The uncalcified part of the capsule, always much softer than cartilage, is very readily detached from the calcified part, and to the naked eye the separated surface seems entire, and might readily pass, as with Cuvier, for a secreting surface. But the fine vascular processes which have been torn from the medullary canals of the calcified part are conspicuous, and resemble villi, when the detached surface is examined, even with a moderate magnifying power, under water.
Calcification extending from the numerous centres, the different portions coalesce and progressively add to the thickness of the ce ment until all the interspaces of the coronal plates and the whole exterior of the crown is covered with the bone-like substance. The enamel-pulp ceases to be developed at the base of the crown, but the capsule continues to be formed pari passu with the partial formation of the pulp, as this continues, pro gressively contracting, from the base of the crown, to form by its calcification the roots. The calcification of the capsule going on at the same time, a layer of cement is formed in immediate connection with the dentine. The circumscribed spaces at the bottom of the socket to which the capsule and dentinal pulp adhere, where they receive their vessels and nerves, and which are the seat of the progres sive formation of these respective moulds of the two dental tissues, become gradually con tracted, and subdivided by the further localis ation of the reproductive forces to particular spots, whence the subdivision of the base into roots. The surrounding bone undergoes cor responding modifications, growing and filling up the interspaces left by the dividing and contracting points of attachment of the re siduary matrix. All is subordinated to one harmonious law of growth by vascular action and cell-formation, and of molecular decre ment modifying form by absorption. Me chanical squeezing or drawing out have no share in these changes of the pulp or capsule; pressure at most exercises only a gentle stimulus to the vital processes. Cuvier be lieved that there were places where the den tinal pulp and the capsule were separate from each other. I have never found such except where the enamel-pulp was interposed be tween them in the crown of the tooth, or where both pulp and capsule adhered to the perktsteum of the socket, below the crown.
Cuvier affirms that the number of fangs of an elephant's molar depends upon the number of points at which the base of the gelatinous (dentinal) pulp is attached to the bottom of the capsule; and that the interspaces of these attachments constitute the under part of the crown or body of the tooth, the attachments themselves forming the first beginnings of the fangs. True to his hypothesis of the forma tion of the dental tissues by excretion, he says * that the elongation of the fangs is pro duced by two circumstances : first, the pro gressive elongation of the layers of osseous substance (dentine) which force the tooth to rise and emerge from its socket ; secondly, the thickening of the body of the tooth by the addition of successive layers to its inner sur face, which, filling up the interior cavity, leaves scarcely room for the gelatinous pulp, and forces it down into the interior of the roots.
This pulling up of the fang on the one hand, and squeezing down the pulp on the other, are forces too gross and mechanical to be admitted in actual physiology to explain the growth of the root of a tooth or of any other organised product ; such modes of explanation were, however, inevitable in adopting the excretion theory of dental development.
With regard to the homologies of the com plex molars of the Proboscidian quadrupeds, a species of insight which may come to be deemed, in the course of anatomical science, as of equal import to the knowledge of the formative processes of parts, I must admit that the mere fact of the marked and dispro portionate increase of size of the first of the three last molars over its predecessor— the last of the first three that are developed — may appear but a feeble support to the analogical evidence on which, chiefly, I have classed the three last developed molars of the elephant, in a category distinct from that of their smaller predecessors. But the value of such indica tion and analogy will begin to be apparent when we examine the condition of dental development in the primeval forms of Pro boscidians. I have already shown that the typical character of the Diphyodont dentition was more closely and generally adhered to in the genera that existed during the oldest ter tiary periods in geology than in their actual successors : it became of course highly inter esting to inquire whether the miocene Mas todons, the earliest of the great Proboscidian quadrupeds of which we have any cognizance, manifested any analogous closer adhesion to type than their elephantine successors, and whether they would afford any actual proof of the true deciduous nature of the first, second, or third molars, by the development of a vertical successor or premolar. Cuvier first ascertained the fact, though without ap preciating its full significance, in a specimen of the upper jaw of the Mastodon angustidens from Dux, in which the second six-lobed deciduous molar was displaced by a four lobed or quadricuspid premolar developed above it and succeeding it vertically.* The same important fact was subsequently con firmed by Dr. Kaup in observations of the Mastodon longirostris of the miocene tertiary deposits of Eppelshehmt This satisfactorily proves the true deci duous character of the first and second molars ; and that the third molar in order of appear ance is also one (the last) of the de ciduous series, is indicated by the contrasted superiority of size of the ante-penultimate tooth, which I regard as the first of the true molar series.