540 
MR. TOMES ON THE STRUCTURE OF 
themselves and return to the pulp-cavity not far from whence they started. The 
canals in radiating from the pulp-cavity pass directly outwards and a little upwards 
parallel with the course of the dentinal tubes, excepting where they turn to reverse 
their course, in doing which they do not occupy more than twice or thrice their own 
diameter. I have not observed that they ever branch or anastomose with neigh- 
bouring canals ; many indeed so quickly return to the pulp-cavity, that there would 
scarcely be space enough for branching, while others advance a considerable distance. 
The dentinal tubes destined for the anterior part of the tooth commence with a 
diameter of about the 7500th of an inch and give off branches in the earlier part of 
their course, which connect themselves with the vascular canals. The disposition to 
branch ceases however after they have passed the vascular portion of the dentine, 
and is not resumed until they come near the enamel, when they form a plexus, towards 
the surface of which is a layer of elongated cells, placed obliquely both to the course 
of the tubes and the surface of the dentine ; the cells are shown in their relative po- 
sition in fig. 14. Previous to the formation of the peripheral plexus, the dentinal 
tubes make several curves in a contour line with the length of the tooth. This point 
is not shown in the figure, as it occurs internal to the part represented. The 
dentinal tubes of the posterior part of the tooth commence with a diameter of about 
the 6000th of an inch. 
The vascular canals measure from the 750th to the 2500th of an inch in diameter. 
Those situated near the worn surface are the smallest, from being lined or filled wfith 
a transparent tissue, into which the branches of the dentinal tubes do not penetrate. 
In the enamel we find a further deviation from the Sciuroid type than was observed 
in the Marmots. The layers of enamel fibres no longer lie at a right angle with the 
surface of the dentine, but are directed upwards at an angle of 60°, and moreover de- 
scribe a slight sigmoid curve, which terminates a little short of the centre of the 
enamel, from whence the fibres become parallel and proceed at an angle of 30° with 
the surface of the dentine. These appearances are best seen in a longitudinal section, 
and are shown in fig. 14E. In a transverse section made parallel with the course 
of the layers, as seen in a longitudinal section, the arrangement of the fibres composing 
the layers becomes apparent. Each layer is composed of a single series of fibres, 
which pass in straight lines alternated to the right and left in the adjoining layers, 
and produce an infinite number of minute square tracings over the inner portion of 
the enamel. When near the middle the decussation ceases, the fibres become parallel 
and proceed upwards and outwards ; but instead of proceeding directly outwards, 
they bear towards the median line of the skull, as shown in fig. 15. In examining 
the fibres in the terminal part of their course, a section must be made parallel with 
their length. 
The enamel fibres, as seen in a transverse section, measure about the 6000th of an 
inch, and may be traced to the surface. 
Professor Owen says, “ In a transverse section of the incisor (of the Beaver), the 
