DENTITION. 723 



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 primitive 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 of the upper jaws may be divided 

 in the typical case into four sets. Most anteriorly, associated 

 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 calcined successors, or, as others maintain, 

 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 

 connection between the teeth and the subjacent bones is a secondary 

 matter ; there is often little to differentiate canine from premolar. 

 Moreover, the teeth of the lower jaw, which is a single bone on each 

 side, cannot be so certainly classified as those of the upper jaw. Plere 

 the lower canine is defined as the tooth which bites in front of the 

 upper, and the incisors as the teeth in front of this tooth. 



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