the Teeth of graminivorous Quadrupeds. 251 
different kind of food from that of other hogs, and is an animal 
of greater longevity. 
Upon comparing the internal structure of the elephant's 
tooth with that of the horse, cow, and sheep, it was found, 
that they were similar in having an intermixture of bone with 
the substance of the tooth, but that they differed materially 
from one another in the proportions and situations of the bony 
portions. Each of these animals having the grinding surface of 
their teeth adapted for particular kinds of food, the parts com- 
posing that surface are variously combined, so as to answer 
the purpose for which the teeth are intended. In all of them, 
the mode of growth is the same ; the substance of the tooth is 
first formed, and the bony part is afterwards adapted to the 
irregularities of that surface. 
In the horse’s grinding teeth, the processes are two in num- 
ber; and, in an early stage of their growth, they appear, as 
well as those of the elephant, to be separate teeth : they differ, 
however, extremely in their shape, forming irregular cylindrical 
tubes, the central part of which is filled up by the projecting 
membranes from the gums, that are to be changed for bone. 
This division of the tooth into two parts, is very distinct in the 
shedding teeth, but not in the second set or permanent teeth. 
These two portions of bone in the middle of the tooth, have 
frequently a hole in them, (probably the passage of a blood- 
vessel, never completely filled up,) and the food getting into it, 
as the tooth is worn down, considerably increases its size. 
Besides which, there is a portion of bony substance surrounding 
a great part of the outside of the tooth. 
In the cow’s grinding teeth, there are two portions of bony 
