as indicative of the Age of the Animal. 
287 
pig, that we are in the habit of speaking of this animal as having 
"28 molars. 
Tlie molar teeth of the ox, sheep, and pig increase in bulk from 
before backwards, while, on the contrary, in the horse they 
diminish in the same direction. This is especially seen in the 
molars of the lower jaw. The last permanent molar in the lower 
jaw of botli the ox and sheep differs from the others in having 
three lobes or principal parts united together in the place of two. 
These peculiarities will be fully described hereafter. In the 
upper jaw this difference is not observed, the tooth not varying 
essentially in its form from those that stand before it. In the 
pig, the last molar teeth in both jaws are similarly formed, being 
trilobular, and far exceeding in size any of the others. 
The office of the molar teeth is that of reducing the food 
to a pulpy mass by grinding it between their faces. It is this 
process which is called mastication. The aliment, being received 
into the mouth, is conducted, by the movements of the tongue, 
upon the molar teeth, and while being ground down, is mixed 
with the saliva and with other fluids found in the mouth : in 
other words, it is both masticated and insalivated, each of these 
processes being of the first importance to healthy digestion and 
consequently to the well-being of the animal. It is, however, 
only in the vegetable feeders, the herbivora, that we see the per- 
fection of the grinding movement. In the omnivora the action 
is very limited, while in the carnivora it does not exist at all, 
the molar teeth being in this class used simply for crushing. 
This different action of the molars upon the food in different 
animals mainly depends upon the manner in which the lower jaw 
is united to the upjier. The joint forming the union is modified 
in the herbivora so as to allow of a side movement of this jaw. 
It is a less restricted hinge joint than in the flesh feeding class. 
Even among vegetable feeders the movements vary, as we may 
see in comparing the ox with the horse. In the ox, the mouth 
being slightly opened, the lower jaw is first moved to one side, next 
elevated so as to approximate the faces of the upper and lower row 
of molars, and then carried in the opposite direction, bruising the 
food between them. Should this action, which depends on the will 
of the animal, be so commenced as to carry the jaw from right to 
left, it will be thus continued, or vice versa. In the horse, on the 
contrary, the jaws are not separated from each other to the same 
extent as in the ox, and the food is ground down by an alternate 
movement of the lower jaw from side to side. 
To secure an irregular surface upon the face of the molars, 
which is necessary for the complete grinding of the food of 
herbivorous animals, we find that the three constituents of a 
tooth, enamel, dentine, and crusta, are so arranged as to enter 
u 2 
