NERVES TO ODONTOBLASTS, AND ON THE GROWTH OF DENTINE. 325 



the whole skin, the ordinary cells of the epidermis merely acting as conductors to convey 

 impressions to these sensory cells. Such may possibly be the case with the odontoblasts. 

 It is highly probable also that the nerves of the tooth have a trophic as well as a sensory 

 function. Since nerves pass to odontoblasts, their trophic influence probably extends 

 alonff the dentinal fibres and influences the nutrition of the tooth. 



Growth of the Dentine. 



While working at the histology of the pulp it was suggested that I might at the same 

 time investigate the manner in which the tooth increases in size. How does the small 

 tooth of the young mammal grow into the large tooth of the adult animal ? We know 

 that the increase in its bulk is chiefly confined to the dentine, so to this tissue I confined 

 myself in this investigation. There has been much controversy as to whether the 

 enlargement of a bone is to any extent dependent on interstitial growth. To find out 

 whether in the nearly-allied dentine there is or is not interstitial growth seemed an inter- 

 esting question ; for in this tissue there is no possibility of external deposition, the 

 dentine being formed from without inwards. If there were an interstitial growth in the 

 dentine, we should expect to find the dentinal tubules becoming further separated in the 

 developing tooth by an increased amount of matrix between them, and thus fewer in a 

 given area in the adult tooth than in the young tooth. To determine whether or not this 

 mode of growth obtains in dentine was one of the questions I have tried to solve. 



Method of Investigation. — I chose for the purposes of this inquiry the lower incisor 

 teeth of the rabbit, for these teeth grow from persistent pulps and are therefore never 

 shed. To observe their condition at different stages of growth, I examined them in (1) 

 a rabbit newly born ; (2) in a rabbit one month old ; and (3) in an adult rabbit. These 

 teeth, while still in situ in the lower jaw, were decalcified and sections made in an antero- 

 posterior direction parallel to their long axis. The sections from the very centre of each 

 tooth were alone used for measurement, as these contained the largest pulp cavity and 

 went directly through the centre of the crown. These teeth, as they are worn down in 

 front, are always being added to from behind and thus pushed forwards. The enamel is 

 only found on the anterior and lateral surfaces, and is always thickest in the former 

 position, where also the dentine is harder. Consequently, as the crown of the tooth is 

 worn down, the anterior part, being harder, is not worn so fast, and thus the tooth becomes 

 chisel-shaped. In each tooth I determined with as great accuracy as possible the 

 following : — 



1. The greatest length of the pulp cavity from the root of the tooth to the apex of 



the pulp. 



2. The greatest breadth of the pulp cavity. 



3. The thickness of the wall of dentine at the middle of the tooth. 



4. The greatest thickness of the dentine at the crown of the tooth. 



5. The width of the dentinal tubules at their origin from the pulp cavity. 



