280 



ANATOMY OF VERTEBRATES. 



The exterior of the long tusk is marked by spiral ridges, which 

 wind from within forward, upward, and to the left. About fourteen 

 220 inches is implanted in the socket ; 



it tapers gradually from the base to 

 the apex. The pulp-cavity, as shown 

 in the longitudinal section of the 

 tusk, in fig. 220, is continued nearly 

 to the extreme point, but is of vari- 

 able width : at the base it forms a 

 short and wide cone ; it is then con- 

 tinued forward, as a narrow canal, 

 along the centre of the implanted 

 part of the tooth, beyond which the 

 cavity again expands to a width 

 equalling half the diameter of the 

 tooth ; and finally, but gradually, 

 contracts to a linear fissure near the 

 apex. Thus, the most solid and 

 weighty part of the tooth is that 

 which is implanted in the jaw, and 

 nearest the centre of support, whilst 

 the long projecting part is kept as 

 light as might be compatible with 

 the uses of the tusk as a weapon of 

 attack and defence. The portion of 

 pulp, in which the process of the 

 calcification has been arrested, re- 

 ceives its vessels and nerves by the 

 fissure continued from the basal ex- 

 pansion of the pulp-cavity. In a 

 few instances, both tusks have been 

 seen to project from the jaw. 



In Delphinus griseus the dentition 

 of the upper jaw is transitory, as in 

 Hyperobdon, but at least six pairs of 

 teeth rise above the gum and acquire 

 a full development at the forepart 

 of the lower jaw. The crowns of 

 these teeth soon become obtuse, and 

 their duration is limited : aged indi- 

 viduals of this species have been 

 taken with the dentition reduced to 

 two teeth in the lower jaw. 



Base of skull of male Narwhal, with a section 

 of the Tusk. 



