1890.] MICROSCOPICAL JOURNAL. 271 



the enamel ; and it becomes a matter of necessity to mount the sections 

 in such a manner as, while preserving the specimen, will not obliterate, 

 the structure. It is found that if a section of either of these substances 

 is put directly into balsam, the air in the internal cells becomes absorbed 

 by the balsam, which, running in to fill the vacuum, reduces the his- 

 tological detail to the dead level of one uniform transparency, and all 

 evidence of the structure is lost. Means must therefore be adopted by 

 which this disadvantage may be obviated, and two plans are offered, 

 either of which entirely prevents this destruction of detail. 



It is obvious that in all structures having internal cells, the fluid media 

 in which they may be mounted will, by endosmosis, run into them, and 

 therefore means must be adopted of keeping this medium out. Thus 

 Canada balsam, used to fasteri the tooth to the slide in grinding the sec- 

 tion, must not be warmed to any such extent as will liquify it ; neither 

 must it be dissolved oft' in such menstrua as chloroform or benzole, but 

 it may be removed by soaking in absolute alcohol, which, softening the 

 Canada balsam without dissolving it, enables the section to be removed, 

 after which the Canada balsam may be cleared away with a camel's 

 hair pencil. When perfectly clean, it should be transferred to clear 

 distilled water, and allowed to remain until the spirit in the spaces has 

 given place to the water. It may then be carefully dried, and mounted 

 in balsam of a moderate degree of stiffness, when the water in the tub- 

 ular structure w^ill prevent the balsam running in and obliterating it. 



In the effective mounting of bone, teeth, and other similar structures 

 in Canada balsam there are three methods for the occlusion of the min- 

 ute spaces, lacunie and canaliculi in bone, and the dentinal tubuli and 

 interglobular spaces in teeth, prior to mounting the sections in balsam. 

 I. Collodion fflm answers very well, but is troublesome and sometimes 

 fails. 2. Mucilage film strained, B. P. solution of pale acacia, answers 

 perfectly, but is apt to harbor dust, unless extreme care is taken. 3. 

 The laccic method, coat with an alcoholic solution of white shellac. 

 This is practically the best and simplest method, provided the slide is 

 not heated to more than the melting point of shellac ; a mishap not 

 likely to occur. 



The finished sections, when ready for mounting, are dipped momen- 

 tarily in the shellac solution, and withdrawn, when a thin coating of 

 the lac is left over the surface, occluding the spaces but not running in. 

 Instructive sections may be made of many structures which contain 

 loose parts, by allowing them, after dehydrating in absolute alcohol, to 

 soak in a thin solution of balsam in benzole till it has permeated all the 

 parts, when the specimen may be removisd and set aside to evaporate 

 and harden ; thus corals, teeth in jaws of small mammalia, and the 

 like, may be rubbed down thin without fear of dislocating their integral 

 constituents. 



Sections of Echinus spines are among the most interesting objects, 

 but on account of their friable nature difficult to make. This may be 

 almost overcome by soaking the spine in Canada balsam dissolved in 

 benzole, and if it is previously steeped in the benzole, and then placed 

 in the solution of balsam, it runs into the structure, thus supporting it 

 during grinding. This may be greatly facilitated by afterwards plac- 

 ing it in a gentle heat, such as a slow oven, until the balsam becomes 

 hard. There is no difficulty in cutting the section with a fine piercing 



