328 DR W. G. ALTCHISON ROBERTSON ON THE RELATION OF 



the pulp (Fig. 9). No odontoblasts remain in this connecting channel ; therefore 

 since the dentinal fibres in the crown of dentine have lost their connection with nerves 

 the grinding surface of the rabbit's incisor has lost sensitivity. These laminae of bone 

 which help to block up the remains of the pulp-cavity at the apex of the tooth may be 

 part of the layer of cement which, in the persistently growing teeth of many animals, 

 covers over the crown of the tooth, and which may when worn away sink into the almost 

 occluded apex of the pulp cavity and grow there. It may, however, be developed directly 

 from the tissue of the pulp. 



In the adult rabbit's tooth, then, the growth of dentine at the " formative ring," the 

 continual deposition of new dentine on the inner surface of the old, and the extent to 

 which the tooth is worn down externally, exactly balance one another, and thus the tooth 

 remains of the same size throughout life. In the young growing animal, however, the 

 first two of these processes exceeds the third, and so the tooth grows greatly in length, 

 diameter, and thickness of dentine. 



Having seen how a simple conical tooth increases in size, the next question which 

 naturally arose was, How do flask-shaped teeth, such as the canine tooth of a cat, 

 increase in size ? To answer that question I examined the canine tooth of the lower 

 jaw in (1) a newly-born kitten ; (2) in a kitten of one month old ; and (3) in the adult 

 cat. These teeth in the cat, as in all carnivora, are shed at an early period of existence. 

 This introduces a slight fallacy, for it compels us to compare deciduous with permanent 

 teeth. I made the same measurements in this case that I had made in the rabbit's 

 incisor tooth. 



(1) Total length of tooth. 



(2) Greatest length of pulp cavity. 



(3) Greatest breadth of pulp cavity. 



(4) Thickness of dentine at middle of tooth. 



(5) Thickness of dentine at crown. 



(6) Diameter of dentinal tubules at their origin from pulp cavity. 



(7) Width of intertubular dentine. 



[Measurements. 



