363 ANATOMY OF VERTEBRATES. 



Only in the Mammalian orders with two sets of teeth do those 

 organs acquire fixed individual characters, supporting the appli- 

 cation of special denominations ; and this iiidividualisation of the 

 teeth is significative of the high grade of organisation of the 

 animals manifesting it. 



Originally, indeed, the name f incisors,' ' laniaries ' or 6 canines,' 

 ' molars,' ' tubercnlars,' were given to the teeth in Man and 

 certain Mammals, as in Reptiles, in reference merely to the shape 

 and offices so indicated ; but names of teeth can now be used as 

 arbitrary signs, in a more fixed and determinate sense. In some 

 Carnivora, e.g., the front teeth have tuberculate summits, adapted 

 for nipping and bruising, while the principal back teeth are 

 shaped for cutting, and work upon each other like the blades 

 of scissors. The front teeth in the Elephant project from the 

 upper jaw in the form, size, and direction of long pointed horns. 

 In short, shape and size are the least constant of dental cha- 

 racters ; and the homologous teeth are determined, like other 

 parts, by their relative position, by their connections, and by their 

 development. 



Those teeth which are implanted in the premaxillary bones, 

 and in the corresponding part of the lower jaw, are called ' in- 

 cisors,' whatever be their shape or size. The tooth in the 

 maxillary bone, which is situated at, or near to, the suture with 

 the premaxillary, is the ( canine,' as is also that tooth in the lower 

 jaw which, in opposing it, passes in front of its crown when the 

 mouth is closed. The other teeth of the first set are the f de- 

 ciduous molars ; ' the teeth which displace and succeed them ver- 

 tically are the f premolars ; ' the more posterior teeth, which are 

 not displaced by vertical successors, are the f molars,' properly so 

 called. 



The premolars must displace deciduous molars in order to rise 

 into place ; the molars are a continuation, backward, of the pri- 

 mary or 4 milk' series. It will be observed in fig. 294 that the 

 last deciduous molar, d 4, has the same relative superiority of 

 size to d 3 and d 2 which m 3 bears to in 2 and m \ ; and that the 

 crowns of p 3 and p 4 are of a more simple form than those of the 

 milk-teeth which they are to succeed : this, however, is not a 

 constant character (see fig. 287, Hyrax\ Teeth of each of the 

 kinds arbitrarily termed ' incisors,' f canines,' ( false molars,' and 

 ' molars,' have received other special names, having reference to 

 certain peculiarities of form or other property. The premolars 

 in the human subject have been called ' bicuspids.' The last 

 upper premolar and the first true molar in the Carnivora are 



