HEAD AND NECK OF THE HORSE 85 



contact (facies contactus). All except old teeth contain a cavity (cavum 

 dentis) entered by an opening at the apex of the root (foramen apicis 

 dentis) and containing the tooth pulp (pulpa dentis). 



With the naked eye it is possible to determine that there are three 

 hard tissues entering into the composition of a tooth. Dentine (sub- 

 stantia eburnea) ^ constitutes the greater part of the structure. Enamel 

 (substantia adamantina) - forms the porcelain-like covering of the crown ; 

 and cement (substantia ossea), a bone-like tissue, covers the root and, 

 especially in the cheek-teeth of herbivora, may form a layer of consider- 

 able thickness over the enamel of the crown. 



Teeth are classified from before to behind as incisors (dentes 

 incisivi), canines (dentes canini), premolars (dentes prsemolares) and 

 molars (dentes molares). For the sake of convenience, it is customary 

 to include the premolars and molars under the common designation of 

 cheek-teeth. The number of teeth in each group may be indicated by a 

 dental formula, of which the following is an example : — 



pm^, m.. _11 



X 2 = 4-4. 



ia, Ci, pm^.m^ 11 



The above formula shows that the animal to which it applies, e.g., 

 the pig, has three incisors, one canine, four premolars, and three molars 

 on each side of both the upper and lower rows of teeth. To save time, 

 the initial letters of the different classes of teeth may be omitted, and 

 the formula written §yiI' 



In mammals in general there are two sets of teeth, namely, 

 deciduous, " milk," or temporary (dentes decidui), and permanent 

 (dentes permanentes) ; and it should be noted that, of the cheek-teeth, 

 only the premolars are represented in both sets. 



The number of teeth in the foal and the adult horse may be best 

 indicated by the following formulae : — 



Deciduous dentition ^ — ^^ — * = ^ x 2 = 82. 

 I3, c^, pm^ -S 



Permanent dentition .^' ^' ' -'- — ■''=-x2 = 40. 

 1.3, Cj, pm.j, niy 10 



In the above formulae, the rudimentary and exceedingly interesting 

 first premolar has been counted as belonging to the deciduous dentition ; 

 but it must not be forgotten that in some instances, especially when its 

 development is greater than usual, it persists long after the other 

 deciduous teeth have been shed, and on this account it might be con- 

 tended that it should be included in the permanent dentition. It will 



1 Ebur [L.], ivory. 



- dbaixav-nvos (adaniaiitiiins) [Gr.], very hard. 



