504 CAJAL 



into the tissues than potassium bichromate. According to 

 Strong (A^. Y. Acad. Sc. Proc, xiii, 1894) lithium bichromate 

 hardens more rapidly than potassium bichromate. The influence 

 on the reaction of the bichromates of ammonium, sodium, calcium, 

 magnesium, rubidium, lithium, zinc, and copper, has been investi- 

 gated 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. zoiss. 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. The 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. 



1033. Ramon y Cajal's Double-Impregnation Process {La 

 Cellule, viii, 1891, p. 130). Sometimes the usual raj^id 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, and put for a second time into 

 the silver bath, where they should remain from thirty-six to 

 forty-eight hours. It is important to find out the proper duration 

 of the first hardening. If it has been too long (four days) or 

 too short (one day) the second impregnation will not succeed. 

 In this case a third impregnation may be resorted to, the objects 

 being again treated with the weak osmium-bichromate mixture' 

 and then again with the silver nitrate solution. I find that this 

 modification, which is the most important that has hitherto 

 been made, gives excellent results if one proceeds by tests, viz., 

 re-transferring into the weak osmium-bichromate mixture those 

 pieces in which the reaction has been found to have succeeded to 

 some extent. 



1034. KoLOSsow's Modification (see Zuschtschenco, Arch. Mikr. 

 Anat., xlix, 1897). Tissues are hardened for one to seven days in 3 to 

 5 per cent, potassium bichromate containing 0-25 per cent, of osmic 

 acid. They are then Avashed quickly in distilled water, dried with filter 

 paper and transferred for two to three days into a bath of 2 to 3 per cent, 

 silver nitrate to which 0-25 to 0-5 per cent, of osmic acid has been 

 added. This is a good modification for sympathetic ganglia. 



1035. GoLGi's Processes for the Rejuvenation of Over-hardened 

 Tissues. Tissues which have been too long in the osmium- 

 bichromate mixture will no longer take on the silver impregnation. 



