132 JoURNAL OF CoMPARATIVE NEUROLOGY. 
simply a widely-spread, digitate reticulum. There are many 
thorn-like branches from all of the strands, and numerous anas- 
tomoses occur between the principal ones. Consult Fig. 57, av. 
The remarkable group of nerve-cells comprising the roof- 
nucleus has been variously interpreted, but its true relations 
were not discovered until quite recently. Rouon (’77) first 
described this collection of cells as the Dachkerne, a name ap- 
plied, of course, from the position occupied by its paired mem- 
bers in the roof of the aqueduct of SyLvius. It was later rec- 
ognized in the brains of various fishes, BURCKHARDT (’92) iden- 
tifying it as the midbrain trigeminal nucleus. It has remained 
for SARGENT (1900) to show that not only is the roof-nucleus 
present in all vertebrates, but that it is part of a most interest- 
ing mechanism, the fibre of REISSNER (’60). REISSNER’S fibre 
is a rod-like body lying in the central canal of the spinal cord, 
extending forward. It had come to be neglected entirely in 
recent years owing to the prevalent view that it merely repre- 
sents a coagulation of the cerebro-spinal fluid. SARGENT (1900) 
demonstrated that such a view is erroneous, that REISSNER’s 
fibre is a real structure, with a perfectly definite character and 
distribution, and, furthermore, that it is found in all classes of 
vertebrates, always extending from the posterior end of the 
canalis centralis to the anterior end of the aqueduct of SyLvrus. 
He has also shown (1901) that the fibre represents, in the main, 
the closely fused axones of the cells of the roof-nucleus. The 
great number of neurones comprising the anterior field of the 
roof-nucleus send their axones into the aqueduct of SyLvius, 
as noted above, whence they pass backward as REISSNER’S fibre 
through the extent of the fourth ventricle and the central canal 
of the spinal cord. Fine processes are given off to the nervous 
matter of the cord as the fibre proceeds. 
An interpretation of the roof-nucleus and of REISSNER’S 
fibre arising from it may now be attempted. In the next to the 
last paragraph, a nexus was traced between the fibres of the 
stratum medullare profundum and the neurones of the roof- 
nucleus. There are here, it is evident, the elements of a tract 
through which quite direct connections may be established be- 
