DIRECTOR   Deforrest Kovačević


The enamel of the teeth of most Marsupial animals presents this remarkable peculiarity, that a large number of the dentinal tubes are continued into the enamel, and constitute a considerable portion of it. This character is present in the teeth of all the species hitherto examined by Mr. Tomes (to the number of ten), excepting the Wombat; and it exists in very few other mammalian teeth, those (namely) of the British Shrews, the Hyrax, and the molar teeth of the Jerboa. A rudiment of it, however, is sometimes traceable in the teeth of man and of several other mammalia. In many marsupial teeth, also, the enamel is studded with small cells, often, but by no means always, arranged in contour lines. The enamel-fibres, too, are in many teeth so intimately united to each other, that their individuality is lost; and this occurs in most teeth in some parts, so that the dimensions of the fibres at such points cannot be taken. Mr. Tomes thinks that these characters may frequently afford valuable assistance in the discrimination of marsupial teeth ; and he also draws the inference " that the enamel and dentine are so closely related that the one should be regarded as a modification of the other, rather than as a tissue of a wholly different nature."—Philosophical Transactions, 1849, Part II. [Having ourselves had the opportunity of examining Mr. Tomes's very beautiful preparations, we can testify to the accuracy of his descriptions; and gladly take the opportunity of withdrawing the objection which we made to his statement of the occasional occurrence of this penetration in the human tooth, in our review of his 'Lectures on Dental Physiology and Surgery' (Vol. Ill, p. 179). We do not feel so well assured, however, of the soundness of his inference respecting the relation between the dentine and the enamel; and we think that, with our present views concerning the formation of the canaliculi of bone, it is not difficult to account for the penetration of the dentinal tubuli into the enamel. For if the canaliculi of bone be radiating extensions of cells, which tend to diverge from their centre, and spread themselves through the soft matrix at the time it is undergoing calcification, we think that the same will hold good of the dentinal tubuli, which may be conceived to be elongated cells, tending to lengthen themselves still more, so long as they have a soft matrix to spread in. Their nisus of growth is ordinarily expended by the time they reach the surface of the dentine; but in these instances they spread onwards, into the enamel, and seem to insinuate themselves between the enamel-prisms, just as the rootlets of plants find their way into minute crevices. In some instances, the dentinal tubes seem to expand into cells, between the dentine and enamel, so as to take (as it were) a fresh point of departure before they begin to extend themselves into the enamel. We have met with a precisely analogous case in the structure of certain bivalve shells, whose exterior layer is composed of prismatic cellular substance, analogous to enamel; whilst the interior is composed of a tubular substance, the analogue of dentine; some of the tubuli of the latter passing onwards distinctly between tlie prisms of the former. There was assuredly here no similarity in the origin of the two layers.]