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 zygomatic-arch, plate viii. fig. 3,/, /, and inserted 
into the lower jaw. 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 whole 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 half 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* 
