534 
MR. TOMES ON THE STRUCTURE OF 
on to the edge of the enamel, where it speedily thins and is lost. The bright brown 
coloured part is very distinct in many teeth, and w^hen the section is thick and a little 
oblique looks like a distinct layer ; but if the section be reduced in thickness, all ap- 
pearance of a layer will be lost. The colour graduates insensibly into the enamel, and 
no defined line of separation between the coloured and colourless parts can be di- 
stinguished. The colour has the aspect of a stain, deepest at the surface, and fading as 
it proceeds inwards. In a favourable section, the enamel fibres may be traced 
through the coloured part to the surface of the tooth, as shown in many of the figures. 
In the molar teeth of continuous growth, the cement may be traced over the whole 
surface of the enamel, and as a thin transparent layer devoid of lacunae. In these 
teeth it is however separated from the enamel by a sharply-marked boundary line. In 
no instance have I found it graduated into the latter tissue in the manner Professor 
Owen supposes it to be in the coloured external portion of the enamel of the incisors. 
In the incisors of the Wombat, the cementum is continued over the enamel, but 
the two tissues are separated from each other by a sharply-marked line. The 
cementum is nowhere graduated into, or “blended with*” the enamel. It is more 
than probable that the thin transparent and almost structureless basement tissue 
of the enamel-pulp becomes calcified with the attached columns of the pulp itself, 
but this tissue is quite distinct from the cementum matrix, and exists in those teeth 
in which we have an external investment of cement. 
The rootless molars of rodents have generally a very close resemblance in structure 
to the incisors, especially in the structure of the enamel. But the rooted molars are 
less like the front teeth, and in many instances cannot be distinguished by their 
structure from other small teeth. In the Rat-tribe, however, the enamel near its 
terminal edge assumes an arrangement similar to that of the incisors. In those molars 
which have an intermediate character, the structure of the enamel in the upper part 
of the tooth resembles that of the rootless molars']-. 
* Odontography, page 405. 
t In describing individual teeth, it will be necessary to repeat frequently the same expression, I will therefore 
state once for all the meaning I attach to such terms. Thus by a vertical or longitudinal section of an incisor, 
I mean a section from back to front through the median line of the long axis of the tooth ; by an oblique section 
through the long axis of the tooth or oblique longitudinal section, I mean that the section should pass from the 
mesian side of the anterior to the outer side of the posterior surface of the tooth, or vice versd ; by longitudinal 
section from the anterior surface, I mean a section by which a portion of the anterior convex surface is removed 
with a portion of enamel at each end of the section ; by a transverse section, a section at a right angle with the 
long axis of the tooth ; by an oblique transverse section, or a section parallel with the surface in wear, a section 
crossing the long axis of the tooth obliquely from back to front. The surface in wear will be described as the 
upper surface, both in upper and lower teeth ; and the angle at which the enamel layer leaves the dentine, will 
be that formed between the enamel lamellae and the surfaces of the dentine immediately above them. 
When the thickness of the enamel and the dentine is given, the measurements will be taken from back to 
front, throuhg the centre of the long axis of a transverse section. 
Teeth obtained from the Zoological Society are indicated by the Society’s name being affixed to the name of 
the species. 
