654 



APPENDIX. 



"but renders also the gray substance much more transparent. 

 The section was then covered with thin glass, and viewed first 



Fig. 347. 



The lower figure represents a choroid plexus with several small tumors at * * *, supposed at 

 first to have been tubercular; they proved to consist of aggregations of concentric corpuscles, chole- 

 .sterine, and pure oil, united by areolar tissue; the concentric corpuscles vi'hich are shown above the 

 plexus are magnified 100 diameters. 



by reflected light with low magnifying powers, and then by 

 transmitted light with higher ones. 



"According to the second method, the section is first mace- 

 rated for an hour or two in the mixture of acetic acid and spirit. 

 It is then removed into pure spirit, £(bd allowed to remain there 

 for about the same space of time. From the spirit it is trans- 

 ferred to oil of turpentine, which expels the spirit in the form of 

 opaque globules, and shortly (sometimes immediately) renders 

 the section perfectly transparent. The preparation is then put 

 up in Canada balsam, and covered with 

 thin glass. By this means the nerve- 

 fibrils and vessels become so beautifully 

 distinct, that they may be clearly seen 

 with the highest powers of the micro- 

 scope. If the section be removed from 

 the tui-pentine when it is only semi-trans- 

 parent, we sometimes obtain a good view 

 of the arrangement of the bloodvessels. 

 This mode of preparation succeeds best in 

 cold weather; for in summer, the cord, 

 fresh when immersed in the spirit, re- 

 mains more or less spongy, instead of be- 

 coming firm and dense in the course of 

 five or six days. The spirit should be 

 diluted with an equal quantity of water 

 during the first day, after which it should 

 be used pure. Certain modifications of this mode of preparation 



