SECTIONS OF HARDENED BONE. 101 



they may be disregarded, for they become almost 

 invisible in the subsequent preparation. This con- 

 sists in well rinsing the piece (when satisfied that it 

 is thin enough) in clean water by aid of a hair 

 pencil, placing it upon a slide to dry, and, when 

 dry, mounting it in Canada balsam. The balsam 

 to be used for this purpose must not be, as is usually 

 the case with that sold in the shops, semi-fluid, but 

 quite hard in the cold a condition which results 

 from long keeping, or may be produced by heating a 

 little of the more recent resin in a capsule over a 

 sand- or w T ater-bath, until all the volatile matters are 

 driven off. A drop of the melted balsam is then 

 placed upon a slide^ which is warmed over a flame 

 until the resin has diffused itself pretty evenly over 

 the central part of the slide. This is then placed 

 on the table, and whilst cooling, but before the 

 balsam has time to become quite hard again, the 

 thin piece of bone is placed upon it. A clean cover- 

 glass is then taken up by the forceps, a drop of the 

 balsam placed upon it also warmed in like manner, 

 and quickly inverted over the preparation. By 

 this mode of proceeding the balsam fills up and 

 renders invisible the scratches on the surfaces of the 

 section of bone, and some of the Haversian canals 

 may also be filled by it ; but it becomes solidified 

 before it has time to penetrate into the lacunae and 

 canaliculi, which remain, therefore, filled with air, 

 and present the black appearance which is char- 

 acteristic of any small cavities containing air when 

 they are viewed under the microscope by trans- 

 mitted light. 



Most of the structural points with regard to bone 

 can be seen much better in these preparations of the 

 hard tissue than in sections of a decalcified bone. 

 But it is important in mounting the pieces after 

 they have been ground down to be careful that the 

 balsam does not remain fluid long enough to have 

 time to penetrate into the thickness of the section ; 

 if this should happen, the whole is rendered too 



9* 



