258 THE MICROSCOPE AND ITS REVELATIONS. 



Long Diameter. Short Diameter. 



Proteus . 1-570 to 1-980 1-885 to 1-1200 



Siren 1-290 to 1-480 



Menopoma 1-450 to 1-700 



Lepidosiren 1-375 to 1-494 



Pterodactyls 1-445 to 1-1185 



1-540 to 1-975 ' 

 1-1300 to 1-2100 

 1-980 to 1-2200 

 1-4000 to 1-5225 ' 



654. In preparing Sections of Bone, it is important to avoid the 

 penetration of the Canada balsam into the interior of the lacunae and 

 canaliculi; since, when these are filled by it, they become almost invisible. 

 Hence it is preferable not to employ this cement at all, except it may be, 

 in the first instance; but to rub-down the section beneath the finger, 

 guarding its surface with a slice of cork or a slip of gutta-percha ( 196); 

 and to give it such a polish that it may be seen to advantage even when 

 mounted dry. As the polishing, however, occupies much time, the bene- 

 fit 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 following 

 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 sur- 

 face 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 sys- 

 tem. 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 

 Mtn> hydrochloric 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 imperfectly-fibrillated material, allied in its 

 essential constitution to the ' white fibrous ' tissue ( 668). 



655. 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 succes- 

 sion. 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 appro- 

 priately 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 con- 

 tinuous with the Haversian canals of the subjacent bone, and receive 

 blood-vessels from them (Fig. 443); and each of these canals being 

 surrounded by a system of tubuli (Fig. 444), which radiate into the sur- 

 rounding solid substance. 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 



1 See Prof. J. Quekett's Memoir on this subject, in the Transac. of the Microsc. 

 Soc.," Ser. 1, Vol. ii.; and his more ample illustration of it in the " Illustrated 

 Catalogue of the Histological Collection in the Museum of the Royal College of 

 Surgeons," Vol. ii. 



2 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. 



