134 



Treat for a short time — 20-30 seconds — with a. }^% aqueous 

 solution of potassium permanganate and decolorize in Pal's bleacher 

 until the cinerea is grey (white), the myelinic fibers blue. The 

 action will be very rapid and must be carefully watched ; a few sec- 

 onds will sufiice. Wash the sections thoroughly ()^ hour or more) 

 in running water. If it is desired, counter-stain with a red stain, — 

 eosin, erythrosin or carmine. 



This method is not as reliable as the method first given ; great 

 care must be exercised in the difierentiation with the bleacher. 



§ 139. Chrome-silver impregnation methods. In this 

 group of methods the tissue is hardened a certain length of time in a 

 fluid containing a chronie salt, especially potassium dichromate, and 

 then placed in an aqueous solution of silver nitrate {,%% and %% 

 solutions generally employed). If successful, the cells and their 

 processes, amyelinic and, to a certain extent, myelinic nerve fibers, 

 are outlined by a black precipitate (black by transmitted light ; 

 brown by reflected light). 



§ 140. Success depends on (a) the kind of animal ; difierent 

 parts and tissues react more satisfactorily in some animals or classes 

 of animals than in others, (d) The age of the animal ; young or 

 fetal animals give better results than do adults, {c) The time of 

 hardening ; it is necessary that the tissue be hardened a certain 

 length of time, constant (relatively) for a certain kind of tissue under 

 the conditions above (a and U). It is necessary that the correct de- 

 gree of hardening be carefully regulated, (rf ) DifiFerent organs and 

 regions of the central nervous system vary greatly in the ease with 

 which they can be made to furnish satisfactory impregnations. Al- 

 most certain impregnations of hippocamp can be gained ; cerebral 

 cortex is likewise quite easy to stain. With the olfactory bulb the 

 action is not constant though fairly complete. The optic lobes and 

 retina of birds and large reptiles are more satisfactory than those of 

 mammals. The myel (spinal cord) of embryo birds (7-14 day chick 

 best) is generally more satisfactory than that of mammals ; in any 

 case, fetal or new-born animals should be employed. Difficult are 

 satisfactory impregnations of sympathetic ganglia, organs of special 

 sense and the intrinsic nerves of the viscera. 



§141. Golgi's Rapid Method. This is the most generally 

 serviceable of the different methods. 



