830 DR W. G. AITCHISON ROBERTSON ON THE RELATION OF 



(1) By the gradual dilatation of the "formative ring" of cells at the base of the 

 dentine it is increased in diameter. 



(2) It is increased in length by the addition of new dentine at the base of the tooth 

 and the consequent elevation of the whole tooth. This also is due to the action of the 

 formative ring. 



These two processes go on simultaneously, and so the base of the tooth is always 

 growing larger while the tooth is growing in length. This outward extension of the basal 

 formative ring of odontoblasts goes on till a maximum is reached. This broadest part of 

 the pulp in the growing tooth of the kit-ten is at 'the base, while in the adult cat it re- 

 mains about the middle of the tooth. Thus in the newly-born kitten the broadest 

 diameter of the pulp cavity was at the base of the conical tooth, and measured 0*056 inch. 

 In the kitten one month old the basal diameter of the pulp was still the greatest, the 

 tooth still being conical, and measured 0"074 inch. It had not yet become flask-shaped, 

 but about this time the pulp cavity attains its greatest breadth and afterwards diminishes. 

 The elongation of the tooth still continues, but the formative ring now gradually con- 

 tracts, and thus forms an inverted basal cone and so leads to the production of the flask. 

 The narrowing of this basal ring continues until in the adult it becomes a small ring- 

 surrounding the vessels and nerves going to the pulp. The eloDgation of the tooth has 

 also caused its broadest part to be situated about midway between crown and base. 

 Thus the tooth is made up of two cones joined at their bases, the "crowD-cone" being 

 formed by a dilatation of the "formative ring" and the "fang-cone" by the gradual 

 narrowing of the ring. 



(3) During the whole time that the tooth is growing in length, a constant deposition of 

 new dentine is taking place on the inner surface of the old. Thus the maximum diameter 

 of the pulp cavity in the young tooth becomes lessened, till, in the adult, the original 

 pulp cavity is much reduced in size compared with its width in the newly-born kitten. 

 Having reached this stage the processes of growth cease, and thus we have a typical flask- 

 shaped tooth produced. We see now how the apparent anomaly regarding the width of 

 the pulp cavity arises. From the table we find that the width of this cavity is less in 

 the adult tooth than it is in the new-born kitten. This is due to the large deposit of new 

 dentine on the inner surface of the old causing such a narrowing of the pulp cavity that 

 the above condition is produced. 



(4) It is also shown that there has been an interstitial change. The dentinal tubules 

 are smaller and closer together near the crown of the adult tooth than near the base. At 

 the base the amount of intertubular dentine remains the same as it is in the younger 

 cat's tooth, though the tubules themselves are a good deal larger in diameter than in the 

 earlier conditions. 



Regarding fang-formation, we have seen how a single fanged tooth, as the canine, is 

 developed by the gradual narrowing of the basal dentine-forming ring (Fig. 13). If, 

 however, this formative ring, having reached its maximum dilatation, becomes con- 

 stricted at two opposite points till these meet like a figure of eight, then two smaller 



