238 A MANUAL OF DENTAL ANATOMY. 



merely rudimentary, as in Man or the Carnivora ; or the 

 investment with enamel may be only partial, as upon the 

 front of a Rodent incisor; or a tooth may be composed 

 solely of a mass of hard unvascular dentine, as in the teeth 

 of the Wrasses. 



And just as endless varieties of teeth may be produced 

 by the suppression, or partial suppression, of certain of the 

 tissues, so differences may be brought about by the occur- 

 rence of other than the three usual tissues. Thus the 

 remains of the central pulp cavity often becomes occupied 

 by calcined pulp, forming " osteodentine ; " this, which 

 occurs in man as an almost pathological condition, is per- 

 fectly normal in many animals; in the sperm whale, for 

 instance, or in the constantly growing teeth of the sloth, the 

 central axes of which are occupied by dentine permeated by 

 medullary canals. 



It is not so much the complexities induced by variation 

 in minute structure that concerns us here, as those brought 

 about by the arrangement of the different tissues. 



If we take a simple conical tooth with one cusp, such as 

 a canine, and grind or wear down its apex till the terminal 

 portion of enamel is removed, its blunted end will present a 

 more or less circular area of dentine, surrounded by a rim 

 of enamel. If we imagine a tooth with four long similar 

 cusps, we shall at a certain stage of wear have four such 

 areas, while eventually, as the tooth gets worn down below 

 the level of the basis of the cusps, there will come to be a 

 single larger area of dentine surrounded by enamel. Thus 

 in those teeth the grinding surfaces of which are rendered 

 complex in pattern by the presence of several cusps, the 

 pattern changes from time to time as the tooth wears down ; 

 while the addition of thick cementum filling up the inter- 

 spaces of the cusps, adds a further element of complexity, as 

 is seen in the teeth of most herbivorous creatures. The 

 change of pattern induced by the wearing down of the surface 



