TEETH. DEVELOPMENT OF ENAMEL. 



289 



Fig. 81. 



surface of the dentinal papilla, the cells multiply, and are developed into the form 

 of a prismatic epithelium (Fig. 81, j) ; but above this, the cells assume the stel- 

 late form, and their radiating prolongations coa- 

 lesce, so that a very curious tissue is formed, which, 

 though not uncommon in Plants (as, for instance, 

 in the Rush), is very unusual in animals. 1 In the 

 early stage of its development, the enamel-pulp is 

 traversed throughout by bloodvessels, which are 

 prolonged into it from the inner lining of the cap- 

 sule; but these gradually retreat, and when the 

 calcification of the enamel-matrix is going on, they 

 have entirely withdrawn themselves from the pulp, 

 the membranous lining of the capsule, however, 

 being itself highly vascular and somewhat villous. 

 One purpose of the stellate tissue would seem to 

 be, to afford a space for the columnar tissue and 

 the dentinal pulp to extend themselves into with- 

 out resistance, and to serve as a protection to these 

 structures during their growth ; but it may also 

 select from the nutrient materials supplied by the 

 blood, those which the cells of the enamel-matrix 

 require, and may prepare it for being finally appro- 

 priated by them. These cells gradually fill them- 

 selves with calcareous salts, which would seem to 

 be deposited in them in a purely crystalline condi- 

 tion, and not to be conjoined (as in Bone and Den- 

 tine) with organic matter; for the small proportion 

 of the latter, which chemical analysis shows to exist 

 in Enamel ( 27^), is probably employed wholly 

 in forming the walls of the prismatic cells, which 

 themselves become penetrated by the consolidating 

 substance. That so large a proportion of the calci- 

 fying material of Enamel consists of the phosphate 

 of lime, is evidently the cause of its extraordinary 

 hardness ( 76). The calcification begins on the 

 surface of the dentinal papilla, which is excavated 

 into shallow cups that receive the ends of the ena- 

 mel columns; and these columnar cells are at first 

 so short, as to constitute but a very thin layer. 

 As calcification takes place at their bases, however, 

 their apices lengthen; and this either by the for- 

 mation, addition, and coalescence, of new cells fur- 

 nished by the enamel-pulp, or else by the subdivi- 

 sion of the prismatic cells at their extremities (just 

 as the increase in length of a Conferva is usually 

 accomplished by the repeated fission of the terminal cell), and the elongation of 

 the new cells thus formed, the process being continually repeated. In either 

 case, it seems that the entire length of each enamel-prism is made up by the 



1 This curious tissue was first described by Messrs. Todd and Bowman, in their "Phy- 

 siological Anatomy," p. 533, Am. Ed. ; these authors, however, do not speak of it as com- 

 posed of stellate cells, but describe it as consisting of a mesh of short fibres, meeting in 

 numberless points, at each of which a transparent clear nucleus is visible. The Author 

 agrees with Mr. Tomes in the interpretation he has given to it ; more especially since, as 

 Mr. Tomes remarks, the large floating cells of parts of the pulp in which this tissue does 

 not yet exist, present indications of an approach towards the stellate form. 



19 



Formation of Enamel: h, primary 

 cells suspended in fluid blastema g ; 

 t, the same more fully developed and 

 become angular ;j, the same becoming 

 prismatic; k, the nucleus disappear- 

 ing; I, the modified prismatic cells, 

 filled with calcareous salts, forming 

 the fibres of enamel. 



