3G8 SPECIAL HISTOLOGY. 



of the gray substance of the cord, a gray, soft substance, consisting 

 chiefly of round, nucleated, pale cells, like nerve-cells, and measuring 

 0005-0'006 of a line. Besides these there occur, in its upper part 

 among the cells, true dark-bordered nerve-fibres, of various, and for. the 

 most part small diameter, and also numerous, fine, pale fibres, the nature 

 of which is not clear to me ; that is to say, whether they are processes 

 of cells or belong to the finest nerve-fibres. Remak (" Observ.," p. 18) 

 supposes, that in the Mammalia the true nerve-fibres of the filum all 

 go ofi" 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 potassa, affords the principal 

 aid. It is not easy to hit upon the proper proportion 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 sub- 

 stance of the cord remains soft in the interior and spoils, if too concen- 

 trated it becomes fragile and friable, and larger sections of it cannot be 

 obtained. I have unfortunately neglected to estimate precisely the 

 proper percentage of acid or salt in the solution; and can only say this 

 much, that one of a wine-yellow color 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 magnify- 

 ing powers may be procured and examined, either with or without 

 pressure or reagents, under various degrees of enlargement. The gray 

 substance is scarcely altered by the chromic acid, except that its ele- 

 ments are more easily separated, and I have in hardly any 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 

 gray 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 potassa, which renders all the nerve-cells pale. 

 To those who may consider the application of these reagents as too 

 powerful for such delicate organs as the spinal cord, I would remark : 

 1, that, as stated by Hannover, chromic acid alters the nerve- fibres, 

 especially of the gray substance, so little, that most of them do not even 

 become varicose ; and 2d, 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 to render them more transparent and their medullary contents 

 fluid. I have never, under any circumstances, seen more beautiful 

 nerve-tubes of the gray substance, than in preparations made with 



