19 



THE MICROSCOPE. 



Fig. 344.— Transverse section of Tooth oj 

 Myliobates, Eagle-ray, viewed as an 

 opaque object to show its radiating 

 librous structure. 



substance of the last-formed layer of the tooth-substance 

 appears, at least partly, in the form of balls, which are 

 fused one among another, and with the balls of the penul- 

 timate layers ; and one also perceives that in general their 

 diameter becomes less and less, somewhat in the form of a 

 point, towards the periphery of the tooth-substance. To 

 obtain specimens, procure a tooth of which the fang is 

 half-grown; then introduce the point of a penknife into its 

 open extremity, and scraping the inner surface, detach, 

 small portions, which exhibit the globules admirably." x 



The cementum is the 

 cortical layer of osseous tis- 

 sue, forming an outer coat- 

 ing to the fangs, which it 

 Bometimes cements toge- 

 ther. It commences as a 

 very thin layer at the part 

 where the enamel ceases, 

 and increases in thickness 

 towards the ends of the 

 fangs. Its internal surface 

 is intimately united with the dentine, and in many teeth, 

 it would appear as if the earliest determined arrangement 

 of the fibres of the dentine started from the canaliculi, as 

 they radiate from the lacunae in the cement. The inter- 

 lacunar layer is often striated, and exhibits a laminated 

 structure : sometimes it appears as if Haversian canals 

 were running in a perpendicular direction to the pulp 

 cavity. The canaliculi frequently run out into numerous 

 branches, connecting one with another, and anastomising 

 with the ends of the dentine fibres. The thick layers of 

 cement which occur in old teeth show immense quantities 

 of aggregated lacunas of an irregular and elongated form. 

 Professor Owen believes that by age the pulp ceases to pro- 

 duce or nourish the dentine, which then becomes converted 

 into osteo-dentine, and thereby the layer of crusta is so 

 much increased as often to fill up the pulp cavity of the 

 tooth. Professor Simonds assures us that this is not the case 

 in the Herbivora. For instance, in the horse, the oblitera- 



(1) Czermak: translated by James A. Salter, M.B., Quarterly Journal o§ 

 Microscopical Science, July, 1S23. 



