THE DENTAL TISSUES OF THE ORDER RODENTIA. 
545 
seen crossing each other at something short of a right angle ; in the inner and in the 
outer part of the enamel they are seen proceeding in straight lines to the surface, 
without inclining to either side. 
A thin layer of cement may be distinguished on the posterior part of the tooth, but 
is lost when it has passed over the terminal edge of the enamel. 
In the lower incisors of the Jerboa the enamel lamellae are arranged in the long- 
axis of the tooth, much in the same manner as in the corresponding teeth of the 
Dormouse. A longitudinal section will expose the decussation of the fibres of the 
contiguous layers, the one set proceeding upwards and outwards, and the other 
downwards and outwards. The fibres in the external division of their course lose 
the lamelliform arrangement and proceed upwards and outwards in a gentle curve 
(fig. 26). 
An oblique section in the length of the tooth will cut across the layers in their 
breadth, and show them arranged in lines parallel with the surface of the dentine. 
Each layer has in this view a denticulated margin, similar to the larnellse in the 
Rats. 
A transverse section of a lower incisor exposes the enamel lamellee divided trans- 
versely to their length. Those which arise from the median side of the tooth are 
directed forwards and towards the median line, while those that start from the an- 
terior and outer surface of the dentine, are directed in a curved line outwards in an 
opposite course. Each layer thickens slightly as it advances outwards, and exhibits 
slightly serrated margins. 
The component fibres have their long axes placed obliquely (fig. 28). Transverse 
sections made with different degrees of obliquity, will give variety in the appearance 
of the enamel layers. Thus a longitudinal section of one set of fibres, and a trans- 
verse section of another may be obtained, as partly shown in fig. 28. But if the sec- 
tion be oblique to each axis of the tooth, the enamel may present a confused inter- 
section of lines, from which little or nothing can be made out. This observation 
may be applied with equal truth to similar sections of other rodential teeth. 
The dentinal tubes in the lower incisor are oval in section, and have the long 
diameter placed transversely to the length of the tooth, hence they appear larger in 
the transverse than in the longitudinal section (figs. 26 and 28). In the molar teeth, 
the dentinal tubes leave the pulp-cavity for the crown of the tooth with a diameter 
of about the 10,000th of an inch, and are very closely packed. They give off very few, 
if any branches, till near the periphery of the dentine, and here they are far from nu- 
merous, as compared with those seen in other molar teeth. The branches are short 
and bristle-like, and the parent tubes, or a large branch, is usually continued into the 
enamel. The dentinal tubes of the fangs commence with a similar diameter to those 
of the crown of the tooth, but they speedily dilate to the 7500th of an inch, and give 
off branches daring the greater part of their course. 
Previous to entering the enamel the dentinal tubes are reduced to about the 
4 A 
MDCCCL. 
