COMPARATIVE ANATOMY. 
distinguished this by the name of crusta 
petrosa; and Cuvier calls it cement. 
The physiological explanation of this dif- 
ference in structure is a very easy and 
clear one. The food of the carnivora re- 
quires very little comminution before it 
enters the stomach ; hence the form of 
their grinding teeth is by no means calcu- 
lated for grinding ; and as the articulation 
of the jaw admits no lateral motion, the 
molares, of which the lower are overlapped 
by the upper, can only act like the incisors 
of other animals. The food of graminivo- 
rous quadrupeds are subject to a long pro- 
cess of mastication, before it is .exposed to 
the action of the stomach. The teeth of the 
animals suffer great attrition during this 
time, and would be worn down very rapidly 
but for the enamel, jvhich is intermixed 
with their substance. As this part is harder 
than the other constituents of the teeth, it 
resists the attrition longer, and presents the 
appearance of prominent ridges on the 
worn surface, by which the grinding of the 
food is much facilitated. 
The distinction of the three substances is 
seen better in the tooth of the elephant 
than in any animal. The best method of 
displaying it is by making a longitudinal 
vertical section, and polishing the cut sur- 
face. The crusta petrosa will then be dis- 
tinguished by a greater yellowness and opa- 
city in its colour ; and by an uniformity in 
its appearance, as no laminse or fibres can 
be distinguished. 
The pulp of a grinding tooth of a grami- 
nivorous quadruped is divided into certain 
conical processes, which are united at their 
bases. These vary from two to six in the 
horse and cow. On these the bone of the 
tooth is formed, as on the single pulp of the 
human subject, but it is here divided into 
as many separate shells, as there are pro- 
cesses of the pulp : all of them however in- 
closed in a common capsule. The ossifica- 
tion commences, as in all teeth, on the 
points of the pulp, and extends towards the 
basis : when it has arrived there, the shells 
unite together ; and they also join at their 
outer margins. Between the processes of 
the pulp other productions descend from the 
capsule in a contrary direction; and depo- 
sit on the surface of the shells enamel dis- 
tinguishable by its crystalline appearance, 
and hence denominated by Blake cortex 
striatus. When these membranous pro- 
ductions have formed their portions of ena- 
mel, they secrete the crusta petrosa within 
the cavities left between these productions 
of enamel. The outer surface of the bone 
of the tooth is covered by enamel, which 
may be compared to that which invests the 
crown of a human tooth, except that it is 
deposited in an irregular waving line, in 
order to render the surface better calculat- 
ed for grinding : and the inequalities of tliis 
surface of enamel are filled up by crusta pe- 
trosa. Tlie exterior enamel, and crusta 
petrosa, (which may be so named, by way 
of distinguishing them from the processes 
within the tootli), are formed by the surface 
of the capsule. 
If then we make a transverse section of a 
grinding tooth of the horse or cow, the ex- 
terior surface will be found to consist of an 
irregular layer of crusta petrosa : this is suc- 
ceeded by a waving line of enamel, within 
which is the proper bone of the tooth. But 
the silbstance of tlie latter is penetrated by 
two productions of enamel, in the interior 
of each of which is crusta petrosa. 
The crusta petrosa which fills these inter- 
nal productions of enamel, is sometimes not 
completely deposited before the tooth cuts 
the gum : hence cavities are left in the cen- 
tre of the tooth, which become filled with 
a dark substance composed of the animal’s 
food, and other foreign matters. This sel- 
dom happens to any considerable extent in 
the grinders of the horse. In the cow and 
sheep these cavities are constantly filled 
with the dark adventitious matter; the 
crusta petrosa being confined to the exte- 
rior surface of the tooth, and not existing 
even tliere so plentifully as in the horse. * 
The lower grinders <Sf the horse differ 
veiy much in their formation from those of 
the upper-jaw. Ossification commences in 
these by four or five points, which increase 
into as many small shells ; yet they unite 
without any processes of the capsule passing 
down between to form internal productions 
of the enamel. That substance is however 
deposited in a very convoluted manner on 
the bone of the tootli, so that the same end 
is attained, as if productions Of the cortex 
striatus had existed in the centre of the 
part. The crusta petrosa fills up the irre- 
gularities of this waving line of enamel. An 
horizontal section of such a tooth presents 
the three substances aivanged within each 
other : the crusta petrosa is external ; then 
comes the enamel, which includes nothing 
but the proper bone of the tooth. 
The incisors of the horse have a produc- 
tion of enamel in their centre ; but the ca- 
vity which this forms, containing no crusta 
petrosa, is merely filled by the particles of 
