698 
PROFESSOR WILLIAMSON ON THE STRUCTURE AND 
ternal enamel, consists of an aggregation of very minute spherical granules, similar 
to those existing in the Cycloid scales of fish, and still more elosely resembling those 
seen in the external half of each osseous rod or prism in the rostrum of the Saw-fish, 
I am unable to detect any well-defined lacunse, with regular radiating canaliculi, 
such as are represented by Dr, Muller, It appears to me, that owing to the incom- 
plete coalescence of the minute granules, an irregular network of minute angular 
interspaces is left, varying in their size, and typically resembling those seen in some 
scales, and iri the portion of the Saw-fish just referred to. From these small inter- 
spaces, the twigs of the dentine-tubes take their rise. The same granular aspect 
of the calcareous intertubular strueture continues to be visible for some distance 
along the course of the tubes, but nearer the pulp-cavity the amalgamation of the 
granules has been so complete, excepting where the dentinal tubes have been left 
open, that all traces of what I have just described completely disappears, I suspect 
that this is the structure which caught the eye of Mr. Nasmyth, and led him, at the 
sixth meeting of the British Association, to declare that the intertubular portions of 
teeth were cellular. 
In the foetal condition of the tooth, according to Pukkinje and Raschow, the pulp 
is surrounded by a membrane, between which and the pulp, these observers believe 
the formation of the dentine to be carried on. It may be worthy of inquiry whether 
the calcareous structure is not developed in rather than beneath this membrane; a con- 
tinual growth of the latter at the pulp-surface, however thin and inappreciable, would 
supply all that was wanting to carry on the corresponding growth of the calcareous 
tissues of the tooth. But even should there be no evidence that this has been the 
case, and we are compelled to fall back upon the pulp itself as the matrix in which 
the calcifying process is carried on, it does not necessarily follow that the cells of the 
pulp are involved in the process. We have already pointed out the probability that 
the new membrane, lining the cavities produced by absorption of the cartilage in many 
fish, and designed to be ultimately converted into osseous cancelli, is probably nothing 
more than an altered condition of the intercellular portion of the cartilage. A similar 
state of things exists at the surface of the cartilage forming the pulp of the defence 
spines of the Picked Dog-fish, where some new lamellse are successively added to the 
interior of the kosmine structure. This suggests a second subject for investigation, 
viz. whether the soft blastema, in which the cells, vessels and nerves of the tooth-pulp 
are distributed, may not undergo a similar change prior to the calcification of its 
outermost portions. Whatever may be ultimately proved to be the true rationale of 
this process, there is no question whatever, that the various peculiarities attending 
the growth and development of kosmine, indicate the propriety of a careful review 
of the evidence upon which the generally received hypotheses respecting the produc- 
tion of dentinal tissues are based. 
Somewhat allied to the above subject is the collateral one respecting the true nature 
of what I have designated Haversian canals. The canals permeating the bones of 
