INTRODUCTION. XVll 



contour of the tooth ; for, when the surface of a longitudinal section 

 of a tooth is viewed with the naked eye, the light is differently reflected 

 from the different parts of the oblique secondary curves of the 

 tube on which it falls ; but the curves being parallel to each other and 

 to the superficial contour of the section, they appear like the cut 

 edges of a series of parallel and super-imposed lamellae. In many 

 teeth, moreover, and especially in the tusks of the elephant, the se- 

 condary branches of the dentinal tubes dilate into intertubular cells 

 along lines, which in like manner are parallel to the coronal contour 

 of the tooth ; hence another cause of the appearance of concentric 

 lamellse and of the actual decomposition of such teeth into super-im- 

 posed lamelliform cones. 



Such appearances and modes of decomposition are peculiar to 

 the dense or unvascular dentine ; but are by no means common to 

 that modification of the tissue. They are never witnessed in any of 

 the varieties of vascular dentine. (1) The prolongation or persistence 

 of cylindrical canals of the pulp-cavity in the dentinal tissue, which 

 is the essential character of vascular dentine, manifests itself under a 

 variety of forms. In mammals and reptiles these canals, which I have 

 termed 'medullary' (2) from their close analogy with the so called canals 

 of bone, are straight and more or less parallel with each other ; they 

 bifurcate, though rarely; and when they anastomose, as in the megathe- 

 rium, it is by a loop at, or near, the periphery of the vascular dentine. 

 In the teeth of fishes, in which the distinction between the dentinal 



(1) This substance was first characterised as a component of tooth, * distinct from ivory, 

 enamel, cement, and true bone, and as easily recognisable,' in my paper communicated to the 

 British Association, in 1838; loc. cit. p. 137. 



(2) Ibid. 



b 



