CHAPTER XXXIV. 461 



help the stain, but are injurious to it. It has been maintained that 

 the stain is merely superficial, and the method has been called an 

 " incrustation method." But it is easy to realise that it generally 

 extends throughout the whole thickness of the impregnated elements, 

 though in special cases or by slight modifications of the original 

 method, the stain may be limited to certain constituents of the 

 nerve-cell body, such as Golgi's pericellular investment and 

 intracellular network. 



The chemical nature of the stain has not as yet been discovered. 



A critical review of the Golgi method by WEIGERT may be found in 

 Ergebn. d. Anat., v, 1895, p. 7. See also HILL (Brain, xix, 1896, p. 1 ), and 

 KALLIUS (op. cit.}. 



Modifications of Golgi's Bichromate and Silver Nitrate Method 

 concerning the Impregnation of Tissues. 



885. Instead of potassium bichromate, ammonium bichromate has 

 been recommended by GOLGI and sodium bichromate by KALLIUS. 

 Both these salts appear to penetrate more quickly into the tissues 

 than potassium bichromate. According to STRONG (N. Y. Acad. Sc. 

 Proc. xiii, 1894) lithium bichromate hardens more rapidly than 

 potassium bichromate. The influence on the reaction of the bichro- 

 mates of ammonium, sodium, calcium, magnesium, rubidium, 

 lithium, zinc and copper, has been investigated by L. SALA (see 

 Kallius, op. cit., i, p. 564), but he came to the conclusion that they do 

 not offer any particular advantage, with the exception of calcium 

 bichromate, this last to be preferred for the staining of the tangential 

 fibres of the cerebral cortex. 



RAMON Y CAJAL (Ztschr. wiss. Mikr., vii, 1890, p. 332) gives 3 per 

 cent, as the strength of the bichromate in the mixture for the rapid 

 process, but in numerous other places has given it as 3-5 per cent. 

 This latter strength has been adopted by many workers for the rapid 

 process, and the mixture containing this proportion of bichromate is 

 generally known as the RAMON Y CAJAL mixture. 



886. RAMON Y CAJAL'S Double-Impregnation Process (La Cellule, 

 vii, 1891, p. 130). Sometimes the usual rapid method fails to give 

 a good impregnation. This, however, may frequently be obtained 

 by putting the tissues back for a day or two into the osmium- 

 bichromate mixture used for the first hardening, or into a fresh but 

 weaker one containing 2 parts of 1 per cent, osmic acid and 20 parts 

 of 3 per cent, potassium bichromate. Tissues are then washed 

 quickly with distilled water or with a weak solution of silver nitrate, 



