OF NUTRITION. 



cases, the production of the new structure takes place most readily in the parts 

 that are in apposition with the old ; but we are not warranted in saying that 

 the presence of the latter is essential. The different membranous structures 

 belonging to the Bone contribute to its regeneration ; probably in proportion 

 to their vascularity. The Bone does not recover its perfect structure for a 

 long period after the Callus has become firm ; the latter at first commonly fills 

 up the medullary canal; but after a time, cancelli are formed in it by intersti- 

 tial absorption, and the canal becomes again continuous by their coalescence, 

 as in the first growth of the Bone.* 



I 633. The Teeth are nearly allied to Bone in structure ; and in some of the 

 lower Vertebrata, there is no separation between the Bone of the jaw, and the 

 Teeth projecting from it. In Man and the higher animals, however, there is 

 an obvious difference both in their structure and in their mode of develop- 

 ment. These subjects have lately received much attention ; and the practical 

 importance of an acquaintance with them renders it desirable that they should 

 be here treated somewhat fully. The Teeth of Man, and of most of the higher 

 animals, are composed of three very different substances ; Dentine or Ivory, 

 Enamel, and Cementum or Crusta Petrosa. These are disposed in various 

 methods, according to the purpose Avhich the Tooth is/ to serve : in Man, the 

 whole of the crown of the tooth is covered with Enamel ; its root or fang is 

 covered with Crusta Petrosa ; whilst the substance or body of the tooth is com- 

 posed of Dentine. In the molar Teett of many Herbivorous animals, however, 

 the Enamel and Cementum form vertical plates, which alternate with plates of 

 Dentine, and present their edges at the grinding surface of the tooth ; and the 



[Fig. 119. 



[Fig. 120. 



A view of an Incisor and of a Molar Tooth, 

 given by a longitudinal section, and showing that 

 the enamel is striated and that the stria? are all 

 turned to the centre ; the internal structure is also 

 seen; 1, the enamel; 2, the ivory; 3, the cavitas 



pui;. * 



A vertical section of an adult Bicuspid, cut from 

 without inwards magnified 4 times; 1, 1, the cor- 

 tical substance which surrounds the root up to the 

 commencement of the enamel ; 2, 2, the ivory of 

 the tooth, in which are seen the greater parallel 

 curvatures, as wel] as the position of the main 

 tubes ; 3, apex of the tooth, where the tubes are 

 almost perpendicular ; 4, 4, the enamel ; 5, the ca- 

 vity of the pulp, in which are seen, by means of 

 the glass, the openings of the tubes of the dental 

 bone.] 



* A very good account of the different opinions which have been entertained in regard 

 to the reparation of Bone, and of the facts that may be regarded as established, is given 

 by Mr. Gulliver in the Edinb. Med. and Surg. Journal, vol. xliv. p. 42. 



