A third kind of complication is produced by an aggregation of many simple teeth into a single mass.
The examples of these truly compound teeth* are most common in the class of Fishes, but the illustration here selected is from the Mammalian class. Each tooth of the Cape Ant-eater (Orycteropus) presents a simple form, is deeply set in the jaw, but without dividing into fangs; its broad and flat base is porous, like the section of a com mon cane. The canals to which these pores lead contain processes of a vascular pulp, and are the centres of radiation of as many inde pendent series of dentinal tubules. Each tooth, in fact, consists of a congeries of long and slender prismatic denticles of dentine, which are cemented together by their ossified capsules, the columnar denticles slightly de creasing in diameter and occasionally bifur cating as they approach the grinding surface of the tooth.
A figure of a longitudinal section of the molar teeth is given in Pl. 76, fig. 10. of my " Odontography," and a magnified view of a similar section in Pl. 77. ; fig. 555. gives a magnified view of a portion of the transverse section of the fourth molar, showing c the cement ; d the dentine ; p the pulp-cavity of the denticles ; and d' a section of one of the denticles just beyond its bifurcation.
The pectinated incisors of the flying Lemur of the Indian Islands (Galeopithecus) are ex amples of teeth, the crowns of which are composed of denticles consisting of hard den tine, with a covering of true enamel. The layer of cement over this is too thin to show its characteristic structure, and does not fill up the intervals of the denticles, which stand out as free processes from the base of the crown. Tubular prolongations of the pulp cavity are continued up the centre of each denticle.
Fig. 556. exhibits a longitudinal section, magnified, of this kind of compound tooth; d is the dentine; e the enamel ; p the pulp cavity. The originally detached summits of the crown of the human incisor are homo logous with these columnar processes, or denticles of the incisor of the Galeopithecus.
In the compound molars of the great Afri can wart-hogs (Phacochcerus) the columnar denticles are in three rows, and their inter spaces are filled up by cement: each denticle consists of a slender column of hard dentine inclosed in a thick sheet of enamel, the whole being bound together by the cement ; and the denticles, as in the Galeopithecus, blending together into a common base in the fully-de veloped tooth.
long diameter of the tooth. When the tooth is bisected vertically and lengthwise, the three substances, d dentine, e enamel, and c cement, A figure is given of the grinding surface of the third true molar of the Phaeochcerus Pal are seen interblended as in fig. 557., in which p is the common pulp-cavity, and r one of the roots of this complex tooth.
A still more complex grinding apparatus is found in certain fishes. The lower pharyngeal bone of the parrot-fish for ex ample, supports a dental plate with a tri lasii, in Pl. 140, fig. 4, of my " Odontography." In the elephant the denticles of the pottlid molars are in the form of plates, vertical to the grinding surface and tranverse to the turating surface like that of the compound molars of the Phacoelurrus. The interlocked upper pharyngeals (fig. 565.) support dental masses with a grinding surface more like that of the compound molars of the elephant.
When a vertical and longitudinal section is made of one of these upper pharyngeal compound teeth, each denticle is seen to be composed, as in fig. 558., of a body of very hard and unvascular dentine d, with a thick sheath of enamel e, the denticles being united together by the cement c, and supported and further united together, and to the pharyngeal bone, by a basal mass of vascular osteo-dentine.
Such are some of the prominent features of a field of observation which comparative anatomy opens out to our view ;—such the varied nature, and such the gradation of complexity of the dental tissues, which, up to December 1839#, continued, notwithstanding successive approximations to the truth, to be described in systematic works as a " pha neros," or " a dead part or product exhaled from the surface of a formative bulb!" The truth may be slowly but is surely established, subject to the usual attempts to mask or detract from the merit of the discovery. By no systematic authors has the hypothesis of the formation of dentine by transudation or secretion been more frequently or more ex plicitly enunciated than by the Cuviers. Baron Cuvier repeats, in both editions of his elaborate work—the " Ossemens Fossiles " —" C'est dans ce vide concevable que-se de poseront les matieres qui doivent former la dent, savoir : la substance vulgairement ap pelee osseuse, qui sera transudee par des pro ductions gelatineuses venant du fond de la capsule, et l'email qui sera depose par les cloisons membraneuses," t. ii. p. 61., ed. 1812.; t. i. p. 33., ed. 1821. See, also, M. F. Cuvier, " Dents de Mammiferes," 8vo, 1825. " L'ivoire se depose par couches concentriques," p. xxvii. ; " L'email se depose dans un sens contraire a l'ivoire," ib. p. xxviii. And Baron Cuvier again, in the second edition of his d'Anatomie Comparee," t. iv. 1836, p. 214: "L'ivoire se depose par couches, par une sorte de transudation." In the first edition of this classical work, Cuvier had illustrated the peculiarity of the teeth of certain fishes, which are at first detached and afterwards united to the jaw-bone, by com paring their growth to that of the epiphyses of the long bones : " Mais les dents qui ne tiennent qu'h la gencive seulement, comme celle des Spades, croissent a la maniere des epiphyses des os, c'est-h-dire que toute leur substance osseuse est d'abord tendre et po reuse, et qu'elle se durcit uniformement, et finit par devenir entierement dure comme de l'ivoire," t. iii. 1805, p. 112. Whether the
great anatomist meant to imply that the osseous tissue of the epiphyses of bones was developed differently from osseous tissue in general, e. g. by the uniform and simultaneous hardening or calcification, obscurely referred to in the above quotation, may be questioned, for such is not the way in which the teeth of the shark are calcified. But this is certain, that the idea, whatever it might have been, had no influence on the fixed belief of the developement of the dental tissue by trans udation expressed in their later and more elaborate works by Baron Cuvier and his ac complished brother ; and, in point of fact, the passage which I have quoted is expunged from the second edition of the " Lecons d'Anatornie Comparee," 1835: the successive stages of calcification in the different teeth of the same vertical series in the jaw of the shark, having probably been noticed in the interim by Cuvier. The author of the article " Secretions" in the "Dictionnaire Universe' d'Histoire Na turelle," has, however, reproduced Cuvier's ob scure comparison of certain fishes' teeth to the epiphyses of bone, as evidence of the need lessness of any ulterior researches for the demonstration of the theory of dental de velopement by conversion and calcification of the pulp. The passage from the third vol. of the old edition (1800) of the " Lecons d'Anat. Comp.," p. 112, is cited to show that it naturally conducts to the knowledge of such mode of developement of dentine : " En 1840 et 1841 (the Comptes Rendus del'Acad. des Sciences' give the true date) l'etude des dents de Squale par M. R. Owen, lui a de montree leur accroissement par intussuscep tion, comme elle avait ete a G. Cuvier trente cinq annees auparavant." How or why G. Cuvier came to abandon the theory so demon strated, and how it happened that none of his contemporaries adopted it, M. Duvernoy does not explain. He does give a reason for the omission, in the second edition of the " Lecons d'Anat. Comp." of the passage which he affirms to contain the demonstration : " Malheurehse ment, le copiste de cet ancien texte pour la 2de edition a omis ce passage, par oubli." It was natural to conclude that its obscurity and seeming contradiction to the theory of dental developement, formally propounded by Cuvier, as well as to the facts shown by nature in the sharks, had been the cause of its omission ; hut even had the misfortune to which M. Duvernoy now attributes that omission (for in the copious list of addenda and corrigenda to the fifth, 1837, and final, 1846, volumes it is not noticed) not occurred, the coincidence of such passages as the fol lowing would still have been inexplicable and irreconcilable with the deductions that M. Dumeril is now enabled to draw from the comparison of the shark's tooth with the epiphyses of long bones. " L'ivoire se depose par couches, par une sorte de transudation." Lecons d'Anat. Comparee, t. iv., 1836, p. 214. To which proposition Cuvier has himself added a note : " Je me suis assure recemment, sur des germes de dents d'elephant, que la substance osseuse de la dent se forme comme les coquilles." And the editor (M. Duvernoy), in order to obviate any possibility of miscon ception, has himself subjoined a note to that passage, as follows : " L'iroirc a 6t6 aussi appelle substance osseuse, h cause de son analogie de composition chirnique et de durete avec les os. Mais la nature inerte et inorganique de cette substance, mieux appreciee dans ces dernicrs temps, surtout par les travaux de M. Cuvier, ne permet plus de la designer, avec justesse, par cette seconde expression. Du moins est-il necessaire de premunir le lecteur contre l'idee fausse pourrait en tirer, qu'elle strait organisee, qu'elle se develop perait a la maniere des os."—Tom. cit. p. 201, (1836). In the same spirit in which M. Du vernoy sees (in 1848) that a true idea, instead of a false one, may be drawn from casual expres sions and similies loosely applied in the old Le cons of 1800 and 1805 ; others have sought to depreciate the value of the establishment of the truth by citing the doubts, or tentative approxi mations made by Purkinje and Schwann to my theory, interpreting such approximations by the light of the established truth. So far from finding such a resting-place for doubt in Cuvier's early simile, cited by M. Duvernoy in 1848, or in the interrogatories of Schwann, nothing short of the investigation of the whole of this vast subject, zootomically, develop mentally, and microscopically, as narrated in my " Odontography," sufficed to settle my own doubts ; and nothing short of the evidence and illustrations given in that work appeared to me adequate to convert anatomists from the excretion-hypothesis to the intussuscep tion theory.