TRANSACTIONS OF SECTION D, 507 
5. Reissner’s Fibre and the Sub-commissural Organ in the Verlebrate 
Brain. By Professor ArruuR Denpy, F’.R.S. 
The sub-commissural organ is a groove, or pair of grooves, lined by much 
elongated, ciliated, epithelial cells, and situated beneath the posterior com- 
missure. From these cells originate a large number of very slender fibrillae— 
probably elongated cilia—which unite together to form Reissner’s fibre, which 
extends as a highly elastic, tightly stretched thread backwards through the 
brain cavities and the canalis centralis of the spinal cord to the extreme end 
of the latter, where it is attached to a plug of connective tissue lying in the 
sinus terminalis and blocking up the terminal neural foramen. Both Reissner’s 
fibre and the sub-commissural organ are well developed in all the great verte- 
brate groups from cyclostomes to primates. 
Reissner’s fibre was interpreted by Porter E. Sargent as a bundle of nerve- 
fibres which form a short circuit for motor optic reflexes, enabling animals to 
turn more rapidly away from a sudden source of danger. The sub-commissural 
organ (ependymal groove) was regarded as a mere attachment plate for the 
constituent nerve-fibres. This view, though somewhat widely accepted, was 
based upon altogether insufficient evidence and cannot be maintained. The 
recent researches of Nicholls in particular have shown that Reissner’s fibre is 
not of a nervous nature. Dendy recently suggested that the sub-commissural 
organ might be a kind of intra-cerebral sense-organ, concerned, together with 
Reissner’s fibre, in automatically regulating the flexure of the long axis of the 
body. Variations in the flexure of the body might cause variations in the 
tension of Reissner’s fibre, and these might act as stimuli upon the cells of the 
sub-commissural organ. Such stimuli might be transmiitted to ganglion cells, 
by which in turn the flexure of the long axis of the body might be con- 
trolled. This view was supported by the fact, demonstrated by Dendy and 
Nicholls, that in man, with his erect posture and but slightly flexible vertebral 
column, the sub-commissural organ is reduced to a mere vestige—the mesoccelic 
recess—while Reissner’s fibre is probably absent. 
Nicholls has recently experimented with regard to the function of Reissner’s 
fibre and the sub-commissural organ in fishes, and finds his results in harmony 
with Dendy’s suggestion. He finds that in some fishes the filum terminale of the 
spinal cord projects beyond the vertebral column into the tail fin, and that a 
very slight puncture with the point of a scalpel in this region is sufficient to 
sever Reissner’s fibre. After the operation the fish assumes a characteristic 
attitude, with the tail bent upwards, which it retains for some days. He con- 
cludes, from the examination of serial sections, that in these cases Reissner’s 
fibre has sprung forward in the spinal cord for a longer or shorter distance, and 
coiled itself, as is its wont, into a knot; this knot catches in the canalis centralis 
and prevents further recoil, and the jibre then probably regenerates and forms a 
new attachment. The histological character of Reissner’s fibre, its highly 
elastic nature, and the character of its anterior and posterior connections lend 
no support to the view that it is a nervous structure. 
6. On the Cestode Parasites of Trout, with special reference to the 
Plerocercoid Disease of Trout from Loch Morar. By J. W. 
CHALONER. 
During the past six months I have been investigating the parasites of fresh- 
water fish, and especially the cestode parasites of the Salmonide. The trout 
(Salmo fario) from Loch Morar, Inverness-shire, are infected by a larval Bothrio- 
cephalid (plerocercoid larva), which appears to cause considerable damage to 
the fish. Through the kindness of W. H. Caldwell, Esq., of Morar- Lodge, I 
have been supplied with an abundance of material for investigating the life- 
history of this particular form, which is quite unknown. 
These plerocercoids are found encysted in the wall of the intestine and of 
adjacent organs, the cysts being formed by the tissues of the trout, not by the 
worm. The majority of the cysts were found in the region of the pyloric ceca, 
