THE DENTAL TISSUES OF THE ORDER RODENTIA. 
547 
near their termination, when they form a plexus, but less dense than that in the ante- 
rior part of the tooth. 
The enamel is seen to be composed of fibres arranged in serrated lamellae, which 
leave the surface of the dentine with a short but gentle curve, and then proceed 
upwards and outwards in nearly a straight line till about to terminate in the outer 
division of the enamel, when they make a slight sigmoid curve with the terminal 
portion directed upwards, their general course being at an 'mgle of 50° with the sur- 
face of the dentine (fig. 30). 
At the outer fifth of the thickness of the enamel the layers are broken up, and 
their component fibres take a parallel course upwards and outwards, and with a 
slight curve, which is strongest near the outer extremity, when they reach the 
surface of the tooth, their general courses being at an angle of 30° with the surface 
of the dentine. 
Each lamella commences at the surface of the dentine with a diameter of about the 
7500th of an inch, which gradually increases till at the distal extremity it attains the 
6000th of an inch. The margins of the lamellee are strongly serrated, and the pro- 
jections of one layer fit into corresponding depressions in the contiguous lamellae. 
This peculiar form and arrangement, as exhibited in a longitudinal section, may be 
regarded as typical of the enamel of this family of Rodentia. 
In an oblique transverse section cut nearly parallel with the worn surface of the 
tooth, it may be seen that each layer is composed of a single series of enamel fibres, 
and that the fibres of contiguous layers cross each other at such an angle as to produce 
a diamond pattern over the lamelliform portion of the enamel ; and also that the fibres 
curve a little in their course outwards (fig. 31). The upper and lower surfaces of the 
component fibres of each lamella appear to be a little uneven, and these irregularities 
no doubt contribute to the development of the serrations observed in the longitudinal 
section of the tooth, but in the section now under consideration these appearances are 
seen but obscurely. When the crossing sets of fibres have arrived within the 750th 
of an inch of the surface, the decussation ceases and they proceed outwards in parallel 
lines directed obliquely towards the median line of the skull ; and in the latter 3000th 
of an inch of their course they turn directly out’:/:-rds in a line with the long axis of 
the section. 
If a section be so made as to display one set of fibres in their length, and the ad- 
joining ones divided transversely, the latter will exhibit an oval section, while the 
former are seen to be armed with minute processes which fit into the small interspaces 
that would otherwise be left between the non-touching surfaces of the oval fibres. 
These conditions are shown in fig. 32. To these lateral processes the serrated mar- 
gins of the lamellee, as seen in the longitudinal section, are due, as are the irregularities 
of surface seen in the transverse section. 
The outer ends of the enamel fibres are best seen in the middle and lower part of 
the tooth. Near the cutting edge, in the outer portion of the enamel, the union between 
4 A 2 
