THE FUNCTION OF REISSNER’S FIBER 185 
usual, yet it was extremely difficult to decide wherein the 
difference lay. 
There is yet another possible explanation. It has been seen 
that, where retraction did not follow the breakage of the fiber, 
there was no obvious reaction. It has been assumed that this 
was due to the maintenance of the tension of the fiber by the 
firm grip of the adpressed walls of the filum terminale upon the 
severed end of the fiber. Not only, however, might the tension 
be maintained but the connections with the numerous sensory 
cells in the central canal, whose filamentous processes con- 
tribute to the substance of the fiber, are preserved intact. The 
absence of the reaction may be partly or wholly attributable 
to this latter fact for, although the terminal plug at the hinder 
end of the fiber is clearly to be regarded as the principal insertion 
of the fiber, yet there can be little doubt that the attachmenis 
of the fiber by the component fibrillae throughout the length 
of the spinal cord must afford a very considerable support. 
Such evidence as these experiments have afforded suggests 
that the greater the extent of the retraction forward the 
greater is the degree of the reaction. It may well be, then, that 
as soon as the forward retraction is checked and the repairing 
process has brought about the unwinding and siraightening 
out of the fiber, the component fibrillae may forthwith begin 
to renew their attachment to the fiber. In this way while 
they may assist in restoring the tense condition of the whole 
fiber a constantly increasing number of sensory cells may be 
coming into action again, the diminution of the visible reaction 
being attributable to the restoration of these connections. 
Sargent has stated (04, p. 230) that “sharks have shown 
almost no capacity to heal wounds or regenerate skin.”” During 
the progress of this investigation several rays were taken in 
which there was evidence of the loss of part. of the tail but the 
stump had healed perfectly. While it may be that, as regards 
this power of regeneration, rays differ from sharks and dogfish, 
it must be remembered that my rays were, in general, quite 
small specimens and it is exceedingly likely that the injury had 
been inflicted when the specimens were very small, indeed. 
