MINUTE STRUCTURE OF RONE AND TEETH. 697 



that it may be seen to advantage even when mounted dry. As 

 the polishing, however, occupies much time, the benefit which is 

 derived from covering the surfaces of the specimen with Canada 

 balsam may be obtained, without the injury resulting from the 

 penetration of the balsam into its interior, by adopting the follow- 

 ing method : — a quantity of Balsam proportioned to the size of the 

 specimen is to be spread upon a glass slip, and to be rendered 

 stiffer by boiling, until it becomes nearly solid when cold; the 

 same is to be done to the thin -glass-cover; next, the specimen being 

 placed on the balsamed surface of the slide, and being overlaid by 

 the balsamed cover,- such a degree of warmth is to be applied as 

 will suffice to liquefy the balsam without causing it to flow freely ; 

 and the glass cover is then to be quickly pressed-down, and the 

 slide to be rapidly cooled, so as to give as little time as possible 

 for the penetration of the liquefied balsam into the lacunar 

 system. — The same method may be employed in making sections of 

 Teeth.*' — The study of the Organic basis of Bone (commonly, but 

 erroneously, termed Cartilage) should be pursued by macerating a 

 fresh Bone in dilute Nitro-Hydrochluric acid, then macerating it 

 for some time in pure Water, and then tearing thin shreds from 

 the residual substance, which will be found to consist of an imper- 

 fectly-fibrillated material, allied in its essential constitution to the 

 White Fibrous tissue (§ 558). 



545. Teeth. — The intimate structure of the Teeth in the several 

 Classes and Orders of Vertebrata, presents differences which are no 

 less remarkable than those of their external form, arrangement, and 

 succession. It will obviously be impossible here to do more than 

 sketch some of the most important of these varieties. — The prin- 

 cipal part of the substance of all Teeth is made-up of a solid tissue 

 that has been appropriately termed Dentine. In the Shark tribe, 

 as in many other Fishes, the general structure of this Dentine is 

 extremely analogous to that of Bone ; the tooth being traversed 

 by numerous canals, which are continuous with the Haversian 

 canals of the subjacent bone, and receive blood-vessels from them 

 (Fig. 358) ; and each of these canals being surrounded by a system 

 of tubuli (Fig. 359), which radiate into the surrounding solid sub- 

 stance. These tubuli, however, do not enter lacunae, nor is there 

 any concentric annular arrangement around the medullary canals ; 

 but each system of tubuli is continued onwards through its own 

 division of the tooth, the individual tubes sometimes giving-off 

 lateral branches, whilst in other instances their trunks bifurcate. 

 This arrangement is peculiarly well displayed, when sections of Teeth 

 constructed upon this type are viewed as opaque objects (Fig. 360). 

 — In the Teeth of the higher Vertebrata, however, we usually find 

 the centre excavated into a single cavity (Fig. 361), and the 

 remainder destitute of vascular canals ; but there are intermediate 



* Some useful hints on the mode of making these preparations will be 

 found in the "Quart. Journ. of Microsc. Science," Vol. vii. (1859), p. 258. 



