COMPARATIVE ANATOMY. 
food, &c. As these processes of enamel 
descend only to a certain extent in the 
tooth, they disappear at last from the con- 
stant wear of the part in mastication. This 
is improperly called the filling up of the 
teeth ; and hence a criterion arises of the 
horse’s age. 
The grinding teeth of the elephant con- 
tain the most complete intermixture of the 
three substances, and have a greater pro- 
portion of cnista petrosa than those of any 
other animal. The pulp forms a number of 
broad flat processes, lying parallel to each 
other, and placed transversely between the 
inner and outer laminae of the alveoli. The 
bone of the tooth is formed on these in 
separate shells, commencing at their loose 
extremities, and extending towards the 
basis, where they are connected together. 
The capsule sends an equal number of 
membranous productions; Which first cover 
the bony shells with enamel, and then in- 
vests them with crusta petrosa : which lat- 
ter substance unites and consolidates the 
different portions. The bony shells vary in 
number from four to twenty-three, accord- 
ing to the size of the tooth, and the age of 
the animal : they have been described un- 
der the terra of denticuli, and have been 
represented as separate teeth in the first 
instance. It must however be remembered 
that they are formed on processes of one 
single pulp. 
When the crusta petrosa is completely 
deposited, the different denticuli are conso- 
lidated together. The bony shells are united 
at their base to the neighbouring ones ; the 
investments of enamel are joined in like 
maimer: and the intei-vals are filled with 
the third substance, which really deserves 
the name bestowed on it by Cuvier, of 
cement. The pulp is then elongated for 
the purpose of forming the roots or fangs 
of the tooth. From the peculiar mode of 
dentition of this animal, which will be ex- 
plained in a subsequent note, the front por- 
tion of the tooth has cut the gum, and is 
employed in mastication, before the back 
part is completely formed, even before some 
of the posterior denticuli have been conso- 
lidated. The back of the tooth does not 
appear in the mouth until the anterior part 
has been worn down even to the fang. 
A horizontal section of the elephant’s 
tooth presents a series of narrow bands of 
bone of the tooth, surrounded by corres- 
ponding portions of enamel. Between 
these are portions of crusta petrosa; and 
the whole circumference of the section is 
composed of a tliick layer of the same sub- 
stance. 
A vertical section in the longitudinal di- 
rection exhibits the processes of bone, upon 
the different denticuli, running up from the 
fangs : a vertical layer of enamel is placed 
before, and another behind each of these. 
If the tooth is not yet worn by mastication, 
the two layers of enamel are continuous at 
the part where the bone terminates in a 
point ; and the front layer of one denticu- 
lus is continuous with the back layer of the 
succeeding one, at the root of the tooth ; 
so that the enamel, ascending on the ante- 
rior, and descending on the posterior sur- 
face of each denticulus, forms a continued 
line through the whole tooth. Crusta pe- 
trosa intervenes between the ascending and 
descending portions of the enamel. 
As the surface of the tooth is worn down 
in mastication, the processes of enamel, re- 
sisting by their superior hardness, form pro- 
minent ridges on the grinding surface, which 
must adapt it excellently for bruising and 
comminuting any hard substance. 
The grinding bases,when worn sufficiently 
to expose the enamel, present a very dif- 
ferent appearance in the Asiatic and African 
elephants. The processes of enamel in the 
former species represent flattened ovals, 
placed across tlie tooth. In the latter they 
form a series of lozenges, which touch each 
other in the middle of the tooth. 
It does not appear that crusta petrosa is 
an essential part in the grinders of gramini- 
vorous animals. For those of the rhinoce- 
ros do not possess it, although the enamel 
descends into their substance, and forms a 
cavity, which is filled with the food, &c. 
Home and Blake likewise state, that it 
does not exist in the hippopotamus, where 
there are internal productions of enamel : 
but Mr. Macartney, the learned and inge- 
nious lecturer on comparative anatomy at 
St. Bartholoniew’s Hospital, has found it in 
small quantity on the exterior surface of the 
tooth, near its root. 
The want of satisfactory observations 
prevents us from saying much on the change 
of the teeth, particularly in wild animals. 
Some erroneous opinions of former times, 
as, for instance, that the domesticated pig 
changes its teeth, and that the wild animal 
does not, hardly require an express contra- 
diction in the present day. There is no 
animal of the class Mammalia, where the 
first appearance and subsequent removal 
of thfe deciduous teeth take place at so late 
a period of life as in man. 
