142 SUTHERLAND SIMPSON 



was cut into slices from 2 to 3 mm. thick and placed in Marchi's 

 fluid made up according to the formula of Van Gehuchten ('06) 

 as follows: 



Osmic acid, 1 per cent 1 part 



Potassium bichromate, 3 per cent 4 parts 



In this solution they were allowed to remain for three weeks. 

 Before being sliced up, photographs of the brain were taken, in 

 each case, to show the position of the lesion. 



In Marchi staining it is essential that a large excess of the 

 fluid be used. The pieces of tissue while in the fluid, should be 

 kept in tightly stoppered bottles in a dark and cool room in order 

 to prevent the evaporation of the very volatile osmic acid and 

 its decomposition by the action of light. 



The original fluid employed by Marchi consisted of 1 per cent 

 osmic acid one part, MuUer's fluid two parts, and in this the tissue 

 was kept for ten days after ten days previous hardening in Mul- 

 ler's fluid. It is claimed, however, that better penetration is 

 secured by using the weaker solution and allowing it to act for a 

 longer time; this I believe to be the case. Further, at the end 

 of the staining process the fluid should not be thrown away; it 

 may be used over and over again with the best results, all that 

 is necessary being the addition of some fresh osmic acid and a 

 small quantity of concentrated solution of potassium bichromate. 



Several modifications of this method have been introduced 

 from time to time, for example, by Orr COO), Vassale ('96), 

 Busch ('98) and others, but in my experience the original Tvlarchi 

 fluid, as used by Van Gehuchten, in its more dilute form, is better 

 than any other. If due attention is paid to the details mentioned 

 above, Marchi staining seldom fails to give good results. 



The survival-time after operations is also an important con- 

 sideration. According to Lange ('00) the optimum time comes 

 between the tenth and fourteenth days. No doubt this will 

 depend, to some extent, on the species of animal used. Most 

 observers are inclined to allow a longer interval than fourteen days. 

 Lange believes that the best results are obtained from adult 

 animals. He fixes his tissue in a mixture of 10 per cent formalin 



