542 
MR. TOMES ON THE STRUCTURE OF 
upwards, and from their commencement at the pulp-cavity give off rectangular 
branches which take a downward course. When near the enamel, the dentinal tubes 
bend upwards and emit branches from the convex surface only, as shown in fig. 16D. 
These are lost at the junction of the enamel and dentine, and terminate without the 
presence of peripheral cells, such as are found in the Beaver. 
The enamel in the inner part of the tissue is arranged in transverse layers, which 
in a longitudinal section are seen to proceed from the surface of the dentine at an 
angle of 73°. Each layer measures about the 6000th of an inch in thickness, and 
extends about half-way to the surface, where the component fibres of the different 
layers assume a more parallel arrangement. The layers in a favourable section have 
even margins, but if the section inclines slightly from the centre of the tooth in either 
direction, the margins will be a little irregular ; indeed this observation applies with 
equal force to similar sections of other rodential teeth. 
In a transverse section the contiguous layers cross each other at a right angle, and 
at places look as though plaited, as in the Sciuridae. In the outer division of the 
enamel, the fibres proceed in straight lines to the surface (fig. 17). An oblique sec- 
tion in the length of the incisor will expose alternate layers of straight fibres, and 
fibres divided transversely ; the appearances thus produced are delineated in fig. 16 A. 
On the whole, the enamel is more transparent and the structure is less distinctly 
marked than in any other incisor I have as yet described ; indeed the peripheral 
division in a longitudinal section frequently seems structureless. 
The enamel varies a little in thickness in different parts of the tooth, the average 
being about the 200th of an inch, the inner or decussating and outer or parallel being 
nearly equal in breadth. 
The rooted molars of the Spalax present a slight structural resemblance to those 
of the Sciuridae. The dentinal tubes leave the pulp-cavity with about the 7500th of 
an inch, which is preserved till they arrive near the enamel. In the upper and middle 
part of the tooth the tubes describe a sigmoid curve in passing outwards, and give 
off branches during the two outer thirds of their course. In a longitudinal section, 
the enamel is seen to be composed of fibres which pass upwards and outwards with 
a slight curve, and have a diameter of the 6000th of an inch. 
The cement resembles that of squirrels’ molars in being composed of rods arranged 
transversely to the length of the tooth. 
In the family Murids of Waterhouse*, a distinguishing character in the enamel 
runs through the various sections, excepting the three first, the fifth and seventh, and 
in the first of those (genus Myoxus) it exists partially. The teeth of the Dormice 
are in structure intermediate between those of the Squirrel-tribe and the Rat-tribe. 
The incisors of the Jerboas are however wholly different from any other members of 
this family, and the incisors of Bathyergina resemble those of the Hystricidse. The 
Spalax I have already described. 
* Johnston’s Physical Atlas. 
