4 ATLAS OF NERVE CELLS 



an example, three pieces cm. thick, and about 3 cm. by 4 cm., after being in a small 

 quantity for an hour, were placed in a dish containing 100 cc. of the hardening fluid, which 

 had to be renewed the next day. This probably represents the maximum quantity of tissue 

 which could be hardened in such a quantity of fluid. 



If the pieces have flat surfaces which lie quite closely applied to the bottom of the 

 dish, a small quantity of absorbent cotton should first be placed in the dish, so that the 

 hardening fluid may have easy access to all sides. 



The crucial point in the method lies in the duration of hardening. How long the tissue 

 shall be hardened depends upon (a) the kind of impregnation aimed at, (b) upon the kind 

 of tissue, (f) upon the stage of development of the tissue, whether adult or embryonic, (d) upon 

 the kind of animal, and (e) upon the temperature in which the hardening takes place. 



(a) According to the rule laid down by Cajal for the cords of embryo chicks, and by 

 Kolliker and Lenhossek for the human embryonic cord, the different elements are impreg- 

 nated, according to the degree of hardening, in the following order: (i) Neuroglia (2-3 days), 

 (2) Nerve cells (3-5 days), (3) Nerve fibres and collaterals (5-7 days). The times given apply 

 to the human cord. This is only true in a very general way. Both neuroglia and 

 nerve fibres usually appear in any preparations in which impregnation has taken place, irre- 

 spective of the duration of hardening. One factor of uncertainty, often overlooked, is caused 

 by differences in the ratio between the bulk of tissue and quantity of hardening fluid. In 

 proportion as the former is increased, the fluid becomes less efficient during the hardening, 

 and thus a dish containing a certain quantity of tissue and hardening fluid may really be 

 in a more advanced stage of hardening than a greater quantity of tissue placed in the same 

 amount of hardening fluid for the same period of time. 



If the tissue be underhardened, the result is often that it is filled, after impregnation, 

 with a diffuse red precipitate of silver chromate. The effect of overhardening is usually a 

 complete absence of any precipitate of any kind in the tissue or, possibly, the formation of 

 numbers of clean, sharply defined black crystals. As the tissue progresses toward the maxi- 

 mum period of hardening, the tendency is for fewer elements, perhaps, to be impregnated, 

 but those that are stained are brought out with greater cleanness. The tendency is also 

 for the tissue to be more uniformly impregnated at all depths. This is probably due to 

 the fact that the tissue is hardened more uniformly throughout when left in the hardening 

 fluid longer. 



(b) Respecting the kind of tissue, it is within the scope of the present work to deal 

 with the central nervous system only, and no very definite rule can be laid down, owing to the 

 factors just mentioned. I am inclined to believe that the cortex of the cerebellum requires 



