THE DENTAL TISSUES OF THE ORDER RODENTIA. 
535 
In the genera Sciurus and Pteromys, I have examined the dental tissues of many 
species, and find the teeth so like the one to the other, that the microscope affords no 
aid in the distinction of species. Under these circumstances it will be necessary to 
describe minutely the structure of the component tissues of the teeth of one species 
only. 
Sciurus niger, Linn. (Zoological Society). — The anterior surface of the incisors is 
coated with an extremely thin layer of enamel, scarcely exceeding in thickness the 
428th part of an inch. The dentine measured from the front to the back, is about 
the 8th part of an inch in thickness. A longitudinal section, taken from the centre 
of an upper incisor, exhibits the dentinal tubes in their length. Those destined for 
the anterior half commence at the surface of the pulp-cavity, and proceed with slight 
secondary undulations, and a decreasing calibre upwards towards the enamel. In 
the earlier part of their course, a few short, minute, rectangular branches are given 
off, and are soon lost ; but when nearing the enamel, the tubes break up into a lash 
of branches, which pass onwards with but slight divergence, and after becoming ex- 
cessively minute, are lost at the juncture of the external tissue ; a few, however, ter- 
minate by forming loops. The dimensions of the dentinal tubes vary at different parts 
of the same tooth. At the lower part of the pulp-cavity the tubes destined for the 
anterior and posterior surfaces have a diameter of the 6000th of an inch, while those 
that proceed from the upper and narrowed part of the pulp-cavity, and from the cen- 
tral line of the solid part of the tooth, seldom exceed the 14,000th of an inch, and are 
often reduced to a scarcely perceptible line. Usually they do not preserve this small 
size, but quickly dilate to the 10,000th or 12,000th of an inch; these conditions are 
not however peculiar to the teeth of this group. The tubes that form the posterior 
part of the incisors give off branches pretty freely throughout the whole of their 
course, and when near the surface break up into a rich plexus of anastomosing tu- 
bules, in addition to which they occasionally dichotomize. Many of the earlier 
branches are sm.all and short, but a few are large, go off at a right angle, and may 
be traced for some distance crossing the course of the neighbouring tubes. In the 
median line of the sides of the tooth, independent of the secondary curves or undula- 
tions, the dentinal tubes pursue a tolerably straight course, as do those in the median 
line of the anterior and posterior surface. But the dentinal tubes of the anterior 
halves of the sides describe one large curve, the convexity of which is turned towards 
the anterior surface of the tooth ; and those composing the posterior part of the tooth 
follow a similar curve, the convexity of which is directed towards the posterior sur- 
face ; similar relations between the tubes and the several parts of the incisor may be 
observed in the teeth of other rodents. 
Professor Owen*, when treating on the teeth of rodents, says, ‘‘The substances of 
the incisor diminish in hardness from the front to the back part of the tooth ; the 
enamel consists of two layers, of which the anterior and external is denser than the 
* Odontography, page 399. 
