588 



VERTEBRATED ANIMALS. 



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. 



406. Teeth. The intimate structure of the Teeth in the 

 several classes and orders of Yertebrata 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 principal 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 bloodvessels from them (Fig. 302) ; and each 

 of these canals being surrounded by a system of tubuli (Fig. 



FIG. 302. 



FIG. 303. 



Fig. 302. Perpendicular section of tooth of Lamna, moderately enlarged, showing network of 

 medullary canals. 



Fig. 303. Transverse section of portion of tooth of Pristis, more highly magnified, showing 

 orifices of medullary canals, with systems of radiating and inosculating tubuli. 



303), which radiate into the surrounding solid substance. These 

 tubuli, however, do not enter lacunae, nor is there any concentric 

 annular arrangement arouijd the medullary canals; but each 

 system of tubuli is continued onwards through its own division. 



