34 
MR. 0. S. TOMES ON THE STRUCTURE AND 
mention of this peculiarity can be made in a definition of vaso-dentine, as in the teeth 
of Pleuronectidse, which are obviously of the same type of structure, the dentinal tubes 
exist near to the tip of the tooth, as seen in fig. 10 .'* 
And although, as a rule, the outermost portion of the dentine is dense, and is not 
permeated by the vascular canals, this dense external layer is not found in all such 
teeth. For instance the teeth of Ostracion (fig. 10 ) are composed of this vaso-dentine, 
but the vessels extend right up to the surface of the dentine, and the thick and strongly 
coloured enamel which clothes their teeth takes the place of the dense dentine of the 
surface. The transition between vaso-dentine of the type just described, and the 
ordinary hard dentine of Mammalia teeth is tolerably gradual. Thus in the Pleuro- 
nectidse (fig. 10 *) we have a tooth the apex of which is composed of hard unvascular 
dentine with true dentinal tubes, whilst its lower two-thirds have abundant vascular 
capillary canals, but no dentinal tube. In Serrasalmo (fig. 1 1 and 1 1 *) we have a tooth 
the upper half of which consists of ordinary (at least for the present purpose ordinary) 
fine tubed dentine, and in it the dentinal tubes permeate the dentine of the base as 
well as that of the upper portion of the tooth. 
But near to the base of the tooth there are a few capillary canals ; by the suppression 
of these we should get ordinary unvascular dentine. The interpolation of capillary 
tracts in dentine is not unknown amongst Mammalia ; thus it is found in the Mega- 
therium, in the Tapir, and in the Manatee, though whether red blood really does circulate 
through them in the completed tooth is not definitely ascertained. From this variety 
of dentine, most appropriately called Vaso-dentine, the relationship of which to 
unvascular dentine has been shown, we pass to the consideration of a very distinct 
variety, which although hitherto universally known as vaso-dentine, has in reality 
very little relationship to that tissue. 
Of the teeth of the Pike, Ketzius (as quoted by Nasmyth, ‘ On the Teeth,’ 1839, 
p. 104) says, “ The dental bone itself in the Pike is properly divided into an internal 
kernel provided with large tubes, and into an external thinner part, which latter forms 
the covering of the first, and contains minute and parallel tubes. The large main 
tubes which occupy the internal more imperfect part of the dental bone, are in their 
widest part about -^ 5 - p.m. ( 9^0 °f an inch) in diameter. They run almost parallel 
with each other, and with the axis of the tooth, and form with each other numerous 
larger and smaller anastomoses. 
“ Near the base of the firmly fixed teeth, the larger transverse anastomoses are so 
near to each other that the interstices are scarcely as wide as the diameter of the 
large tubes. In some few recent teeth these tubes contained here and there a blood- 
red substance, and may hence be regarded as divisions of a cavity of the pulp. 
“ Here, too, those (larger tubes) which are near to the apex run almost parallel 
with the axis of the tooth ; but those which are nearest the root transversely to it, 
and so on. They divide at their commencement into bundles of larger and smaller 
branches, which enter into numerous reticular anastomoses with each other, but which 
