128 THE STRUCTURE OF THE HORSE 



gradually rises towards the surface of the socket, the 

 bottom of which fills up with bone, so that in very old 

 horses the crowns are entirely worn away, and nothing 

 but the roots of the teeth remain, loosely implanted in the 

 jaw. If from any accidental cause one of the teeth is 

 absent, the tooth opposite to it in the other jaw, having 

 nothing to wear against, will gradually rise high above 

 the level of its fellows. 



Temporary or Milk-Teeth. — The first, temporary, 

 deciduous, or milk set of teeth of the horse, though on 

 the whole resembling the permanent set, having the 

 characteristic enamel foldings arranged on the same 

 genera] principle, present several interesting peculiari- 

 ties. 



The incisors are in number the same as the perma- 

 nent teeth of this class. They are, however, not only 

 smaller, but, as they are only required to be in use for a 

 limited time, they have not need of the lengthened 

 crowns passing indefinitely into the root possessed by 

 the latter, and therefore show distinctly the broad crown, 

 contracted neck and definite root of the brachydont 

 type of tooth. In this respect they resemble the ances- 

 tral form from which the permanent teeth have been 

 derived. The infundibulum or pit is present, but of com- 

 paratively little depth. 



The canine teeth of the horse evidently belong to 

 the permanent set, not coming into place until the 



