128 



THE ESSENTIALS OF HISTOLOGY 



course of certain lines or clefts which are seen traversing die dentine 

 across the direction of the tubules (incremental lines, fig. 150, shown 

 magnified in fig. 154). 



FlG. 154. A SMALL PORTION OK THE DENTINE WITH INTERG1.OBULAH SPACES. 



(350 diameters.) 



c, portion of incremental line formed by the interglobular spaces, which are, here filled up 

 by a transparent material. 



The pulp consists of a soft, somewhat jelly-like, connective tissue, 

 containing many branched cells, a network of blood-vessels, and some 

 nerve fibres which pass into the pulp-cavity along with the blood- 

 vessels by a minute canal at the apex of the fang. The superficial 

 cells of the pulp form an almost continuous layer, like an epithelium. 

 They are known as odontoblasts , from having been concerned in the 

 formation of the dentine. 



The crusta petrosa (fig. 152, i) is a layer of lamellated bone in- 

 cluding lacunae and canaliculi, but without Haversian canals, at least 

 normally in the human teeth. It is covered with periosteum (dental 

 periosteum], which also lines the socket, and serves to fix the tooth 

 securely. 



Formation of the teeth. The teeth are developed in the same 

 manner as the hairs. A thickening of the epithelium occurs along the 

 line of the gums, and grows into the corium of the mucous membrane 

 (common enamel-germ, fig. 155, A). At regular intervals there is yet a 

 further thickening and growth from the common enamel-germ into the 

 tissue of the mucous membrane, each of these special rudiments swelling 

 out below into a flask-shaped mass of cells, the special enamel-germ, 

 fig. 155, B). A vascular papilla grows up from the corium into the 

 bottom of the special enamel-germ (fig. 155, C, D) ; this papilla has 

 the shape of the crown of the future tooth. Each special enamel- 

 germ, with its included papilla, presently becomes cut off from the 

 epithelium of the mouth, and surrounded by a vascular membrane 

 the dental sac. The papilla becomes transformed into the dentine of 

 the future tooth, and the enamel is deposited upon its surface by the 

 epithelial cells of the enamel-germ. The root of the tooth, with its 

 covering of cement, is formed at a later period, when the tooth is 



