THE TEETH AND THEIR DEVELOPMENT. 211 



great strength, and constituting the cheek ; it has its origin 

 at the superior maxillary bone, under the ridge continued 

 from the zjgomatic-arch, plate viii. fig. S,f,f, and inserted 

 into the lower jaAv. This acts in conjunction with the 

 temporal muscle in closing the jaw, and in producing its 

 direct cutting, or champing motion. 



Inside of the lower jaw, on each side, and occupying the 

 entire of the hollowed portion, and opposite the masseter, 

 or cheek-muscles, are the pterygoid-muscles, which proceed 

 from the jaw-bones to those more in the centre of the 

 channel, also shutting the mouth, and likewise by their 

 alternate action giving that grinding motion so necessary in 

 preparing the food of the horse. 



The channel which occupies that portion of the lower 

 jaw between the branches, is of considerable importance 

 in the conformation of the head of the horse. If this part 

 is too wide, it always gives a clumsy, heavy appearance to 

 the face ; and when too narrow, it prevents the animal from 

 bending his head with freedom and grace. Horses with 

 such heads are unpleasant in the hand either of the rider 

 or driver, as they constantly keep pulling and stretching 

 their head, which also prevents them from being well 

 reined in. 



The changes which successively take place in the incisory 

 teeth throughout the Avhole period of the life, form the 

 most certain test of the age of the horse. 



The teeth of a horse are forty in number, and in the 

 mare thirty-six, being destitute of the four canines. The 

 incisory, or cutting teeth, are temporary, and are replaced by 

 others from two and a hiilf years, to four or five. The 

 marks on them which distinguish the age become obliterated 

 at eight years. The life of a horse may be divided into 

 three periods ; first, from birth to two and a half years, 



