as indicative of the Age of the Animal. 
311 
Fig. 10* 
lowing engraving, fig. 19. This figure, which has been drawn 
a size larger than natural to make the several parts of the tooth 
more apparent, sliows the depth to 
which the enamel cup descends, and 
also the extent of the outer layers of 
the same substance. The pulp ca- 
vity, which can be traced to nearly 
the top of the tooth, is marked a, at 
its bottom part, and h, where it is 
about to close, above; the crusta has 
the letter c, and the dentine, d; the 
enamel cup is distinguished by e*, 
and the outer layer of this substance 
by e. 
In fig. 20 we have a view of a 
transverse section of the upper part 
of a permanent molar, also of a sheep, 
magnified two diameters. It shows 
that the J^ulp cavity, b, is nearly 
closed in the part where the section 
is made by the dentine, D, the radi- 
ating lines in which map out the 
size of the original cavity from which 
they spring. The external layer of 
enamel, as in many other of these 
illustrations, is marked E*, the crusta, 
c ; and the inner enamel, as filled 
with the crusta, e. ^ 
From these explanations of the arrangement of the structures 
in compound teeth, I pass to the changes the teeth undergo when 
brought into daily use. It has been shown that in all animals, 
with which we are familiar, there are two sets of teeth ; the 
temporary and the permanent. Tlie temporary set in the ox 
and sheep consists of twenty teeth ; eight being incisors and 
twelve molars ; fourteen of this number, that is, the eight in- 
cisors and six molars, are placed in the lower jaw, the remain- 
ing six occupying the upper jaw. The number of temj)orari/ 
teeth in the pig is twenty-eight, namely, twelve incisors, four 
tushes, and twelve molars ; a moiety of each being located in 
either jaw. It may be also necessary to repeat that it is while 
the temporary teeth are being formed that nature provides for 
* Fig. 19. A vertical section of the posterior half of the //rt molar of a sheep in 
its short diameter, showing the gi-eat depth of the enamel cup and that it is 
filled with crusta. a, the inferior part of the pulp cavity; h, its superior portion 
■which is being closed; c, the crusta in the enamel cup; d, tlie dentine; e*, the 
outer layer of enamel ; e, the inner layer of the same substance, forming the cup. 
