GENERAL HISTOLOGY OF THE NERVOUS SYSTEM. 115 



cal hole a little larger than the trunk of the nerve, then im- 

 merse the whole in water, and the pith will begin to swell. As 

 soon as it has firmly embraced the nerve, sections may be 

 made with the knife. Ammonia-carmine will stain the axis- 

 cylinder well, but the outline of the cut will appear irregular 

 rather than round. This appearance is doubtless artificial. In 

 my hands, borax-carmine ' has proved much better than the 

 ammonia-carmine, as it diffuses very little, and much of the 

 excess may be removed by dilute acetic acid (about J percent.), 

 in which the specimen should remain, from a few seconds to a 

 minute or two, until it has become bright to the eye. The fur- 

 ther steps in the process of making a permanent preparation 

 are the same as those for other specimens ; i.e., it may be 

 mounted in glycerine and water, or clarified by clove-oil and 

 mounted in dammar varnish or Canada balsam. 



Preparation by tlie bichromate of ammonia. Ranvier em- 

 ploys of this a 2 per cent, solution, allowing the specimen to 

 remain, with frequent changes of the 

 fluid, from two or three months to a 

 year. The sections are to be stained 

 in ammonia-carmine or picro-carmine, 

 and mounted in glycerine. It will 

 then be seen that immediately about 

 the axis-cylinder is a sheath. This is 

 called by Ranvier the sheath of Mauth- 

 ner, from the author who described it. 

 (See Fig. 46, 5.) Specimens prepared 

 in the ordinary way, by long immer- +****' *' 8beathl 

 sion in Mueller's fluid alone, or sub- 

 sequently in the chromic acid solution (gr. ij. j.) and stained 

 with ammonia-carmine, occasionally show the same thing. 



Sometimes histologists find that embedding in gum succeeds 

 best in securing these transverse sections of nerves. The diffi- 

 culty of the task is one of considerable moment. The method 

 is as follows : Take a fresh nerve, harden it in osmic acid (1 per 

 cent., if it is desirable to expedite the process, or ^ per cent, 

 if it is not necessary to conclude the examination the same 

 day). Then, when the nerve is thoroughly blackened all through, 



'The powder is prepared by Eimer & Amend, of this city (205 to 211 Third 

 Avenue), according to Arnold's formula. The strength required is gr. xv. 5J- 

 distilled water. 



