APPENDIX. 297 



in Miiller's fluid), and to leave them in it for a week or more ; but if necessary 

 sections can be stained ; in this case they are left in the solution for a few hours. 

 In either case they are mounted by the usual process in Canada balsam. 



14. Weigert-Pal method. This method is of great value for the central 

 nervous system. By it all medullated nerve-fibres are stained dark, while 

 the grey matter and any degenerated tracts of white matter are left un- 

 coloured. The following modification of the original method can be very 

 strongly recommended : Pieces which have been hardened in Miiller's fluid 

 and afterwards kept a short time in alcohol (without washing in water) are 

 embedded in celloidin, and sections are cut as thin as possible. Or sections 

 may be made of the tissue direct from Miiller's fluid, if it is first soaked in 

 gum-water for a few hours. In either case they are placed in water, and 

 from this are transferred to Marchi's fluid (see above, sec. 13), in which they 

 are left for a few hours. They are then again washed in water and trans- 

 ferred to acetic acid hsematoxylin (see above, sec. 3). In this they are left 

 overnight, by which time they will be completely black. After again wash- 

 ing in water they are ready to be bleached. This is accomplished by Pal's 

 method as follows : Place the overstained sections, first in j per cent, solu- 

 tion of potassic permanganate for 5 minutes ; rinse with water and transfer 

 to Pal's solution (sulphite of soda 1 gramme, oxalic acid 1 gramme, distilled 

 water, 200 cubic centimeters), in which the actual bleaching takes place. 

 They are usually sufficiently differentiated in a few minutes : if not, they can 

 be left longer in the solution without detriment. If after half an hour they 

 are not differentiated enough, they must be put again (after washing) into 

 the permanganate for some minutes, and then again into Pal's solution. 

 After differentiation they are passed through water, alcohol, and oil of ber- 

 gamot (or xylol), to be mounted in Canada balsam. The advantages which 

 this modification has over the original methods are (1) the very finest 

 medullated fibres are brought to view with great surety ; (2) the staining 

 of the fibres is jet black, and offers a strong contrast to the colourless grey 

 matter ; (3) the sections are easily seen and lifted out of the acid hsema- 

 toxylin, which has very little colour ; (4) it is difficult to overbleach the 

 sections ; (5) the stain is remarkably permanent. 



15. Stainmg with chloride of gold. a. Cohnheim's method. Place the fresh 

 tissue for from 30 to 60 minutes in a J per cent, solution of chloride of 

 gold ; then wash and transfer to a large quantity of water just acidulated 

 with acetic acid. Keep for 2 or 3 days in the light in a warm place. This 

 answers very well for the cornea. If it be principally desired to stain the 

 nerve-fibrils within the epithelium, the cornea may be transferred after 24 

 hours to a mixture of glycerine (1 part) and water (2 parts), and left in this 

 for 24 hours more. 



/3. Lowifs method. Place small pieces of the fresh tissue in a mixture of 

 1 part of formic acid to 2 to 4 parts of water for to 1 minute ; then in 1 per 

 cent, chloride of gold solution for 10 to 15 minutes ; then back again into the 

 formic acid mixture for 24 hours and into pure formic acid for 24 hours more. 

 After removal from the gold, and whilst in the acid, the tissue must be kept 

 in the dark. 



