THE DENTAL TISSUES OF THE ORDER RODENTIA. 
553 
to see that the enamel fibres pursue a serpentine course, and in the lamelliform por- 
tion describe three tolerably uniform curves (fig. 36 E.). Then again, by altering the 
focus of the microscope, without changing the position of the section, it may be seen 
that the fibres immediately above and below those already observed, pursue a similar 
serpentine course, but arranged so that the concavities and convexities point in op- 
posite directions, like the two sides of the figure 8. The appearance thus produced 
is shown in fig. 39, which in this particular must be regarded as a diagram rather 
than an accurate portrait. This crossing and re-crossing in curved lines, together 
with the cropping out of the curves of the contiguous fibres, produces a most complex 
appearance ; and if the section be not parallel with the length of the fibres, the struc- 
ture appears, to one who has not given attention to the subject, so confused as to defy 
explanation. If the section be so made as to expose in their length the enamel fibres 
of the middle of the anterior part of the tooth, those near the sides will be cut ob- 
liquely and present a penniform arrangement. An oblique longitudinal section will 
exhibit the fibres of alternate layers divided almost transversely, while those of the 
intermediate ones are exposed, taking an oblique course : this point is shown in fig. 37- 
In addition to the peculiarities already enumerated, small rounded cells are scattered 
through the enamel of this and most other Hystricine teeth. 
The dentine of the molar teeth of Hystrix cristata is peculiar in having its tubes 
unusually free from lateral branches during the greater part of their course. The se- 
condary undulations are strongly marked ; and on approaching the enamel, the 
terminal branches are comparatively few in number and large in size, and commonly 
pass the 750th of an inch into the enamel. 
Near the termination of the dentine in the root of the tooth, the dentinal tubes 
radiate from a number of centres, and the tissue graduates into the cementura, which 
is very abundant in this situation, and forms a considerable portion of the root. 
In a longitudinal section the enamel fibres are seen to proceed upwards and out- 
wards in parallel lines till near the surface, when they assume a similar arrangement 
to that seen in the incisor teeth. When the enamel is thin, the lamelliform arrange- 
ment is scarcely seen ; but where it exists in a tolerably thick layer, the confluent 
laminae commence near the dentine. It should be remarked that in the Hystricine 
molar teeth, the usual position of the fibrous and lamelliform divisions of the enamel 
is reversed. Hitherto we have found the fibrous enamel placed externally; but here 
and in the succeeding family, the lamelliform enamel occupies the external position 
in the molar teeth. 
Hystrix preliensilis (Linn.). — The incisors in microscopic structure closely resemble 
those of the common Porcupine. In the molar teeth, the dentinal tubes are con- 
tinued about the 1000th of an inch into the enamel. 
Dasyprocta Agutl (Linn.). — A longitudinal section of an incisor shows that tiie 
dentine of the anterior part of the tooth is bordered by a layer of obliquely placed 
elongated cells, about the 750th of an inch broad. The enamel is composed of con- 
4 B 
MDCCCL. 
