280 A MANUAL OF DENTAL ANATOMY. 



we may at once pass to the homologies of the teeth with 

 one another. 



Teeth are divided into incisors, canines, premolars, and 

 molars, but these classes do not all admit of quite satis- 

 factory definition. Incisors are denned as teeth implanted 

 in the intermaxillary bone, a definition which has the merit 

 of being precise ; and on the whole there is a certain resem- 

 blance running through incisor teeth in most animals, but 

 the definition of lower incisors as being the corresponding- 

 teeth in the lower jaw is a good deal less satisfactory, because 

 they are not situate upon any distinct bone. And it has 

 even been denied that there can be a true homology between 

 a maxillary and a mandibular tooth. 



Molars are teeth at the back of the mouth, which come 

 up behind the milk teeth (when there are any), and which 

 are generally subservient to grinding the food. 



Premolars are teeth in front of the molars, usually differ- 

 ing from them by being more simple in form and being- 

 smaller, and in most animals by having displaced deci- 

 duous predecessors. But they are not always simpler in 

 form, nor smaller (e.g., the horse, fig. 138), nor do they 

 always displace deciduous predecessors (e.g., they do not all 

 do so in the Marsupials), so that this definition is not abso- 

 lutely precise. Still, as a matter of practice, it is usually 

 easy to distinguish the premolars, and the division into 

 premolars and molars is useful. 



Any objection that can be raised to the name of premolar 

 on the score of a short logical definition being impossible, 

 applies with tenfold force to the canines. (Cf. Messrs. 

 Mosely and Lankester, Journ. Anat. and Physiology, 1869.) 



The nearest approach to a good definition is that which 

 describes the canine as the next tooth behind the intermax- 

 illary suture, provided it be not far behind it; and the 

 lower canine as the tooth which closes in front of the upper 

 canine. 



