APPENDIX 319 



or b. 25 p.c. formic acid in the dark, i.e. 25 c.c. formic acid, of 

 sp. gr. 1'12 and 75 c.c. water (cp. p. 119, septum auricularum ; 

 p. 120, Auerbach's plexus ; p. 128, end-plates of snake) 

 or c. in distilled water for 6 to 20 hours until it is of a steel- 

 grey colour. It is then warmed at 40 to 50 C. in a strong solution 

 of tartaric acid until it becomes dark ; this may take five or ten 

 minutes only, or an hour or two (cp. p. 73) 



or d. an aqueous solution containing 1 p.c. formic acid and 

 1 p.c. alcohol (used for the cochlea). 



ii. The tissue is placed in a mixture of 4 parts of 1 p.c. gold 

 chloride, and 1 part of formic acid, which has been boiled and 

 cooled. 



The tissue is either exposed to light in dilute acetic acid, or 

 in 20 p.c. formic acid (Ranvier). 



iii. The tissue is placed for about 5 minutes in fresh lemon 

 juice which has been filtered through flannel. It is rinsed with 

 water, treated with gold chloride as in i., washed with water, and 

 either exposed to light in acidulated water, or kept in the dark 

 in 20 p.c. formic acid (Ranvier). 



iv. a. The tissue is placed in 50 p.c. or undiluted formic 

 acid for a few minutes until it is fairly transparent ; then in 

 1 p.c. gold chloride for 20 to 30 minutes ; it is washed well with 

 water, placed for a day in 50 p.c. formic acid in the dark the 

 acid being renewed at the end of the first hour and in undiluted 

 formic acid for a day, to decolourize it (Lowit). The strength 

 of the formic acid before and immediately after gold chloride 

 does not appear to be of much consequence (cp. p. 127, end- 

 plates, rat). 



b. In the preceding method, 25 p.c. formic may be used 

 before and after this gold chloride ; the tissue is then left for 

 a day in 50 p.c. formic acid, and for two to three weeks in 

 glycerine containing 20 p.c. formic acid. 



v. The tissue is placed in *5 p.c. arsenic acid for about 

 15 minutes ; in *5p.c. chloride of gold and potassium for half-an- 

 hour, washed and exposed to light in 1 p.c. arsenic acid for a day, 



