420 NERVOUS SYSTEM SPECIAL METHODS. 



even to five days at a temperature of about 35 C. In summer, with 

 a temperature constantly over 22 C., the stove may be dispensed 

 with, provided the impregnation is prolonged for two to three days 

 more. The tissues are known to be ripe for reduction when a freshly 

 cut surface shows a brownish-yellow colour. 



They are then washed for one to two minutes in distilled water 

 and put into 



Pyrogallol or hydroquinone . . . 1 2 grms. 



Distilled water 100 c.c. 



Formalin 5 10 c.c. 



The formol is not necessary but useful. One may use pyridine 

 instead (1 to 3 per cent.). The addition of a small quantity of 

 sodium sulphite (0-2 to 0-5 per cent.) has been abandoned by Cajal. 

 The stronger the pyrogallol, the greater the contrast, so that it may 

 be useful to take, sometimes, as much as 3 per cent., but then the 

 over-impregnation of the outer layers will be increased. Hydro- 

 quinone reduces more energetically than pyrogallol. 



The pieces remain in the reducing fluid for about twenty-four hours 

 and are then quickly washed, hardened in alcohol and embedded in 

 paraffin or celloidin. The sections (15 to 20 //, thick) are mounted 

 in dammar after toning with a solution of gold chloride if the reaction 

 is rather weak, without toning if the impregnation is a good one. 



Faintly impregnated sections can be advantageously toned with 



Distilled water 100 c.c. 



Ammonium sulphocyanide ... 3 grms. 



Sodium hyposulphite . . . . 3 



1 per cent, gold chloride ... a few drops. 



If subsequently found to be too dark they can be bleached by 

 Veratti's potassium permanganate and sulphuric acid mixture 

 (see 846). 



The sections from the outer layer are generally too dark for 

 study, those from the innermost too pale, whilst those from the 

 intermediate layer are good. The over-staining of the outer layer 

 can be diminished by diluting the silver nitrate with 1 volume of 

 water for the last twelve hours. 



The method has the defect of giving an imperfect fixation of the 

 nervous tissue and of impregnating, almost exclusively, cell bodies 

 and dendrites. It is not good for ganglia and large cells of adult 

 subjects, but excellent for small and medium-sized cells of very 

 young subjects and early embryos. 



Formula la, A. As the last, but pieces are fixed in 3 to 6 per cent. 



