290 On the Teeth of the Ox, Sheep, and Pig, 
Fig. 2 * 
compound one. (See figs. 3, 13, 14, and 19.) Dentine holds the 
second place in the order of hardness, Crusta the last. The 
cause oi this difference of density and the advantages springing 
therefrom will appear by-and-by. 
In fig. 2, which represents a vertical section of an incisor 
of the horse, I have exhibited the relative 
amount as well as tlie situation of each of 
these constituents. The enamel is marked 
E ; the dentine, D ; and the crusta, c. This 
tooth is, truly speakinjf, a compound one, in 
as much as the enamel, besides forming the 
lateral boundary to the dentine, descends in 
a cup-like form into the upper part of this 
structure. The crusta is found filling this 
enamel cup, and when discoloured by the 
secretions of the mouth and the juices of the 
plants on which the animal feeds, it cqpsti- 
tutes " the mark " by which the age of the 
horse is partly ascertained. Professor Owen, 
in his description of the incisor of the horse, 
says that " a lai/er of cement is reflected into 
the deep central depression of the crown, 
and that a coloured mass of tartar and ^ja;-- 
ticles of food, which fill up the cavity, form 
' the mark ' of the horsedealer." f It is, 
however, blackened crusta, and not tartar, 
which produces the mark, as has been just explained. 
A similar thing is observed in the molars oi the ox and sheep; 
these teeth being almost always blackened on their sides above 
the gum, where a layer of crusta exists. 
The central portion of a tooth (a, fig. 2) is holloAved out to receive 
the pulp from which the organ chiefly derives its nourishment. 
The size of this cavity depends upon the age of the tooth. It is 
always large in a young tooth, becoming gradually smaller as age 
advances. The pulp cavity is bounded by the dentine, D, which 
makes up the chief part of the organ. Before, however, directing 
further attention to this cavity, I shall describe the special cha- 
racters of the hard parts of a tooth. First, of the enamel : 
On inspecting an incisor tooth of an ox, as an example, 
we observe a white incrustation, bearing in the young subject 
more or less a pearly appearance, covering the crown of the 
* Fig. 2. Vertical section of an incisor tootli of the horse, a, pulp cavity; d, . 
dentine ; e, enamel ; c*, enamel cup ; c, crusta filling the cup ; e*, crusta covering the 
enamel. Natural size. 
t Cyclopa-dia of Anatomy and Physiology, vol. iv. p. 8G7. The italics in the 
extract are our own. 
