CHAP. XXIII.] DEVELOPMENT OF THE TEETH. 



175 



and lap over, and finally meet and unite, so as to form a lid or oper- 

 culum to the now closed cavity. Thus the epithelium of the lining 

 membrane of the mouth may be shewn to form the lining of the 

 follicle, and to be reflected thence over the surface of the papilla. 



Now, the tooth papilla must be regarded as homologous with, or 

 answering to the tactile or hair papillae of the skin, already de- 

 scribed at a former page ; and it would, therefore, be expected that 

 its main part would consist of a peculiar sub-mucous tissue, covered 

 by a homogeneous basement membrane, and surmounted by a tissue 

 answering to the epithelium ; and this seems actually the case. The 

 substance of the papilla is at first a congeries of granular nuclei, dis- 

 persed irregularly through a firm homogeneous sub-granular matrix, 

 or blastema, in which vessels and nerves are by degrees developed. 

 This is bounded by a definite transparent membrane, on which rests 

 a reflection of the epithelium lining the sac, modified in structure, so 

 as to present a series of columnar nucleated particles, the matrix of 

 the future enamel. It would appear 

 that the lining and reflected layers of 

 the epithelium become blended toge- 

 ther, and constitute but one, which is 

 more adherent to the sac than to the 

 papilla, so that on opening the sac its 

 wall generally seems to be unattached 

 to the surface of the papilla, and the 

 latter to be limited by what we have 

 regarded as the basement membrane. 

 Or, it may be that the epithelium re- 

 flected over the papilla disappears, 

 leaving only that which lines the sac. 



Between the columnar epithelium thus 

 lining the sac, and the surface of the 

 alveolar cavity, that is, apparently in 

 the wall of the sac itself, is now found 

 a thick, semi-transparent, pulpy tissue, 



A . vertical section of the enamei-puip, 



which has been termed the enamel- tac B he * olmnnar epithelium seen endwise> 



about five 



pulp. It presents towards the pulp of 



the tooth (dentinal pulp) a series of ele- 



vations and depressions, precisely the reverse of those of the dentinal 



pulp on which they rest, and answering mutually to these, with 



only the columnar epithelium intervening. The structure of this thick 



pulpy tissue is very beautiful and peculiar, as is seen in the annexed 



woodcut (fig. i 53, A, a) . It consists of a mesh of short fibres, meeting 



