26 
MR. 0. S. TOMBS ON THE STRUCTURE AND 
described these varieties of dentine in the clearest terms. For example, he says, “this 
form of dental bone presents the most evident similarity to proper bone. There are 
found in it medullary tubes (canals of the pulp) and medullary fibres (fibres of the 
pulp), round which groups of concentric layers have been formed ; from these the 
minute tubes radiate, which in the different layers are as it were pieced together, and 
in these layers concentric rows or rings of cells are again found, just as in bones.” 
And again, when describing the teeth of the Pike, he says of the large tubes that in a 
few recent teeth “ they contained a blood-red substance, and may hence be regarded as 
divisions of a cavity of the pulp.” (Nasmyth ‘ On the Teeth,’ p. 105.) 
As Professor Owen’s definitions and descriptions of vaso-dentine and osteo-dentine 
are generally accepted and followed, it is necessary to gather from his writings extracts 
which will show what precise meaning he attaches to the terms, and indeed little 
exception could be taken to his grouping of the varieties of dental tissue, were it not 
that his classification is not based upon, and indeed sometimes conflicts with, the 
evidence derived from a study of their development. 
Vascular-dentine is described by Professor Owen (‘ Odontography,’ p. xvii.) thus : 
“ The prolongation or persistence of cylindrical canals of the pulp cavity in the dentinal 
tissue, which is the essential character of vascular dentine, manifests itself under a 
variety of forms. In mammals and reptiles these canals, which I have termed 
‘ medullary ’ from their close analogy with the so-called canals of bone, are straight 
and more or less parallel with each other ; they bifurcate, though rarely, and when 
they anastomose, as in the Megatherium, it is by a loop at or near to the periphery of 
the vascular dentine. In the teeth of fishes, in which the distinction between the 
dentinal and osseous tissues is gradually effaced, the medullary canals of the vascular 
dentine, though in some instances straight and parallel and sparingly divided or 
united, yet are generally more or less bent, frequently and successively branched, 
and the subdivisions blended together in so many parts of the tooth as to form a 
rich reticulation. The calcigerous tubes sent off into the interspaces of the network 
partake of the irregular character of the canals from which they spring, and fill the 
meshes with a moss-like plexus. 
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“ If the first described modification of vascular dentine which forms the chief part of 
the teeth of the Sloth and Megatherium be regarded as a fourth dental tissue (i.e., 
enamel, cement, dentine, and this), this second modification of vascular dentine from its 
close resemblance to bone might be reckoned as a fifth ; in proportion, however, as it 
resembles bone, so, likewise, it approaches to the structure of cement.” 
At a subsequent page Professor Owen further subdivides vascular dentine into 
three varieties. The first in which all the medullary canals are parallel, each having its 
own distinct system of dentinal tubes, as in Pristis or Myliobates ; the second in 
which the anastomoses between the medullary tubes are more numerous, and the 
boundaries between the component denticles less distinct, as in Cestracion ; and 
