456 AXIS-CYLINDER AND DENDRITE STAINS. 



P.M. table even twenty-four to forty-eight hours after death. 

 It should be divided into pieces of not more than 1 cm. or 1-J cm. 

 in size. 



The most difficult point of the method consists in finding out 

 the exact degree of hardening after which the material can be success- 

 fully submitted to the further treatment. In summer good results 

 may be obtained after fifteen to twenty days of hardening, and the 

 material may continue to be in a state suitable for the silver 

 impregnation up to thirty, forty or fifty days. In cold weather 

 good results can seldom be obtained under a month ; when this is 

 the -case, the material may continue to give good impregnations 

 for two, three or even four months. The only way to make sure 

 is to pass, at intervals, trial portions of the tissue into the silver 

 nitrate solution in summer frequently, in winter every eight or 

 ten days and observe whether and when the reaction has been 

 obtained. 



It is a good practice to inject the organs (see 806) with the 

 hardening fluid, generally 2-5 per cent, potassium bichromate, to 

 which, according to Golgi, 5 to 6 per cent, of gelatine may be added, 

 in which case, however, the fluid must be injected after warming 

 it to body temperature. Stoving at a temperature of 20 to 25 C. 

 is useful for abridging the hardening, but there is a risk of over- 

 hardening ; and Golgi thinks that the results are never quite so 

 delicate as after hardening at room temperature. 



(b) Impregnation. As soon as the pieces of tissue have attained 

 the proper degree of hardening, they are brought into a large 

 quantity of silver nitrate solution, the usual strength of which is 

 O75 per cent., but 0*50 per cent, may be used for material which 

 has not been quite enough hardened, and 1 per cent, for material 

 that has been slightly over-hardened. 



The moment the pieces are put into the silver bath an abundant 

 precipitate is formed. This, of course, weakens the bath pro tanto. 

 It is, therefore, advisable first to wash them well in a weaker silver 

 solution until, on being put into a fresh quantity of it, no further 

 precipitate is formed. Used solutions will do for this purpose. 

 The final silver bath needs, generally, no further attention ; but it 

 should be changed for a fresh one if it becomes yellowish, as it some- 

 times does, particularly in the case of tissues which have taken 

 up a great deal of bichromate. 



It is not necessary to keep the material in the dark during the 

 impregnation ; in winter it is well to keep it in a warmed room. 

 The time generally necessary for the impregnation is from twenty- 



