THE FUNCTION OF REISSNER’S FIBER 129 
In this undisturbed state the fiber issues (in the lower verte- 
brates at least) from the posterior end of the central canal through 
a terminal opening (the ‘terminal neural pore’) into the peri- 
neural space, where it ends in an elongated conical expansion 
(the ‘terminal plug’). There is, in these forms, a widening of 
the lumen of the central canal at its posterior end to form a 
chamber (the ‘terminal sinus’) which is only incompletely 
enclosed by the walls of the filum terminale, this being, in this 
region, reduced to a simple epithelial tube. The posterior wall 
of the terminal sinus is formed by the meningeal sheath into 
which the terminal plug is inserted (text-figs. 1, 3). 
——— = = 
Mane ge boaecesmees -- 
aM TNT Niel! ae a 
Halfuittbey Uy sitot Za SSH woh Vivant 
PaWR0-509 ede Gernieea.e. 
Text-fig. 1 Slightly diagrammatic median sagittal section through the end 
of the filum terminale to show the normal (undisturbed) arrangement of the 
sinus terminalis and the insertion of Reissner’s fiber. c.c., central canal of the 
spinal cord (and terminal filament); f.t., filum terminale; mn., meninges, form- 
ing the hinder wall of the sinus terminalis; R.f., Reissner’s fiber; s.t., sinus ter- 
minalis; ¢.p., terminal plug. 
Traced forward, the normal fiber is found to pass from the 
central canal of the spinal cord into the fourth ventricle. It 
maintains its position in the middle line and appears, in this 
part of its course, to lie absolutely freely at the level of the middle 
of the height of the ventricle. 
At the anterior part of the hindbrain, however, the fiber 
stretches in contact with the lower surface of the cerebellum. 
There is frequently, upon the lower surface of the rhombo- 
mesencephalic fold, a narrow groove (the ‘isthmic canal’) which 
may show traces of a paired character and which serves for the 
