324 THE SOLIDS. 



According to Mr. Tomes also numerous spaces exist be- 

 tween the fibres in newly -formed enamel ; and it is owing to 

 the presence of these that young enamel owes its opacity and 

 brittleness. 



We thus perceive that the enamel is to be regarded rather 

 as a modification of the epithelium than of bone. 



The development of both dentine and enamel may be well 

 studied upon the teeth of young pigs, or kittens at the birth. 



Formation of Cementum. The cementum pulp is formed 

 between the external surface of the dentine and the internal 

 of the sac of the tooth, it being intimately united to both. 



It consists, like the pulps of the other tissues of the teeth, 

 of nucleated cells embedded in a granular matrix : these cells 

 are described by Mr. Tomes * as resembling those of tempo- 

 rary cartilage, being oval in shape and having their long 

 axes placed transversely, and at right angles to the length of 

 the tooth. 



The cells nearest the surface of the dentine are the first 

 to become ossified ; and when their ossification and develop- 

 ment is completed, they form the stellate or bone cells of the 

 cementum. Some consider that the nuclei of these cells alone 

 give origin to the bone cells, and appearances may be observed, 

 even in adult cementum, which are favourable to this opinion. 



The cementum is often seen to be traversed by fibres 

 derived from the outer layer of the membrane of the tooth- 

 sac, as well as by tubes prolonged into it from the dentine. 



Notice has already been taken of the small hexagonal cells 

 contained in the cementum, and situated principally upon 

 the outer surface of the dentine, and a doubt was expressed 

 whether these were to be regarded as forming a part of the 

 structure of the cementum, or whether they constituted a 

 distinct organisation. 



The cementum is particularly liable to an increased and 

 abnormal development constituting exostosis. 



It would thus appear, on the one hand, that the cementum 

 and dentine are but modifications of each other, and also of 



* See Lecture V. 



