390 CHAPTER XXXII. 



It is washed out for from six to twelve hours (not more in any case) in 

 ;ibsolute alcohol, cleared in clove oil imbedded in paraffin, and cut. It 

 Is said that if sections are mounted by the water method the stain will 

 he extracted ; but why not make the water acid, 291 ? 



The usual histological stains either leave them colourless 

 or stain the surrounding plasma more strongly than the fibrils 

 themselves. There may thus be produced a " negative " 

 image of fibrils which does not really show the true nenro- 

 fibrils, and being taken for them may prove a source of error. 

 The following methods are such as have been recommended 

 as giving true stains of the fibrils. 



767. Neurofibrils : Silver Methods. Those most used are 

 RAMON Y CAJAI/S and BIELSCHOWSKY'S. The essential differ- 

 ence between the two is that Ramon employed a single 

 impregnation bath of nitrate of silver; whilst Bielschowsky 

 employs two one of nitrate and one of silver oxide dissolved 

 in ammonia. 



Fixing agents should in general be neutral or alkaline', for 

 acids or oxidants divert the impregnation from the fibrils to 

 other elements, chiefly tigroid and nuclear elements. 



Sections should be thin not more than 15 ft to 20 ft. 



Toning with gold has the effect of intensifying the stain 

 in the fibrils and lightening it in surrounding elements, thus 

 giving enhanced contrast. It also serves to favour the pre- 

 servation of the stain. 



The object of fixing with hyposulphite is to remove from 

 the tissues any unreduced silver salts which might cause a 

 loss of contrast by darkening the ground of the preparations. 

 With well reduced preparations, such as Ramon's are in 

 general, it is not necessary. 



Bielschowsky's methods have the advantage of being 

 applicable to larger specimens than Ramon's, for they give a 

 more uniform impregnation through the whole thickness of 

 the objects (especially with the central nervous system of 

 man) ; whilst Ramon's (especially his formula la) only gives 

 the desired results in a sometimes very thin layer between 

 an overstained outer and an understated inner one. 



Bielschowsky's are applicable to very old formol material. 

 K\YON (Diehistologiftcheu Uniersuchungs-Methoden r/r.v Ncrvfn- 

 -V//.S// ///y, p. 157) has succeeded with, material four years 



