DENTAL TISSUES. 995 



thickened basement-membrane, and by a layer of cells belonging to the corium 

 outside it. The outer epidermic cells form the root-sheaths, and the central 

 ones, resting upon a little vascular papilla, develop into the hair. This in- 

 creases in diameter and length, and then pierces the cuticle, beneath which it 

 is really formed, sometimes by its point, and sometimes in a bent position. 

 The first hairs are very fine, and form the down or lanugo. All new hairs, 

 when old ones are to be shed, commence by a cluster of epidermic cells, 

 formed at the bottom of the hair-follicle, upon the side of the old papilla ; as 

 these grow, they detach the old and falling hairs. 



Dental Tissues. 



The dentine is a dermoid bone, formed by the gradual transformation and 

 ossification of the superficial portion of the dental papillae or pulps, and not 

 by a mere excretion or deposit on their surface. The pulp is chiefly composed 

 of rounded nucleated cells, in a clear matrix, but contains also a few areo- 

 lar fibres, and many bloodvessels. The outer cells become lengthened, like 

 columnar epithelial cells. By some it is thought that a single layer of these 

 cells may, by elongations and other modifications, develop into the whole 

 length of a dentinal tubule. It is more commonly supposed that successive 

 layers of pulp-cells are developed, coalesce with each other, undergo meta- 

 morphosis, arid become ossified, in order to complete a tubule. Lastly, it has 

 been suggested, that rows of secondary cells developed within one primary 

 cell, and subsequently coalescing, are so transformed. There are differences 

 of statement as to the mode in which this occurs. The nuclei of these sec- 

 ondary cells are supposed to coalesce, and, remaining hollow, to form the 

 dentinal tubuli. All the other parts of the cells and of the intermediate ma- 

 trix become calcified, and constitute the walls of the tubuli and the intertubu- 

 lar dentinal substance. The fine bifurcated ends of the tubuli are formed 

 by branching and anastomosing processes of the cells. Upon the surface of 

 the growing dentine, next to the enamel, is seen a fine basement-membrane, 

 named the preformative membrane; it is supposed to be the seat of commenc- 

 ing calcification, to be very early converted into the more compact superficial 

 dentine, and to assist in connecting this with the enamel. 



The enamel-organ (p. 970), consists of a soft pellucid tissue, entirely epi- 

 thelial in its nature. It is composed, on its inner or deeper aspect, of a layer 

 of columnar epithelial cells, which are applied to the preformative membrane 

 of the dentine. Outside these is a thick stratum, composed of stellate cells, 

 forming a network of fibres, inclosing multangular areolse, filled with trans- 

 parent substance, and having brilliant spots at the junctions of the fibres. Its 

 outer part consists of epithelial cells, arranged in masses, between projections 

 of the inclosing vascular mucous membrane. These masses are sometimes 

 so large and prominent as to appear like white bodies beneath the gum, and 

 have been erroneously regarded as glands the dental glands. (Serres.) It is 

 usually stated that the cells of the enamel-organ become elongated and calci- 

 fied, with gradual absorption of their animal substance at first forming a 

 soft cretaceous mass, but afterwards becoming hard, and being firmly fixed to 

 the surface of the preformative membrane. (ISchwann, Kolliker, and others.) 

 Their nuclei disappear, or leave only a fine linear trace. It has been supposed 

 that the enamel-cells are developed beneath the preformative membrane (Hux- 

 ley) ; but this view is not generally entertained. It is variously imagined 

 that a single prismatic cell serves to form a single enamel prism, running 

 through the whole thickness of that structure ; or that several secondary cells 

 combine to form each prism. As the enamel-organ terminates at the cervix 

 of the tooth, the formation of enamel is limited to the crown. The crusta 

 petrosa is developed upon the fang, probably by mtra-membranous ossifica- 

 tion. 



