132 



1. Sawing the section. Make an exact transection, of a part of 

 the shaft of a long bone. The section should be about i cm. long 

 and include the thickness of the shaft from the surface to the medul- 

 lary cavity. Make the sections about i mm. thick. 



2. Grinding the sections. Place the piece of bone on a cork or 

 piece of soft wood and wet it with water. File it on one side until 

 smooth and then turn it over. Continue the filing till the piece is 

 from .05 to .10 mm. thick, using the cover-glass measurer to deter- 

 mine the thickness. In the beginning one can press quite hard in 

 filing ; as the section thins, more care should be exercised and the 

 pressure should lessen. 



3. Washing and drying the section. When the section is thin 

 enough, rinse it and dry it with lens paper. 



4. Mounting the sections in hard balsam. To prepare the bal- 

 sam, put two or three large drops on the middle of a slide and heat 

 the slide in some way to drive off the volatile constituents. Do not 

 heat the balsam hot enough to produce bubbles. When the balsam 

 chips after cooling, it is ready for use. 



In mounting, have the section and a clean cover so placed that 

 they may be easily and quickly grasped. A cork somewhat smaller 

 than the cover-glass should be within reach, and also a stone or 

 piece of glass upon which to quickly cool the specimen as soon as it 

 is mounted. 



Heat the slide until the balsam is well melted. Put the slide 

 upon a piece of paper, grasp the piece of bone with the forceps and 

 plunge it into the melted balsam ; put on the cover as quickly as 

 possible and press it down with the cork ; finally put the slide on 

 the stone or glass to cool the balsam quickly. All of this should be 

 done as rapidly as possible, and if done rapidly, the air will be 

 retained in the lacunae and canaliculi, and cause them to stand out 

 as black spots and lines. If soft balsam were used it would soon 

 drive out the air, and being of nearly the refractive index of bone, it 

 would obliterate the lacunae and canaliculi. Further, if the hot bal- 

 sam were not cooled quickly, the air would be driven out and balsam 

 would take its place in the spaces. 



CENTRAL NERVOUS SYSTEM. 



§ 135. There are two special groups of methods employed in 

 the microscopical examination of the central nervous system, (a) 



