DENTITION 815 



pace with the rate at which the apex is worn away with use, and it is 

 described as '' rootless " and " with persistent pulp." The incisors of 

 Rodents and of elephants illustrate this condition. 



In the development of most teeth, however, the base is narrowed 

 and prolonged into a root or several roots which become firmly fixed 

 in the socket. Through a minute aperture at the end of the root, 

 blood vessels and nerves still enter the pulp-cavity and keep the tooth 

 alive, but, as the limit of growth is reached, the residue of soft pulp tends 

 to disappear. 



The two most marked characteristics of the teeth of Mammals are 

 that they are typically heterodont — that is, different from one another in 

 form and function — and that the succession is practically reduced to 

 two sets, a condition described as diphyodont as contrasted with the 

 polyphyodont condition seen in Fishes and Reptiles, where the suc- 

 cession is practically unlimited. 



As exceptions, there are cases like that of the dolphins, where the 

 teeth are uniform or homodont and very numerous. This, however, 

 is not a prinutive but a secondarily acquired condition. 



In the typical dentition of Mammals there are forty-four 

 permanent teeth, eleven on each side above and below ; but 

 it is rare in the Eutherian Mammals to find the full number 

 developed, and the dentitions of the Marsupials, of the 

 Edentates, and of the Cetacea cannot be reduced to this 

 type. The eleven on each side of the jaws may be divided 

 in the typical case into four sets. Most anteriorly, associ- 

 ated with the premaxilla, are three simple, single-rooted 

 teeth, usually adapted for cutting or seizing. These are 

 called incisors. Posteriorly there are crushing or grinding 

 teeth, whose crowns bear cusps or cones, or are variously 

 ridged, and which have two or more roots associated with 

 the maxilla. But of these grinders the last three occur as 

 one set, having no calcified successors, or, as others main- 

 tain, having no milk predecessors. They are therefore dis- 

 tinguished, as true molars, from the four more anterior and 

 often simpler premolars, which usually occur in two sets, 

 the milk set being replaced by a permanent set. In many 

 cases, however, the first premolar seems to be only once 

 represented. Finally, the tooth just behind the incisors — 

 that is to say, immediately posterior to the suture between 

 premaxilla and maxilla — is distinguished as the canine, 

 and is often long and sharp. 



This classification of teeth is in great part one of convenience : thus 

 the distinction between incisors and grinding teeth is anatomical, that 

 between molars and premolars refers to the history of these teeth ; the 



