148 EIGHTH REPORT — 1838. 



mosiiig and communicating with minute calcigerous cells immediately 

 beneath the enamel. 



The teeth of those orders of Mammalia in which they present the 

 usual structure of compact ivory, enamel, and ccementum, have been 

 described in several genera with so much accuracy by Professor 7?ctems, 

 that there are few modifications or examples worthy of particular at- 

 tention. 



In the simple teeth of the Marsupial animals, the external layer of 

 cmmentiim covering the enamelled crown is thicker in many of the 

 species than is usually seen. The Phalangers, Koala, and Wombat, 

 offer good examples of the superficial layer of cement on the exposed 

 crown. It possesses the usual high degree of organization, and abounds 

 in the Purkingian cells. 



In the incisors of the Orang-Utan, the main calcigerous tubes of the 

 ivory, which radiate from the central cavity of the pulp, are somewhat 

 larger than those of man ; they present the same primary curvatures, 

 but less numerous and less strongly-marked secondary undulations*. 

 In the crown of the tooth of the Orang, the dental tubes are chiefly 

 branched at their extremities, while towards the apex of the fang the 

 main tubes are surrounded by exceedingly fine and close-set branches, 

 which subdivide in their course. The nearer the crown, the larger are 

 these branches ; they are curved, with the concavity towards the pulp. 

 In the summary of this series of observations which Professor Owen 

 detailed, he observed, that in the human and similarly organized teeth, 

 the analogy of ivory to bone, as to texture, was only seen in the ex- 

 istence and intercommunication of the minute calcigerous tubes and 

 cells ; but that there was no trace of medullary or Haversian canals, 

 with their characteristic concentric laminae, unless the entire tooth 

 were regarded as analogous to a single enlarged Haversian canal, when 

 the cavity of the simple pulp would represent the medullary cavity of 

 the canal ; while the tubes, with the appearance of laminae occasioned 

 by their undulations, might be deemed equivalent to the concentric la- 

 mellae and the calcigerous tubes, which, in bone, traverse these lamellae, 

 and radiate from the Haversian canal. In tlie teeth of many of the 

 lower animals, however, and especially that of the extinct Acrodus, 

 amongst the cartilaginous fishes, the resemblance of the dental tissue 

 to bone was extended to the existence of the characteristic Haversian 

 canals in great numbers. The presence of these canals was explained 

 by the progress of the development of these bone-like teeth, as observed 

 by Professor Owen in recent cartilaginous fishes. The large pulp, at 

 the commencement of the foi'mation of the tooth, had exei-cised its 

 ordinary function in the secretion of a close-set series of calcigerous 



* Tlie primary curvatures Professor Oweu explained to be those which belong to 

 the general course of the dental tube, and which are seen with a lower power ; in 

 man they resemble the curves of the Greek Zeta (Z). The secondary curves are mi- 

 nute undulations in the whole course of the tube, requiring a high power for their 

 perception, and affecting both the main trunks and their branches ; these prol)ably 

 indicate and are due to the movements of the formative pulp during the deposition of 

 the ivorv. 



