Dark or Douhle-hordered Nerve Fibre. 207 



shortly after death on the addition of water. According to my 

 observations, the fibrillous layer consists of those fibrils just 

 described, which are intimately connected, not only to each other, 

 but also to the inner surface of the tubular membrane, by an 

 intermediate substance, soluble in water. The mutual connection 

 of these two membranes forms the original double contour of the 

 nerve fibre. The appearance of the double contour has been 

 ascribed, like that of those clods, hooks, or other figures, to a 

 coagulation of the outermost portion of the nerve-medulla, caused 

 by an access of the atmosphere, — and it is not very long ago that 

 even the formation of the axis cylinder itself was ascribed to this 

 cause, and the entire double-contoured nerve fibre regarded as 

 consisting only of a homogeneous semi-fluid substance, enclosed 

 within the tubular membrane. Again, it is said that the nerve 

 fibre, immediately after its removal from the living organism, and 

 without the addition of reagents causing visible changes, has been 

 seen in the form of a transparent cylinder of a dull lustre, bordered 

 by simple contours and devoid of any other distinguishing character.* 

 In the transparent eyelid of the frog and in the tail of the tadpole, 

 the nerve fibres, it is said, have been seen — but only rarely — in 

 the form of homogeneous, clear, milk glass-like thrcads.f The 

 observation on the tail of the tadpole can hardly be relied upon, as 

 it is not very probable that, in this instance, the double-bordered 

 nerve fibre had obtained its full development, but may rather still 

 have existed in the form of a naked axis cylinder. 



Whether the statements last mentioned, together with others, 

 are well founded, or whether they simply rest upon some optical 

 delusions, I venture not to decide ; but as far as my own researches 

 are concerned, I must say that I have never seen the nerve fibre in 

 question in the form of a clear thread with simple contours, — not 

 even in the uninjured bundles of nerve fibres of the ischiatic nerve 

 of the living frog. Nevertheless, it might be possible that the 

 fibres of an exposed nerve undergo a sudden change by the mo- 

 mentary contact of the atmosphere. But if this were the case, it 

 must also be presumed that the electro-motory behaviour of these 

 fibres would differ from that of those belonging to a nerve still 

 occupying its place in the uninjured animal body, — in consequence 

 of which many observations relating to the electricity of the nerves 

 would appear unfounded. In the same manner, the axis cylinder 

 also should possess a different degree of conducting power in a 

 fluid condition, than in a state of coagulation. 



Considering, therefore, the difficulty of deciding this question, 

 and, further, the different views still held by a number of chemists 

 regarding the chemical composition of the nerve-medulla in general, 



* Funke, 'Lehrbuch der Physiologie,' 4th edit., vol. i., p. 661. 

 t Frey, ' Handbuch der Histologie u. Hifstochemie,' 2nd edit., p. 353. 

 VOL. XI. R 



