MICROSCOPIC STRUCTURE. 513 



The cap of enamel with which the teeth of the Walrus are at 

 first tipped is soon worn off, and, except at the abraded surface, the 

 rest of the teeth, both tusks and molars, are thickly coated with 

 cement. Retzius has left little to add to his detailed and exact 

 account of the microscopic structure of both the canines and molars 

 of the Walrus.(l) 



The dentine, in this species, closely corresponds with that in 

 the ordinary Seals ; in the molar teeth the tubes present the same 

 diameter, the same interspaces, and undulating curvatures ; but their 

 dichotomous divisions are more marked. In the canines the lateral 

 branches terminate in minute opake cells, dispersed throughout 

 almost the w^hole dentine, but most numerous and conspicuous near 

 its periphery, wdiere the dentine is defined by a distinct layer of these 

 cells : only a third part of the periphery of the canine is composed of 

 true dentine, the central thin part of the tooth is filled up by osteo- 

 dentine, which, as in the teeth of the Cachalot, often projects in 

 irregular rounded masses, like stalactites, into the short and wide 

 basal pulp-cavity. The whole mass, indeed, of the osteo-dentine 

 consists of numerous independent calcifications of the pulp, having as 

 many distinct centres, usually hollow, and producing, when the sub- 

 stance is examined by the naked eye, the appearance which Cuvier 

 has compared to ' pudding-stone.' (2) The central cavities are for 



(1) Loc. cit. p. 35, 73. 



(2) " L'ivoire des defenses du Morse est compact, susceptible d'un poll presque aussi beau 

 que celui de I'hippopotame mais sans stries ; la partie moyenne de la dent est formee de petits 

 grains ronds places pele-mele, comme le cailloux dans la pierre appelee poudingue ; c'est ce qui 

 le caracterise. Les dents molaires de cet animal ont leur axe compose des memes petits grains 

 que celui des defenses. Elles n'ont aucun cavite dans leur interieur." Cuvier, Logons d'Anat. 

 Comparee, torn, iii, (1805) p. 106. The cement of the Morse's teeth is distinguished from the 

 osteo-dentine by its continuous uniform structure, by the absence of the detached centres and 

 their concentric lines ; but the radiated cells are disposed in regular layers concentric and 

 parallel with the contour of the body of dentine. The radiating tubes from the cells forming 

 the layer next the dentine commimicate freely with the peripheral ramifications of the dentinal 

 tubes and also with the proper cemental tubes which are dispersed vertically to the plane of the 

 cement : indeed, the evidence of an intercommunicating system of canals, too minute for the 

 gross fluid of the circulating system, is most striking and universal throughout the substance of 

 both the tusks and small teeth of the Walrus. Vascular canals are, however, present in the 

 cement as in the osteo-dentine, from the contents of which it may be presumed that the 

 colourless plasma is elaborated, which meanders through the minuter systems of cells and 

 tubes. 



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