Dark or Double-hordered Nerve Fibre. 203 



proportion to the quantity of the metallic deposit. For the study 

 of the more minute structure of the ganglionic bodies and the dark- 

 bordered nerve fibres, however, I found it very uncertain. Without 

 depreciating the assistance of reagents in general, I nevertheless 

 always prefer the examination of the f7'esh unchanged tissue. In 

 most cases, the advantages gained by the reagent are counterbalanced 

 by the changes which the tissue undergoes ; this is especially the 

 case in the examination of the delicate nervous elements, and like- 

 wise of the embryonic tissues. It cannot be well disputed that those 

 solutions of metalhc salts, or other combinations so much employed 

 in latter years, especially the nitrate of silver and chloride of gold, 

 have proved to be of great advantage to the histologist ; notwith- 

 standing, however, the aspects which are obtained from tissues so 

 prepared cannot be unconditionally relied upon; for the metalhc 

 deposit, as I know from experience, does not always take place 

 evenly throughout all parts of the tissue. The advantage gained 

 from such preparations consists mainly in this : that their various 

 tissues may be better distinguished from each other, and that in 

 consequence the study of their relations is much facilitated. It is 

 thus that the chloride of gold has proved useful to me in the 

 examination of the termination of nerves. For the study of the 

 more minute structure of the tissues themselves, I must, however, 

 always prefer a solution of chromic acid, for the reason that its 

 action is more equally distributed throughout the whole tissue, and 

 that no deposit obscuring the views takes place. Of all preserving 

 agents which I have hitherto used, a weak solution of chromic acid 

 seems to be attended with the least disadvantages. By the loss of 

 a small part of water, which the tissues, as I suppose, must suffer 

 by the action of the chromic acid, they gain, to a certain degree, in 

 consistency, in consequence of which their contours become more 

 sharply defined. Whatever I could possibly discover on speci- 

 mens prepared with the chloride of gold, I have thus far always 

 been able to demonstrate also either on the fresh specimen, or on 

 such as were prepared with a chromic acid solution ; but I could 

 not always demonstrate on chloride of gold preparations what I saw 

 on the former. It therefore appears to me somewhat hazardous to 

 draw definite conclusions from observations made on specimens pre- 

 pared with metalhc salts, without having them confirmed by others, 

 on such specimens as those just mentioned. The best results I have 

 always obtained by minute dissections under the loupe, or by the 

 making of fine transparent sections. 



As I look upon the tissues of man, in general, as the type of 

 the highest development, I therefore took the greater portion of 

 the material used in these researches, in a condition as fresh as 

 possible, from the human body, rarely longer than three or four 

 hours after death, sometimes sooner. But besides this, I also used 



