STRUCTURIi AND DEVELOPMENT OP REISSNER S FIBRE. 
65 
inclucledj is actually coutained iu two adjoining sections. It 
liaSj however, sprung slightly, so that it lies (in tlie fourth 
ventricle) a little out of its proper position, as shown in fig. 
20. This retraction lias permitted some forward displace- 
ment of the point of union of the two halves of the fibre, 
these apparently meeting beneath the sub-commissural organ 
(fig. 58). In front of this point the paired fibres spread out 
iuto a brush-like end of lesser factors, which have broken free 
from their attachments and are slightly displaced in the sec- 
tions. From the cells of the sub-commissural org'an many 
constituent fibrilla) may be seen freely projecting, anti these 
are particularly noticeable in the infra-pineal recess, as Dendy 
has described and figured (’07, p. 17, and fig. 6). 
In anotber series the anterior end of the fibre closely 
resembles that just described, having also been slightly 
displaced anteriorly, so that in neither of these two can 
Reissner’s fibre be actually traced to the cells of the sub- 
commissural organ. Posteriorly, however, the fibre in this 
series has recoiled into a tangled snarl which was apparenily 
brought up at the hinder end of the isthmic canal. 
In the last of these series the fibre has recoiled forward 
from the point of section to fonn a typical tangle beneath the 
])osterior commissure as seen in the photomicrogTaph (fig. 19). 
In front of this point the paired factors of the fibre may be 
well seen. 
In the four remaining series in which Keissner’s fibre was 
i3iade out it had in every case retracted into tightly wound 
knots, in the formation of which the fibre had become torn 
free from its attachment to the sub-commissural organ. In 
one case alone the snarl a])peared as a mass of coiled and 
greatly swollen fibre immediately below the recessus infra- 
piuealis. In two others it occurred in similar form, but at 
the posterior border of the sub-couimissural organ, while iu 
the remaining instance it was less coiled but enormously 
swollen, and lying freely in the mesocoul. In this latter 
specimen the diameter of the piece measured varied between 
9'6 fi and l8/i. The normal diameter of the fibre iu (Jeotria 
VOL. 58, l-AKT 1. — NEW SEEIES. 5 
