296 MICROSCOPIC ANATOMY OF THE TEETH 



the vessels having completely disappeared from the cartilage 

 in the region of ossification. The subsequent removal of 

 this first-formed bone and the re-deposit of bony matter in 

 the interior of the cartilage are however, not described by 

 them in considering this mode of calcification of the cement. 

 In the radicular cement of human teeth, where the 

 process is similar to that seen in membrane bones, the 

 connective tissue of the follicle with its rich vascular supply 

 invests the forming root ; but a layer of epithelial cells, 

 extending from the epithelial elements in the coronary 

 portion of the follicle, and known as the sheath of Hertwig, 

 extends downwards along the margin of the dentine as far 

 as it is formed, and as described on p. 320 is the form- 

 determining organ of the dentine as first shown by Von 

 Brunn (2) . The dentine is laid down beneath this epithelial 

 sheath, which always intervenes between it and the con- 

 nective tissue of the periodontal membrane during the active, 

 growth of the root. Where no cement has begun to 

 form, as can be seen at the tip of the forming root, the 

 epithelial layer is in contact with the dentine. As soon as 

 the development of the cement commences, bundles of 

 connective tissue and osteoblasts in every respect similar to 

 those of bone pass between the epithelial cells of the sheath 

 .from the surrounding connective tissue of the follicle, 

 separating the epithelial masses composing the sheath from 

 one another, and the fibrous bundles become firmly attached 

 to the dentine. The osteoblasts can be seen in sections to 

 occupy little spaces or divisions between the connective- 

 tissue bundles, and as in bone, some of them become included 

 within the forming tissue, and remain as the lacunal cells 

 of the finished cement (figs. 191 and 193). 



As was first pointed out by Kolliker, the calcareous sub- 

 stance is deposited in little flakes or plates which afterwards 

 coalesce (fig. 194). The connective-tissue bundles become 

 incorporated in the forming cement, and form the Sharpey's 

 fibres as in bone (fig. 193, &c.). 



The osteoblasts are probably modified connective-tissue 

 cells, but they are considered by some authorities to be 

 leucocytes derived from the circulating blood ; they are 

 very abundant and lie in the first-deposited cement 



