te 
furtber, it is recorded of one specimen that when swimming vertically 
it would carry its head curved dorsally and also that it swam about 
for a while upon its back. For none of these reactions would the 
supposed retardation of optical stimuli account, nor would this explain 
the general sluggishness which was the invariable consequence of the 
operation. 
The operation, as performed by SARGENT, consisted in drawing a 
curved needle several times across the floor of the fourth ventricle 
and was clearly open to the serious objection that its performance 
involved risk of damage to the brain itself. As a matter of fact it 
was always followed by grave constitutional disturbance — most, if not 
all, of the animals operated upon dying within a few days. 
It appears, therefore, that we can attach but little value to these 
experiments for, although the behaviour which SARGENT observed in 
his animals may have been due in part to the injury to REISSNER’S 
fibre, yet it is not possible to decide to what extent it may have 
been due to the disturbance of the central nervous system itself. 
SARGENT was scarcely justified therefore in his belief that “these ex- 
periments show clearly .... that when Reissner’s fibre is severed the 
power to respond quickly to optical stimuli is lost” (1904, p. 231). 
His observations might perhaps be held to indicate that the operation 
had brought about a partial loss of control over the movements and 
equilibration of the body, but there certainly was not sufficient evi- 
dence even to connect this reaction with ReEIssner’s fibre, much less 
to show that severance of this fibre caused delay in the transmission 
of optical stimuli. 
The only other experimental work upon ReIssner’s fibre, of which 
I can find any record, is that carried out by Sir Victor HoRSLEY 
and Dr. McNALTY upon Macacus. In this animal, minute electrolytic 
lesions were made in the interior of the spinal cord, thus destroying 
the continuity of the fibre (HorsLey, 1908). Although no observations 
are recorded upon the subsequent behaviour of the animal, considerable 
interest attaches, nevertheless, to these experiments, for examination 
of sections of the spinal cord and brain enabled Horsey to state 
that Reıssner’s fibre, when its continuity is destroyed, exhibits none 
of those degenerative changes characteristic of severed nerves. 
In December 1909, Denpy put forward an entirely novel hypo- 
thesis. He suggested that Reıssner’s fibre and the associated sub- 
commissural organ (“Ependymal Groove”) might be concerned in re- 
gulating flexure of the body. He pointed out that the fibre, which 
exists in life under considerable tension, would be subjected to con- 
