26 ZOOLOGY. 



central cavity alone indicates the place of this organ. But 

 when the bulb does not exhibit this arrangement ; when it is 

 not pedunculated, and when the tooth forms only on the upper 

 surface, the growth of the tooth ceases not, and no central 

 cavity is found in its interior. 



Fig. I4.t 



The large teeth found in the front of the mouth of the 

 rabbit (Fig. 12) offer an example of this kind of dentition : 

 and if their length does not constantly increase, it is because 

 they are worn down by trituration on their cutting edge in 

 proportion as they grow at the base. 



50. Teeth are composed of various structures. The 

 substance forming the greater part of the tooth, underneath, 

 is called the ivory or dentine. The dense covering of the 

 corona is called enamel: A third substance is occasionally 

 found towards the extremity of the roots, or even enveloping 

 the enamel and corona (as in the ox), to which the name of 

 cement or cortical substance has been given. 



The ivory of the tooth is composed of an animal matter 

 analogous to gelatine, of phosphate of lime (in the propor- 

 tion of about 64 to 100 in the adult human tooth), of car- 

 bonate of lime (amounting nearly to T ^ parts), and of a 

 small quantity of phosphate of magnesia. The enamel, which 

 differs somewhat in colour from the dentine or ivory, and 

 which is hard enough to strike fire with flint, shows on 

 analysis slight traces of an animal substance. The phosphate 



* Section of a dental capsule, a, capsule ; b, bulb or germ ; c, blood- 

 vessels and nerves entering the germ ; d, first rudiments of the ivory of the 

 tooth. 



. t Lower jaw of a very young infant. The outer table of the jaw has been 

 removed to expose the capsules of the teeth enclosed in its interior, a, the 

 gum ; b, lower edge of the jaw ; c, angle of the jaw; d, dental capsules ; e, 

 coronoid process; f, condyle of the jaw. 



