THE DENTAL TISSUES. 81 



are united to one another by an inosculation of the ter- 

 minal branches of the tubes in some few places, but more 

 generally by a clear layer containing radiate spaces, some- 

 thing like the lacunae of cementum. Hence Professor Owen 

 has described the tooth as consisting of radiating plates of 

 dentine, between which pass in equally convoluted plates 

 of cementum. But, as was pointed out by my father (Phil. 

 Trans. 1850), the mere presence of lacuna-like spaces is 

 not sufficient to prove the presence of cementum, inasmuch 

 as they occur on a small scale in the granular layer of 

 dentine ; moreover, when cementum and enamel are both 

 present, the cementum is always outside the enamel, whereas 

 at the upper part of the tooth of the Labyrinthodon the cha- 

 racteristic inflections take place within a common investment 

 of enamel which does not dip in. Thus the whole of the 

 tissue constituting the very complex pattern of the Laby- 

 rinthodon tooth is dentine, and the cementum does not, as 

 was usually supposed, enter into its composition at all. 



Another form in which plici-dentine may exist is exempli- 

 fied in the teeth of Myliobates, a large Ray ; or in the teeth 

 of the rostrum of the saw-fish (Pristis). 



In the Myliobates (Fig. 41) the flat pavement-like 

 tooth is permeated by a series of equidistant parallel 

 straight canals, running up at right angles to the surface ; 

 from the upper end and sides of these channels systems of 

 dentinal tubes radiate, just as the tubes radiate from the 

 single pulp chamber of a human tooth, save that they run 

 for a comparatively short distance. In transverse sections 

 the tubes are seen radiating from these channels, and at 

 their terminations sometimes inosculating with the terminal 

 branches of the tubes of another system. The channels 

 contain prolongations of the vascular pulp, which are 

 distinct in the upper part of the tooth, but intimately 

 united together at its base, where the disposition of the 

 channels ceases to be regular, and, as a consequence, the 



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