
.1905.| The Structure and Function of Nerve Fibres. 327 
The appearances seen at the nodes of Ranvier have yet to be considered. 
‘It is conceivable that the nodes of Ranvier may represent situations easily 
injured by such stretching of the fibre as is incidental to the process of 
teasing. This supposition may account for the appearances seen at some few 
of the nodes, but not for the greater number of those observed. The only 
nodes consistently affected in every preparation are such as belong to inter- 
‘nodes in which there is, usually at some distance, some other site of injury 
obviously present—for example, the nodes next to the cut ends of the fibres. 
The large majority of the other nodes escape. Again, it is very notable that 
such nodes as are affected are involved at a much later time than the 
ordinary sites of injury. It therefore seems possible that the staining effects 
at the nodes are due not to primary but to secondary accumulations of 
material deposited during the progress of the injury-current. I have 
‘previously stated my inference that the internodal segments of the fibre 
represent a perfectly insulated path, that is to say, that the myelin-sheath is 
‘perfectly impermeable to the ions carrying the injury-current. The nodes of 
Ranvier are therefore left as the only situations through which the current 
can leak. It is possible that, although relatively permeable, they are still not 
absolutely permeable. They would then form the site of secondary accumu- 
‘lations of material in transit, and, therefore, of processes of polarisation. 
Digressing for a moment to consider the bearing of such a statement upon 
‘the polarisation of non-medullated nerve. It is conceivable that these 
fibres, possessing no nodes of Ranvier, are not so perfectly insulated as 
medullated nerves which do, since the non-medullated nerve is one long node 
‘of Ranvier throughout its whole length. 
It will be seen later that all the appearances I have described are due to 
the deposition of potassium salts at the site of injury, and their migration 
from thence with the injury-current. I have now, however, to relate some- 
thing of the difficulty into which I got in attempting to prove this fact 
previous to the great assistance provided by Macallum’s reagent. Before 
doing so, however, I may be allowed to sum up the appearances described 
above in terms of this statement, that they are produced by precipitations of 
dye in solutions of potassium salts. At the site of injury potassium salts are 
found in concentrated aqueous solution, The process giving origin to their 
appearance is a reversible process, they may disappear from this state of 
simple aqueous solution, returning to their original concealed condition. 
From the site of injury this potassium salt migrates at a uniform pace into the 
nerve-fibre. Secondary accumulations of potassium salts occur at the nodes 
of Ranvier. 
Having observed results presumably due to the differential precipitation of 
