THE HISTOLOGICAL TECHNIQUE 3 



not exceed \ cm. in thickness whether from an injected or from a fresh brain. This size is 

 rendered necessary by the fact that the fixation by the hardening fluid extends inwards slowly, 

 and because the diffusion of the silver cannot take place properly beyond a certain depth. 

 This does not exclude the impregnation of large pieces, provided they are of the necessary 

 thinness. Care must also be taken to cut the pieces to be impregnated as accurately as pos- 

 sible in the plane in which it is desired to section them subsequently ; e.g. if it be desired to 

 make sections displaying the whole length of the pyramid cells of the cortex, the cuts should 

 be made perpendicular to the surface of the cortex; if the protoplasmic expansions of the cells 

 of Purkinje, they should be made at right angles to the convolutions of the cerebellum, etc. 

 If this precaution be not observed, much of the thin pieces impregnated will be wasted by 

 cutting off corners with oblique cuts, etc. Furthermore, the impregnation is liable to vary 

 at different depths from the cut surfaces of the piece, being especially full in details nearest 

 the surface ; hence it is obvious that sections passing parallel to these cut surfaces give 

 better and more uniform fields. 



The surface of impregnated pieces is usually covered with silver chromate, which mars 

 the beauty of the preparations, and may also obscure details of structure lying near the 

 surface. To avoid this, it has been recommended to cover the surface of the pieces, before 

 hardening, with gelatine (Sehrwald) or with celloidin or blood (Cajal). Gelatine, however, 

 interferes with the impregnation. Egg albumen might be adapted for this purpose, and 

 should be smeared upon the surface (not necessarily upon the cut surfaces, however) either 

 before or during the hardening. 



The hardening fluid usually employed is potassium bichromate 3! % 4 vols. + osmic 

 acid i % i vol. Instead of this a mixture was sometimes used practically the same as that 

 recommended by Berkley; i.e. osmic acid 2% 16 vols. + potassium bichromate 5 % 84 vols. 

 It is difficult to determine whether this be better than the ordinary mixture. Possibly its 

 use results in the impregnation of a greater number of elements, owing to the fact that 

 there is more bichromate of potassium in the tissue available for the formation of silver 

 chromate. 



Another requisite to success is that a sufficient quantity be used. It is not easy to 

 give an absolute rule regulating the proportion between the amount of tissue and the quantity 

 of fluid. The pieces may first be placed in a small quantity of the fluid. This soon 

 becomes turbid, and after half an hour or an hour should be changed for a larger quantity 

 of fresh fluid. This should be examined now and then, and if at any time it should be 

 found to be turbid, or to have lost, or nearly lost, its characteristic smell of osmic acid 

 (accompanied often by a darkening of the fluid), it should be changed for fresh fluid. As 



