NERVES TO ODONTOBLASTS, AND ON THE GROWTH OF DENTINE. 331 



formative rings are produced. If these both go on forming dentine and diverging from 

 one another, we have two " fang-cones " produced springing from one body and giving us 

 a double fanged tooth. In a similar manner, if the formative ring becomes sub-divided 

 into three or four rings, we have a three or four fanged tooth resulting. The tooth 

 follicles themselves, even of the molar teeth, are quite simple and show no indication of 

 roots. It is only after the body of the tooth has been completed that the roots are 

 produced. 



This inquiry shows that the growth of a tooth is only to a very slight extent inter- 

 stitial. Interstitial growth is seen in the incisor tooth of the rabbit, where the dentinal 

 tubules become further separated by an increase of dentinal matrix, but this appears to 

 take place only in the young tooth. Probably it causes a slight increase in the size of 

 the rabbit's tooth. In the cat, however, it does not cause any increase in the size of the 

 tooth, the width of the intertubular substance remains the same. It is only in the upper 

 part of the adult tooth that the tubules are smaller and more closely packed. All we 

 can affirm in this case is, that the interstitial increase of the matrix simply encroaches on 

 the size of the tubules and so does not cause any increase in the size of the tooth. 



Examination of the Teeth of Young Rabbits fed on Madder. 



"While working at this subject Professor Haycraft kindly gave me the teeth of three 

 young rabbits which had been fed on madder for a fortnight. I carefully examined these, 

 as we thought they might throw some light on the mode of growth in teeth. 



I. The first rabbit was killed after being fed on madder for two weeks. (The diagrams, 

 Fig. 14, show by the darker shading the exact localities where the dentine is stained.) 

 All the stained part of the tooth is that produced while the madder was added to the 

 food. In the section it is seen that this staining reached the very crown of the tooth, 

 but only at the centre. This clearly demonstrates what I have already stated, that there 

 is a constant deposit of new dentine on the inner surface of the old. At the apex of the 

 pulp cavity the colour is deepest, for most of the new dentine was deposited in that 

 situation. It is also seen that there is a narrow band of stained dentine which immediately 

 surrounds the pulp. These teeth also show that the incisor teeth increase in length much 

 more rapidly than the molars ; for, while the incisor is stained in three-fourths of its length, 

 the premolar is stained in only half its length. 



II. The second rabbit was fed for two weeks on madder and then on ordinary food 

 for a similar period. The lower part (Fig. 15) of the incisor tooth, and also a narrow 

 strip of dentine surrounding the pulp cavity and extending up to the grinding surface, 

 is now unstained. This is all new dentine, formed during the last two weeks of the 

 animal's life. In the premolar the axial staining is hardly yet worn away. The deeper 

 staining of the dentine on the concavity of the incisor may be due to the more rapid 

 growth which there is in this situation, and the greater consequent absorption of the 

 circulating stain. 



