130 GEORGE E. NICHOLLS 
reception of the fiber. Emerging from the anterior end of this 
groove, sometimes as a paired structure (12a, figs. 10, 11), 
Reissner’s fiber stretches freely through the midbrain ventricle 
to the neighborhood of the posterior commissure. 
The ventricular surface of the posterior commissure is clothed 
by a band of highly developed epithelium which is often folded 
in both the longitudinal and the transverse planes. It is to this 
remarkable tract of epithelium that the name ‘sub-commissural 
organ’ has been given. Owing to its longitudinal folding it has 
usually, in transverse sections, a horseshoe shape and partly 
encloses a median dorsal groove (the ‘sub-commissural canal’). 
Reissner’s fiber, if it has continued as an unpaired structure so 
far forward, breaks up at the hinder end of the posterior com- 
missure into two or more strands which subdivide within this 
median groove into numerous delicate fibrillae which are con- 
nected with the cells of the sub-commissural organ. 
A study of the development of the fiber indicates that it 
arises by the confluence of numerous filaments springing from 
sub-commissural organ and that the composite thread so formed 
extends backwards into the central canal of the spinal cord. 
Within the central canal it probably receives numerous additional 
components from scattered cells in the epithelium which lines 
the central canal. 
Perhaps the most remarkable characteristic of the fiber is its 
extreme elasticity. In life it appears to exist under quite con- 
siderable tension and to be somewhat prone to accidental break- 
age. In that event, or following artificial section, the free ends 
may recoil sharply to form tangled knots or ‘snarls.’ The re- 
traction is accompanied by a marked increase in the diameter 
of the fiber. 
This elasticity usually disappears very rapidly during the 
process of fixation and the preserved fiber may become distinctly 
brittle (fig. 21). If, however, the fiber be severed before fix- 
ation is completed a retraction will still take place, but much 
more gradually, and it will then be found that the fiber has 
become wound in a more or less open spiral. Even where the 
recoil has been an abrupt one, resulting in the formation of the 
