as indicative of the Age of the Animal. 
323 
Attention must now be directed to the state of the incisor teeth 
tit eis^hteen months. In fig. 24 it was shown that at a year old 
the four middle-placed incisors, in particular, gave indications of 
wear, by the loss of their sharp edges and increasing flatness of 
their crowns. By eighteen months this flatness has considerably 
increased ; it is not now, however, confined to the teeth placed in 
the centre of tlie mouth, but has extended to all. The jaw of the 
animal has also grown wider, thus increasing the spaces between 
the teeth, so as to leave not merely their fangs apart, but likewise 
their crowns. To compensate, in part, for their diminished length, 
the teeth have likewise risen in their sockets ; and as some of 
them are soon to be renewed by the permanent incisors, the pro- 
cess of absorption has commenced in their fangs. These various 
causes, more or less modified in different animals, give to the 
mouth an appearance which is quickly recognised. 
To these indications of age have to be added a diminished 
whiteness of the teeth, the part of their crowns which is exposed 
being that which is covered by a thin layer of enamel ; the exist- 
ence also of yellowish lines on their wearing surfaces, which indi- 
cate the outline of the once open pulp cavities ; and the discoloured 
state of their fangs from the action of the food and secretions of 
the mouth upon the crusta. 
Fig. 26 will convey many of these things to the mind of the 
reader. It repre- 
sents the front part 
of the lower jaw at 
eighteen months. 
With increasing 
evidence of a speedy 
fall of the middle 
incisors, we arrive 
at one year and nine 
months. At this 
time the central 
incisors are often 
replaced by the 
permanent. This 
change is generally 
observed in animals 
whose vigour of con- 
stitution a)id potcer 
of arriving at early 
maturity has been 
uided by a liberal 
* Fig. 2G. The front part of the lower jaw at IS mouths, exhibiting the 
dimmished size of the bodies of the incisors, and the increased space between their 
fangs. Natural size. 
