NERVES TO ODONTOBLASTS, AND ON THE GROWTH OF DENTINE. 327 



are therefore carried up by the rising dentine, for as soon as each has deposited a little 

 dentine at the extreme base of the tooth, it becomes fixed as a permanent odontoblast 

 and is afterwards lifted up. Fresh cells are continually growing below those engaged 

 in the production of dentine, and thus the existence of the " formative ring " is continued. 

 From whence do these new cells arise ? Are they derived from odontoblasts, or are 

 they derived from the connective tissue cells of the pulp ? I am inclined to believe 

 that they arise from the pulp cells. If we trace the layer of odontoblasts downwards, we 

 find that as the dentine becomes thinner so the size of the dentine-forming cells decreases, 

 till at the lower limit of the dentine they are small spindle-shaped cells attached to the 

 dentine by their distal process. Even below the extreme limit of the dentine we can 

 still follow the line of odontoblasts downwards as a layer of fusiform connnective tissue 

 cells gradually becoming smaller till they fade imperceptibly into the pulp tissue. There 

 is no line of demarcation between them and the ordinary small round cells of the pulp. 



The question now is, How are we to explain how the tooth has increased so much in 

 size ? Well, there appears to be four processes all at work at the same time in the 

 growing tooth. These processes are : — 



(1) Increase in length of the tooth by addition of new dentine at the lower end 



of the fang. This addition more than compensates the loss caused by the 

 grinding down of its crown. In adult age, the growth of new dentine and the 

 wearing down balance one another, and the tooth therefore remains of constant 

 length. 



(2) Increase in width of the tooth by the gradual widening of the " formative ring." 



(3) A slight interstitial increase in the dentine, causing the formation of an increased 



amount of matrix between the tubules. This interstitial increase appears only 

 to occur in the very young tooth. 



(4) As the tooth grows, new layers of dentine are deposited on the inner surface of 



the already existing dentine. This deposit is probably due to the influence of 

 odontoblasts, since they are concerned in production of dentine from the 

 beginning. 



As the entire tooth is pushed onwards by the growth of new dentine at its lower end, 

 the crown is continually being worn down in grinding. The upper end of the pulp 

 cavity is very narrow and contracted, owing to the large amount of dentine which has 

 accumulated on its surface, for in this situation the dentine is of oldest date and so is 

 thickest. Unless provision were made to prevent it, the pulp cavity would soon become 

 exposed by reason of the grinding down of the crown. It is here, however, at the upper 

 part of the pulp cavity, that the dentine reaches its maximum thickness, and so reduces 

 the diameter of the pulp cavity that it persists only as a fine channel of considerable 

 length leading from the pulp cavity to the free surface of the tooth (Fig. 8). Osseous 

 tissue is developed in this channel, which, together with many small round cells and 

 capillaries, prevent any direct communication between the surface of the tooth and 



