APPENDIX. 403 



preparation jars, with ground stoppers, meet every requirement. When of 

 the proper height and diameter, they permit three pairs of slides to be im- 

 mersed at one time with convenience and safety and enable volatile fluids to 

 be used repeatedly without deterioration. After the staining has been com- 

 pleted according to the details of the method selected, the sections are passed 

 through jars containing 95 and absolute alcohol to insure complete dehydra- 

 tion prior to clearing and mounting. 



Clearing the sections is necessary to render the otherwise more or 

 less opaque tissue transparent and suitable for microscopical examination. 

 If the tissue contains no trace of water, it may be mounted in pure Canada 

 balsam, the usual medium in which objects are preserved as permanent prepa- 

 rations, directly from fresh absolute alcohol, the latter being gradually 

 replaced by the highly refracting balsam and the required transparency 

 thereby secured. If, however, the alcohol be of insufficient strength, as 

 often occurs after exposure, it mixes with the balsam imperfectly and 

 turbidity results. In order to obviate such accident, it is more con- 

 venient to employ some clearing agent of suitable refraction as a 

 go-between, which will mix with the alcohol, on the one side, and with the 

 balsam on the other. 



The means employed to secure this transparency depends upon the 

 condition of the tissue, whether infiltrated with the embedding mass, as in 

 the case of celloidin sections, or free from such support, as in the case of 

 paraffin sections attached to the slide. 



Celloidin sections are best cleared in a carbol-xylol mixture, consisting 

 of one part of pure carbolic acid to three parts of xylol. The dehydrated 

 sections are transferred to this fluid from the strong alcohol and after 

 a few minutes become transparent. If, however, the required transparency 

 fails to appear after five or ten minutes, it may be concluded that the 

 dehydration has been insufficient to allow the penetration of the clearing 

 solution. The sections must be returned, therefore, to the alcohol. In all 

 cases thorough dehydration must be insured by adequate treatment with 

 strong alcohol. 



Paraffin sections, after the removal of the paraffin by immersion in 

 xylol, are transferred to some clearing fluid, oil of turpentine answering well 

 and being inexpensive. 



MOUNTING AND FINISHING. 



Unless required for only cursory examination, the sections are mounted 

 in some medium suitable for further study and preservation. The most sat- 

 isfactory medium for general use is pure Canada balsam, which is supplied 

 in convenient collapsible metal tubes, from which the required amount may 

 be pressed. In the case of tissues cut in celloidin, the individual sections 

 are transferred from the dish of clearing solution (carbol-xylol) to a clean 

 slide by means of a thin metal section-lifter, a clean mounted needle being 

 used in guiding and holding the section on the lifter, as well as in transfer- 

 ring it on the slide from the lifter. When the latter is broad, the blade 

 should be perforated to facilitate raising the section from the fluid, as well as 

 draining off the clearing solution. After being placed on the slide, the 

 superfluous carbol-xylol having been carefully removed by filter paper, the 

 section is finally arranged by judicious use of the needle and a drop of balsam 

 gently pressed out from the tube upon the middle of the preparation. 



