314 
On the Teeth of the Ox, Sheep, and Pig, 
incisors is still standing between the first and second pairs of 
the permanent. In addition to this, one of the third pair is also 
in situ, the corresponding temporary incisor of the opposite side 
liaving given way to the permanent tooth, producing thereby 
an inequality in the relative number of each set. 
rig. 21 .» 
It is a circumstance worthy of a passing remark, that these 
temporary teeth are not unfrequently hollow, their pulp cavity 
being opened from above by the decaying process they are slowly 
undergoing. The adage that " Nature gives nothing in vain 
is beautifully exemplified in the fact before us. These teeth 
have but a temporary purpose to serve, and therefore, although 
dentine is gradually added from the pulp to keep their wearing 
surfaces solid while attrition is daily going on, their cavity is not 
perfectly obliterated by a conversion of the pulp into dentine, as 
it is in the teeth of old animals. Had this been tlie case, the 
Crusta, to a certain extent, would have taken the place of the 
Dentine, in order that the tooth might still be nourished as a 
perfect organ. Out of its proper situation, however, the socket, 
* Fig. 21. lu this figure the second pair and one of the third pair of the tem- 
porary incisors are represented as having become persistent from their being 
pushed aside by the permanent. Engraved from a specimen in the author's 
possession. 
