ANIMAL TISSUES. TEETH. 377 



germ cavity are terminated. By reflected light, hack-ground illumina- 

 tion, one perceives this stalactite-like condition of the inner surface of 

 the tooth-substance very distinctly, by means of the varied illumination 

 of the globular elevations, and by the shadows which they cast. Here 

 one has evidently to do with a stage of development of the tooth-sub- 

 stance ; for the older the tooth is, the less striking in general are these 

 conditions, and the more even is the surface of the wall of the germ- 

 cavity. In very old teeth considerable un evenness again makes its 

 appearance ; these, however, are not globular, but have a cicatrised, 

 distorted appearance. It is best to make the preparation from a tooth of 

 which the root is not perfectly completed. With such preparations, one 

 is readily convinced that the ground-substance of the last-formed layer 

 of the tooth-substance appears, at least partly, in the form of balls, 

 which are fused among one another, and with the balls of the penulti- 

 mate 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. The majority of th^se balls are 

 pierced through by one or more tubes, crosswise, passing from within 

 outwards. Very frequently, however, they appear homogeneous, and 

 contain no tubes." To obtain specimens, procure a tooth of which the 

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

 Ofien extremity, and scraping the inner surface, detach small portions, 

 which exhibit the globules admirably. 



"Another method of obtaining specimens which further illustrate 

 the internal surface of the dentine is the following : In rubbing down 

 a section of a tooth, as the operator approaches the pulp-cavity the 

 last fibre of dentine frequently bulges into the unresisting cavity ; and 

 instead of grinding up into particles, comes away in a small sheet, — a 

 little film of dentine parallel with the pulp-cavity's surface, the inner- 

 most layer, and the one last formed. This should be carefully preserved 

 and mounted. On viewing such a specimen by transmitted light, one 

 sees the globules scattered about — some isolated, others more or less 

 confluent ; and between them a pale, rather indefinite structure, uniting 

 the whole into a sheet. 



But the most instructive specimens are to be obtained from the 

 very thin cap of dentine found upon the foetal pulp. The thin edge 

 should be cut oflf, and examined on the inner surface ; it should be 

 moist, and never allowed to get dry. In such specimens the globules 

 are very apparent ; but, as Czermak observes, they do not appear 

 superficial, but in the substance of the dentine. It is here, in the 

 moist specimens, that the focus reaches the globules ; and, consequently. 



