LESSON 57.] THE STRUCTURE OF THE TEETH. 



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855. In the class of Reptiles, the Ophidians (Serpents) are en- 

 tirely destitute of enamel, and the same remark applies to nearly 

 the entire class of Fishes. In the latter, the dentine assumes new 

 characters ; in some Fishes the dentinal tubuli have their origin from 

 vascular canals, hence such a tissue is called vaso-dentine ; and in 

 other fishes the medullary canals are wavy, irregular, and prone to 

 anastomose (join), and is usually covered by a hard dentine ; this is 

 called osteo- dentine. 



856. The teeth of the Pristis (Saw-fish), and the Myliobatis (a 

 Ray), well display the vaso-dentinal structure, and as such have 

 been selected for illustration. 



857. In the Pristis the vascular canals are large, and remarkable 

 for their parallelism ; the tubes that are given off from the centre of 

 the tooth, are large in size, and few in number, but those canals 

 which approximate to the sides, give off a dense plexus of fine tubuli, 



FIG. 



FIG. 327. 



Longitudinal section of tooth, Myliobatis. 



Longitudinal section of tooth, 

 Pristis. 



distributed to the sides and crown of the tooth, and forming the 

 vaso-dentine. This is shown in Fig. 326 ; the vascular canals (a) 

 are seen in the centre, giving off (comparatively) but few tubes. 

 The lateral tubes (b) distribute the great plexuses of fine dentinal 

 tubes to the crown and sides ; there is no pulp cavity in this tooth. 



858. The vascular canals of vaso-dentine can only be well seen 

 in longitudinal sections; for this purpose such a section is given 

 of the tooth of Myliobatis (Fig. 327). Here the vascular canals 

 (a, a) are of great size, and, as is usual in this form of tissue, 

 parallel to each other ; the dentinal tubuli (b, b) are given off in 

 remarkably rich clusters of beautiful ramose tubuli, presenting, under 

 the microscope, a very fine appearance. 



