131 
ON THE NATURE, VARIETIES, AND DEVELOPMENT OF TEETH 
IN THE CLASS MAMMALIA. 
By Beverley R. Morris, Esq. 
The meaning of the term tooth being so very generally known, I need not, I 
imagine, give any precise definition of it here: it will be enough to state, that 
the term tooth is usually understood to apply to such bones as are naked, and 
placed, partially at any rate, within the cavity of the mouth. This is sufficiently 
accurate for my purpose, though I am aware that there are deviations from this 
rule in some of the lower classes; with these, however, I have nothing at present 
to do. Teeth in general have only two substances entering into their composition, 
viz., bone sometimes called ivory, and enamel; some, however (as the Grami¬ 
nivorous animals), have a third ingredient, called crusta petrosa , wdiich I shall 
consider hereafter. I shall not enter into the details of their chemical composi¬ 
tion, as I consider that would be a subject more properly discussed in a treatise 
on Animal Chemistry. 
Teeth are formed, not like other bones, on cartilaginous masses, but are the 
secretions of what are called pulps; these are vascular bodies placed in the jaw¬ 
bones, and plentifully supplied with nerves and arteries, derived from those which 
ramify through the jaw or maxillary bones. The mode of their formation is as 
follows:—The pulp at the appointed time begins to throw out lamina of bone, 
the fibres of which are concentric; one of these is deposited on the upper part of 
the pulp, another layer is added underneath this, and so on, not in but on the 
pulp.-—The pulp is encircled as far as the-part that is to correspond to the crown 
of the tooth by a serous capsule, forming a cul-de-sac , and therefore the tooth 
is not absolutely within the capsule, though it is encircled by it. From the 
side of the capsule next the pulp the enamel is secreted ; this falls on the already 
formed bone in lines corresponding to the axis of the tooth, thereby giving it great 
strength. The pulp only secretes lamina of bone at the top to a certain thick¬ 
ness, and then proceeds downwards at the sides, and so goes on to form the root, 
leaving a hollow space throughout the substance of the tooth; this is, of course, 
filled by the pulp. The root, as it continues growing, presses against the jaw¬ 
bone below, but as it cannot advance in that direction, it pushes upwards and 
causes absorption, first of the bone above it, and afterwards of the gum, and thus 
at length it is protruded through the skin. When the tooth cuts the gum it has 
broken through its capsule, and remains exposed to the air, and uncovered by 
any membrane as all other bones are. Molar or double teeth, as they are not 
inaptly called, are formed on three, four, or more of these Pulps, which are 
united above but separated below, thus forming the fangs. In the Cow, Horse, 
vol. hi.—no. XVIII. 
T 
