CAETILAGE. BONE. TEETH. 39 



ossification here are entirely analogous to those we 

 have observed in the deeper strata of periosteum, 

 both in its healthy state, and when inflamed. In all 

 these cases the osseous cell, in the absence of which 

 the tissue of bone cannot exist, is developed from 

 another cell or its nucleus that is to say from the 

 essential organic element which is always present 

 both in the periosteum, and in the contents of the 

 medullary cavities. 



SECT. III. TEETH. A tooth consists of a central Teeth, 

 portion which constitutes nearly the whole of the 

 organ, and of an outer lamina which accurately invests 

 its external surface. The central mass, or ivory, has 

 a cavity in its interior variable in size and shape, 

 which communicates externally through a narrow 

 canal situated at the extremity of the root; this 

 cavity contains the pulp (PL VIII, fig. I. 1, 2). The 

 portion of the external lamina which invests the 

 crown of the tooth is the enamel (fig. I. 4) ; that 

 which covers its root is the cementum (fig. I. 3). 



The ivory is composed of fundamental substance, ivory, 

 traversed by minute canals. The former, structure- 

 less and transparent when viewed in thin section, is 

 identical with the fundamental substance of bone, and 

 the canals which it contains resemble closely the 

 'canaliculi of bone-cells. They take their origin in 

 the central cavity of the tooth, and thence radiate to 

 the surface of the ivory, where they terminate ; it is 

 an exception to the rule for them to pass beyond this 

 limit and to penetrate the exterior lamina (PL VIII. 

 fig. III. 2). At their commencement some are single, 

 others arise from a common trunk, and others again 



