326 Prof. J. S. Macdonald. [Apr. 17, 
current is a consequence of an output of acid waste-products originating 
from some gross chemical change in the proteids of the axis-cylinder. 
The presence of the injury-current in fibres still under observation with the 
microscope can be determined by a resultant formation of deeply stained 
granules, the granules mentioned above. ‘These granules are not structures 
pre-existent in the nerve-fibre that are successively stained by the gradually 
invading dye. They are first observed at a time when the dye has already 
reached situations far in advance of the site of their formation, They may 
be seen to grow from scarcely visible points into irregular spherules of 
considerable size. That they are not formed by any uncomplicated process 
of diffusion is deduced from the rate with which their successive appearance 
involves measured lengths of the axis-cylinder. Equal lengths of the axis- 
cylinder are involved in exactly equal times. The invading material giving 
rise to their formation advances into the axis-cylinder, therefore, at an 
absolutely uniform rate. The rate of advance is of the same order as that 
of known ionic velocities.» The inference is, therefore, drawn that they 
represent the consequences of the conduction of the electrical current along 
a perfectly insulated path. The material giving rise to their formation 
comes from the site of injury and not from outside of the fibre, and is 
probably the material, which at the site of injury forms the newly deposited 
cause of the injury-current. Modifying the probable value of the injury 
E.M.F. by my original expedient of varying the external solution—changing 
the value of the salt solution in which the dye is presented to the fibres— 
the rate of granule formation undergoes anticipated changes. ‘Within the 
small limits of modification practicable, the rate is inversely proportional to 
the strength of this outer solution. It should be noted that there is some 
difference between the behaviour of these granules® when reddened by the 
stain from that of those observed under conditions such that they are yellow. 
The yellow granules disappear with much greater rapidity, leaving no trace 
of their former presence. The conditions leading to their appearance are, 
it may be concluded, therefore, readily reversible, since the accumulation of 
material at the site of injury and at neighbouring points vanishes in this 
case more rapidly than can be explained by its dispersal along the nerve- 
fibre. The inference is drawn that this material reassumes the original 
relation to the colloidal solution of the nerve-fibre which it possessed 
previous to the occurrence of injury. The reddened granules are more 
lasting, although not permanent. They are, therefore, more easily observed, 
and the definite statements made as to the rate of granule formation are 
made from the results of their observation. In the nerve-fibres of cooled 
frogs the granules are yellow, in those of warmed frogs they are yellow. 
