////' Mnl Til. 



347 



Fig. 155. 



e,i:tains. sparingly, tho lacuiUD and canaliculi which characterise true 

 bone : those placed near the surface have the canaliculi radiating from the 

 side of the lacuna: towards the periodoiital membrane ; and those more 

 deeply placed join with the adjacent dental tubnli. In the thicker portions 

 of the crust a petrosa, the lamelhe and llaversian canals peculiar to bone 

 are also found. As ago advances, tho ccmentum increases in thickness, 

 ami gives rise to those bony growths, or exostoses, so common in the t> < t'n 

 of tho aged ; the pulp cavity also becomes partially filled up by a hard 

 substance, intermediate between dentine and bone (osteo-dentine Owen; 

 uUtry dentine Tomes). It appears to bo formed by a slow conversion 

 of the dental pulp, which shrinks or even disappears Gray.) 



Dentnl juilp. Tho pulp, or papilla, is formed by a 

 tibrillar and nuclear mass that fills the internal dental 

 cavity. It receives blood-vessels and nerves, and is 

 enveloped in a very thin membrane which is entirely 

 composed of several layers of beautiful cylindrical or 

 prismatic cells, the most superficial of which send 

 tibrillar prolongations into the dental tubuli. Towards 

 the base of the papilla, this membrane assumes the 

 texture of connective tissue, and is reflected upwards on 

 the fang of the tooth to line the alveolus, and join the 

 gum at the origin of the crown. 



Gum. The gum is a portion of the buccal mucous 

 membrane surrounding the neck of the tooth, and con- 

 curs in consolidating it in the alveolar cavity. Its 

 structure is that of the membrane to which it belongs, 

 being a thick derruis furnished with papillae and tesselated 

 epithelium. It does not contain any glands. 



Alveolo-flental periosteum. This scarcely differs from 



the ordinary periosteum except in being a little softer. 



It lines the alveolus and covers the eemcntum of the fang. 



DKVKLOPMKXT.- Each tooth is developed in tho 



interior of a closed sac named the dental follicle, and 

 lodged in an excavation in tho maxillary bones. Tho 

 sac presents, according to the species of animal and kind 

 of teeth, numerous variations, which wo cannot stay to 

 consider here; but must confine ourselves mcrelv to a 

 brief sketch of the general and constant characteristics "^^"K^J!^,','!' 



of its organisation. IN1 ,_' 



The dental /<///>//: is constituted by an external en- MUI noosum 

 ping membrane of a cellulo-vascular nature (Fig. i. Cn.wn :.',.', Neck; 



loi'i. A). It shows at bottom the simple or compound , or n.t : i, 



puuilla which at a later period is termed the ./ 



/'iilj> (B); this organ, destined for tho secretion of tho 



dentine, then fills nearly the whole of the follicle. 



In its upper part ;.- oi.s, rved the /;;/</ <,rgan, or germ 

 /</ ini'in\trnin'\, formed by a prolongation of the 



gingiviil epithelium, and connected with the latter by a 



small ma-- of cells named the gubentOCtdum tlrnti*. 



Most frequently there is, opposite the bottom of the 



follicle. ..in- or more papilhe which, in some cases, adhere by their 



\\hole length to one of the lateral walls of the follicular sac. and tl. 



\tivmitie- of which cross those of the <li niim papilla-, or are buried in 



' 



<>|.<'!lill_' |,y wllicll 



the vessels and 



numiiiir.it.- 



with tin- pulp; 6, 6, 



-ii.'WMi'.; 



lil.i-.iii-> .-trui-tun- ; 

 i.uni-1 ; 8, 8, 

 ('.-in. nt. 



