THE NERVOUS SYSTEM. 117 



processes of cells or belong to the finest nerve-fibres. Rcmak 

 ('Observ.,' p. 18) supposes, that in the Mammalia tbc true 

 nerve-fibres of the jihtm all go off in lateral branches of it, the 

 existence of which were detected by him. 



[In the investigation of the course of the fibres in the spinal 

 cord, chromic acid, or instead of it, chromate of potass, affords 

 the principal aid. It is not easy to hit upon the proper pro- 

 portion of acid, and so to harden the cord, previously stripped 

 of its dura mater, and cut across with a sharp knife at the 

 points selected, that very thin transverse sections may be 

 taken from it. If the solution be too much diluted, the 

 substance of the cord remains soft in the interior and spoils, 

 if too concentrated it becomes fragile and friable, and larger 

 sections of it cannot be obtained. I have unfortunately 

 neglected to estimate precisely the proper per centage of acid 

 or salt in the solution; and can only say this much, that one 

 of a wine-yellow colour acted the best. Objects thus properly 

 hardened may be cut at pleasure with a razor, or other very 

 sharp instrument, if due care be taken, particularly in the 

 avoiding of any sawing motion ; and sections suitable even for 

 the highest magnifying powers may be procured and examined, 

 either with or without pressure or reagents, under various 

 degrees of enlargement. The grey substance is scarcely 

 altered by the chromic acid, except that its elements are more 

 easily separated, and I have in hardly an}* other way seen its 

 nerve-cells, together with their processes, and the nerve-fibres, 

 more beautifully displayed. If it be desired to examine the 

 former, the grey substance is broken up in water, which now 

 no longer produces any change ; or, what is best, in the 

 solution of chromic acid itself; but if the examination of the 

 latter be wished, it is by far the best to employ diluted caustic 

 soda or potass, which renders all the nerve-cells pale. To 

 those who may consider the application of these reagents a* 

 too powerful for such delicate organs as the spinal cord, I 

 would remark : 1. that, as stated by Haunover, chromic acid 

 alters the nerve-fibres, especially of the grey substance, so 

 little, that most of them do not even become varicose; and 2. 

 that soda, added to a preparation made with chromic acid, 

 does not act upon the fibres for a long time, and only so far as 



i. 27 



