WITH THE MICROSCOPE. 145 



which expels the spirit in the form of opaque globules, and 

 shortly (sometimes immediately) renders the section perfectly trans- 

 parent. The preparation is then put up in Canada balsam, and 

 covered with thin glass. By this means the nerve fibrils and vesicles 

 become so beautifully distinct, that they may be clearly seen with 

 the highest powers of the microscope. If the section be removed 

 from the turpentine when it is only semi-transparent, we sometimes 

 obtain a good view of the arrangement of the blood-vessels. This 

 mode of preparation succeeds best in cold weather, for in summer, 

 the cord, however fresh when immersed in the spirit, remains more 

 or less spongy, instead of becoming 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 may be sometimes 

 employed with advantage by a practised hand. (Phil. Trans., 1851). 

 These processes are more or less- applicable to the investigation of 

 the brain and some ganglia. 



For some time past Mr. Clarke has adopted a modification of his 

 original plan. He has been kind enough to send me the following 

 directions. 



The spinal cord and medulla oblongata of man, and the higher 

 mammalia are to be cut into pieces of half or three quarters of an 

 inch long, and steeped in a solution of one part of chromic acid in 

 200 parts of water, for three weeks or a month. It is then preserved 

 for use in a solution of about one part of bichromate of potash in 200 

 parts of water. For hardening the convolutions of the cerebrum 

 and cerebellum, the solution of chromic acid must be weaker than 

 for the spinal cord or medulla oblongata, that is the proportion of 

 one part of the acid to four, or even five hundred parts of water ; 

 but the portions of brain must be small, not more than half an inch 

 thick, otherwise they become rotten before the acid has reached their 

 centres. A little spirit added to the solution for two or three days, after 

 the first day, will prevent this. The pure solution can then be renewed. 



Spirit of wine is used to wet the knife or razor in making sections, 

 which should be washed in water before they are placed in solution 

 of carmine. When sufficiently coloured, the sections are again 

 washed in water, and placed for ten minutes or a quarter of an hour 

 in strong spirit ; after which, if they be thin, they are floated on the 

 surface of spirit of turpentine, where they remain until they are quite, 

 or nearly, transparent, when they are removed to glass slides, on 

 which a little Canada balsam has been previously dropped. If now 

 examined under the microscope, they frequently show but little 

 traces of either cells or fibres a circumstance which seems to have 



