THE TEETH. 



243 



Cementum. 



of this plexus small terminal branches are given off which termi- 

 nate between the , odontoblasts, or pass through the layer of 

 odontoblasts, to end between these and the dentin (Retzius, 94 ; 

 Huber, 98; Rygge, 1902). Medullated nerve-fibers also terminate 

 in free endings in the peridental membrane. 



Development of the Teeth. In the second month of fetal 

 life the first traces of the teeth are seen in the development of a 

 groove along the inner edge of the fetal jaw, the dentinal or en- 

 amel groove. From the floor of the latter an epithelial ridge 

 is formed constituting the anlage of the enamel organs and 

 known as the dentinal ridge, or enamel ledge. At those points 

 at which the milk-teeth later appear, the enamel ledge develops 

 solid protuberances corre- 

 sponding in number to the 

 temporary teeth. These are 

 known as the dentinal bulbs 

 or enamel germs. In their 

 first stage of development 

 the enamel germs are knob- 

 like, but later their bases 

 spread, and they become 

 flattened and finally cup- 

 shaped by the pushing up 

 into them of connective - 

 tissue projections, the den- 

 tinal papilla. At the same 

 time they gradually sink 

 deeper into the underlying 

 tissue, but still remain con- 

 nected, by means of a thin 

 cord, with the epithelium of 

 the enamel ledge, which now lies on the inner side of the enamel 

 germs. The enamel germs now differentiate into enamel organs. 

 In this stage they consist of an outer layer of columnar epithelial 

 cells, which are to be regarded as a direct continuation of the basal 

 cells from the epithelium of the oral mucous membrane, or still 

 better, of the enamel ledge ; the epithelium in the interior of the 

 organ is derived from the stratum Malpighii of the oral epithe- 

 lium. The cells of this layer, however, undergo a change in 

 shape and structure, in that an increased quantity of lymph-plasma 

 or intercellular substance collects in the interspinous spaces between 

 the cells, pushing the cells apart, and allowing their processes to 

 develop until the cells finally assume a stellate shape. In this way 

 the enamel pulp is gradually formed. The next stage is character- 

 ized by a vertical growth of the dentinal papillae, which soon be- 

 come surrounded on all sides by the cap-like enamel organs. The 

 cylindric cells (enamel cells) of the enamel organs lying next to 



Dentin. 



Fig. 184. Cross-section of human tooth, 

 showing cement and dentin; X 2I2 - At a are 

 seen small interglobular spaces (Tomes' granular 

 layer). 



