THE DENTAL TISSUES OF THE ORDER RODENTIA. 
559 
posterior half, in addition to which the walls of the cavity have two deep indentations 
on the median side and one on the outer side of the tooth. From each of the angles 
a line of small vascular canals is continued a short distance into the dentine, which 
tissue is marked by numerous concentric and broad, but ill-defined and interrupted 
contour lines, which follow the involutions of the surface of the pulp-cavity. They 
seem to be produced by a greater density of the tissue in these than at other parts. 
The dentine has a generally diffused cellular appearance. The dentinal tubes have 
an irregular outline, and have small lateral branches throughout the whole of their 
course. When divided transversely they show thick and strongly-marked parietes, 
and have an internal diameter of about the 6000th and an external one of the 
3000th of an inch. On nearing the enamel, the tubes give off larger and more 
numerous branches, and ultimately terminate in small oval or rounded branching 
cells. 
In the upper incisors the vascular canals are more abundant in the posterior than 
in the anterior part of the tooth. The pulp-cavity is occupied near its apex with 
secondary dentine, the tubes of which proceed from the surface towards the centre 
and give off many branches ; when viewed by transmitted light they resemble tufts of 
moss. The enamel is strongly Hystricine in character. In a longitudinal section of 
a lower incisor the confluent laminse leave the dentine at an angle of 60°, which in 
the external division of the tissue is reduced to 25°. The enamel has a thickness of 
the 79th of an inch, of which jfths is lamelliform. The component fibres of the 
lamellae have a diameter of about the 7500th of an inch. 
The dentinal tubes in the molar teeth proceed from the pulp-cavity in nearly a 
straight course upwards and outwards at an angle of 20°. When within the 500th of an 
inch of the enamel, they turn a little more outwards and afterwards a little upwards, thus 
describing a small final curve, the convexity of which is directed towards the base of the 
tooth. But few branches pass off till the dentinal tubes make their final curve, when 
many leave both their convex and concave sides. Vascular canals are continued from 
the pulp-cavity into the dentine, as in the incisor teeth. The arrangement of the com- 
ponent fibres of the enamel in the molars is peculiar, and probably characteristic of the 
tooth of this creature. In a longitudinal section the fibres in the first two-fifths or 
half of their curve are straight and parallel, and lie at an angle of 40° with the surface 
of the dentine. Afterwards they suddenly fall into the confluent lamelliform arrange- 
ment, and with this disposition reach the surface of the tooth, as shown in fig. 48. 
In the folds which lie between the confluent denticles, the enamel is thinner than on 
the exterior of the tooth ; the loss of thickness is principally at the expense of the 
outer lamelliform portion. In a transverse section the fibres of the lamelliform por- 
tion are seen to make a open letter /'curve, which is reversed in direction in the con- 
tiguous layers, but parallel, or nearly so, in the alternate ones. The appearances seen 
in this section are delineated in fig. 49. Both in the transverse and longitudinal 
sections the lamelliform portion of the enamel is crowded with small branchless cells. 
