Development of vascular dentine. 
35 
most externally give off very beautiful, close, parallel, generally straight tubes of 
about from - r5 Vo to p.m. (23 oof) of ai1 indl in breadth ; ” amongst the latter 
Retzius could discover neither branches, anastomoses, nor cells. 
“ This most external stratum of dental bone gives to the transverse sections of the 
tooth of a Pike a peculiar and pretty appearance, and resembles, slightly magnified, 
a layer of enamel. 
“ This minutely tubular external portion of the dental substance is of the purest 
white, and is also much harder and more compact than the interior of the dental bone. 
To judge by the hardness of the surface in dried teeth, Retzius would have concluded 
that it was invested with an extremely thin layer of enamel ; but he could not detect 
any with the microscope, although accurate authors have asserted that it is present on 
teeth of the Shark.” 
Professor Owen adds nothing to Retzius’ description — there is, in fact, little to be 
added, as it is both accurate and comprehensive. But as I wish to emphasize the 
points of difference between this and the vaso-dentine just described, I will add a 
few words to it. 
Dentine of the Pike . — Both in its structure and in its development this differs 
markedly from the form of vaso-dentine first described ; in fact it is a misnomer to 
call it vaso-dentine at all. The dentine is divisible for descriptive purposes into two 
portions : an outer which is permeated by numerous fine tubes perpendicular to the 
surface, like the tubes of ordinary unvascular dentine, and an inner which is of much 
coarser structure and is permeated by large irregular spaces having a general longi- 
tudinal direction (see fig. 13). 
The tubes of the outer layer are parallel and end apparently short of the surfaces ; 
they are about 2 -oeroo i 11 diameter. 
They spring, through the intervention of short branches of intermediate size, from 
the larger spaces of the coarse core of the tooth (see fig. 14). These latter form 
longitudinal canals of varying diameter and irregular form ; they give off from their 
sides and ends branches which abruptly subdivide and become small ; at the point 
where these merge into the fine dentinal tubes of the exterior they lose their tubular 
form and are dilated into irregular spaces of small size, like small bone lacunse or 
the interglobular spaces of the granular layer of human dentine. From these spaces 
originate the dentinal tubes (see fig. 14). 
There is thus a strongly marked difference between the dentine of the Pike and 
that of the Gadidae, even when the structure of the hard tissues of the tooth alone is 
considered, but the distinction between the two varieties becomes yet more marked 
when the relation of the soft parts to the dentine is also taken into account. 
For the larger longitudinal canals of the Pike’s tooth do not, except as a matter of 
accident, contain capillary blood -vessels ; that is to say, the dentine as it is formed is 
not deposited round capillaries, so as to enclose them within itself, and hence very 
few of the channels do contain capillaries ; when they do, the capillary only forms a 
F 2 
