vii.j THE EXTERNAL SKELETON. 253 



Physiology, but it should here be noted that not only do the 

 permanent canine teeth come into place before the wisdom 

 teeth, but also even before the second molar. Moreover, the 

 last milk molar is shed before the epiphyses of the long bones 

 are united to their shafts, and before the bones of the limb 

 girdles have coalesced. 



All the teeth of the permanent set are thus provided with 

 temporary or deciduous predecessors, except the last three 

 molars of each jaw. It is on account of this absence of 

 vertical predecessors that the three hindmost teeth on each 

 side of each jaw are called "true molars." In extending our 

 view to other animals we shall see that true molars must be 

 defined to be "teeth situated behind the hindmost tooth 

 having a vertical predecessor." 



19. The upper incisors are implanted in what we have seen 

 to be a distinct bone (the premaxilla) from that- in which the 

 other teeth of man's upper jaw are implanted, while the 

 canine is the foremost tooth implanted in the true maxillary 

 bone. In surveying these parts in other animals we shall 

 see that the DEFINITION of an upper incisor is " a tooth im- 

 planted in the premaxilla ; " of an upper canine, " the fore- 

 most tooth of the maxilla, provided it be not at a considerable 

 distance from the anterior end of that bone." The lower 

 canine must be defined as " the tooth which bites in front 

 of the upper canine," and the lower incisors as " teeth placed 

 in front of or on a line with the lower canine, or, if this is 

 absent, teeth corresponding with the upper incisors." 



The bicuspid molars of man (not having always two 

 cusps in other animals) are, in zootomy, termed premolars, 

 because they are placed in front of the true molars. An 

 extended view reveals facts which compel us to give to 

 premolars generally the following somewhat cumbrous defi- 

 nition : " teeth behind the place of the canines and in front 

 of the true molars, or, if the latter are absent, teeth behind 

 the place of the canines and having vertical predecessors, or 

 in front of molar teeth which have such predecessors." 



20. Such being the dentition (i.e. tooth-furniture) of man, it 

 may be conveniently expressed by the following SYMBOLS : 



If, C ^, P M |, M f , for the permanent dentition. I f 

 means " two incisors, above and below, on each side of the 

 jaws ;" C \ means similarly one canine in each case ; P M | 

 means " two premolars on each side of each jaw ; " and M f 

 means "three true molars, both above and below, on each 

 side." Similarly the symbols D I f, D C \, DM f, for the 



