TRANSACTIONS OF THE SECTIONS. 145 



In the class Mammalia the teeth of the animals belonging to the 

 order called Edentata by Cuvier, present the nearest resemblance to 

 the vascular and organized structures above described in the teeth of 

 cartilaginous fishes. The close resemblance, in this respect, between 

 the teeth of Orycteropus and Myliobatis has already been alluded to, 

 but their outward form and mode of attachment are widely different. 

 The teeth of Orycteropus present the form either of a simple cylinder, 

 or of two joined laterally together. In these, as in the tesselated teeth 

 of the Rays, Cuvier had recognised a tubular structure ; but the tubes 

 described by that great anatomist were merely the medullary or pulp- 

 canals which run parallel with the axis of the tooth, at regular distances 

 from each other. These visible medullary canals, which are widest at 

 the base of the tooth, diminish at first rapidly, and afterwards very 

 gradually in diameter, and some of them divide dichotomously in their 

 course from the base to the grinding-surface of the tooth. Throughout 

 their course they send off at right angles and from every part of their 

 circumference the true calcigerous dental tubes. These tubes, at their 

 origin are 7-^th of a line in diameter, but quiciily diminish, as they pro- 

 ceed in a wavy course to the interspace which divides them from the 

 contiguous medullary canals and their systems of calcigerous tubes : the 

 tubes give off numerous branches, which form, near the boundary space, 

 a moss-like reticulation of extremely fine tubes. Nearly the whole ex- 

 tent of the medullary canal is occupied with a vascular pulp, and its 

 parietes near the base is likewise surrounded with a thin vascular cap- 

 sule ; the whole tooth is in fact composed of a closely-packed congeries 

 of slender prismatic elongated miniature simple teeth, each of which is 

 provided with its pulp and capsule, its medullary cavity, and its radiated 

 series of calcigerous tubes. The capsule of each component prismatic 

 'tooth becomes ossified at a little distance from the base of the tooth. 

 A transverse section of the whole compound tooth above this part pre- 

 sents a series of hexagonal, pentagonal, or tetragonal groups of cal- 

 cigerous tubes radiating from an elliptical space occupied by avascular 

 pulp, and separated from each other by a thin boundary line of bone 

 or ccementum, characterized by the presence of Purkingian corpuscles. 

 The vascular pulp, likewise, becomes ossified near the grinding-surface 

 of the tooth, and consequently a transverse section taken near this part 

 presents the centres of the radiation of the calcigerous tubes filled up 

 with bone or c<2me7itum. 



Bradypus didactylus. — The substance in the tooth of this species 

 which corresponds to the true ivory forms only a very thin layer, situ- 

 ated near the superficies of the tooth ; the central yellowish substance 

 of the tooth presents a number of coarse canals, about one-tenth of a 

 line in diameter ; these radiate in a beautiful manner from the upper 

 part of the pulp-cavity, those in the middle proceeding parallel to the 

 axis of the tooth, those at the circumference curving outwards. These 

 canals are unequal, presenting partial dilatations, which, however, are 

 sometimes, though rarely, discernible in the tubuli of human teeth ; 

 they give off numerous tortuous branches of different sizes, and these 

 open into very distinct calcigerous cells scattered about the interspaces 



VOL. vir. 1838. l 



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