF GULLET, SEEN 

 IN SECTION, a, vertebral column ; 6, 

 gullet; c, trachea; d, larynx; e, epi- 

 glottis ; /, soft palate, between/ and e 

 is the opening at back of cavity or 

 fauces ; g, opening of Eustachian tube ; 

 h, nasal cavity; k, tongue; I, hard 

 palate; m, sphenoid bone at base of 

 skull ; n, roof of nasal cavity; o, p, q, 

 placed in nasal cavity. 



