310 
On the Teeth of the Ox, Sheep, and Pig, 
Fig. n 
that its situation upon the tooth pulp might be compared to 
one's finger as covered by a thumbstall. A similar figure may be 
employed to elucidate the arrangements of the structures in a 
molar tooth of an Herbivorous animal. Thus, suppose two fingers 
when held up to represent a cleft tooth pulp. Cover these first 
with two leather caps, say of a yellow colour (this is the dentine), 
place over them two other caps of a white colour (this is the 
enamel), put over these two more caps of a brown colour (this 
is the crusta). Now unite the brown caps by approximating 
the fingers ; and supposing these several coverings to be all 
joined together, and tiiick enough to have a horizontal section 
carried through them, we shall find in reckoning from outside 
to outside, first brown 
(crusta), then white (ena- 
mel), next yellow (den- 
tine), then white again, 
next brown, and after- 
wards white, yellow, 
white and brown. Like 
most similes, objections 
can easily be taken to 
this, but still it is suf- 
ficient to show the com- 
mingling of the several structures ; and as they all differ in 
density, we see how a roughened surface is maintained upon the 
exposed part of a compound tootli. 
As these molar teeth wear away so do they rise in their 
sockets, and consequently they always stand at about the same 
height in the mouth, and the incessant wear occasions such 
an arrangement of the dentine, enamel, and crusta, that the tooth 
of the old animal, when worn nearly to its fangs, may be as efTec- 
tive an instrument for sjrindino' the food as it had been when the 
animal was young. Provision is made for this, by elongatmg 
the bodies of these molars at the expense of their fangs. The 
body of the last molar of the ox is not less than two incites long, 
and that of the same tooth of the sheep an inch and a half long ; 
Mobile the bodies of several of the permanent molars of the horse 
are often from three to three inches and a half long, although little 
more than a quarter of an inch of these teeth may appear above 
the gum. This great length of body of all the horse's molars 
renders them nearly fangless, and the same is the case with 
the fifth molar in particular of the sheep, as seen in the fol- 
* Fig. 18. Face of the third teinporavy molar of a calf, natural size, showing 
that the exposure of the dentine and isolation of the central from the outer enamel 
is caused by the wearing away of tlie original enamel cusps, e, outer layer of 
enamel ; e*, inner ditto forming the cup ; c, crusta ; d, dentine. 
