478 NEUROFIBRILS 



Formula 1, Small pieces of fresh tissue are put directly into 

 1-5 per cent, silver nitrate and kept therein for three to four and 

 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 longer. The tissues are known to be ripe for reduc- 

 tion when a freshly cut surface shows a brownish-yellow colour. 

 In this country it is better to use incubator temperature. 



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

 and jDut into — 



Pyrogallol or hydroquinone . . 1 — 2 grm. 



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 or hydroquinone solution, 

 the greater the contrast, so that it may be useful to take, some- 

 times, as much as 3 per cent, of either the one or the other, but 

 then the over-impregnation of the outer layers is increased. 

 Hydroquinone 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 

 imbedded in paraffin or celloidin. The sections (15 to 20 /it 

 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 grm. 



Sodium hyposulphite . . . 3 ,, 



1 per cent, gold chloride . . Some drops. 



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

 Veratti's potassium permanganate and sulphuric acid mixture 

 (see § 722). 



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 embrj'os. 



