24 
OEOKOE E. NTCnOLLS. 
probably take an important part in tlie growtli of the fibre, 
and may extend backwards in it for an appreciable distance. 
Tlie fibre extends along the entire length of the canalis 
con tral i s , maintaining a practically^ nniform thickness in 
Ihis part of its course, although, perhaps, diminishing slightdy 
in diameter near its posterior end. 
Towards the hinder end of the body the spinal cord under- 
goes a considerable diminution in size, and tapers off into the 
delicate filnm terminale. This becomes a simple epithelial 
tube, and may, in some fishes, pass beyond the enclosing 
vertebral canal, to lie unprotected, except for its meninges, 
just beneath the skin. At its actual extremity the filum 
terminale becomes somewhat dilated, and contains an ovoid 
enlargement of the canalis centralis which is known as 
the sinus (ventriculus) terminalis (fig. 40, s.f.). The 
sinus terminalis is, however, only incompletely surrounded 
by the enlargement of the end of the filum terminale, for 
at about the middle of the sinus the ependymal as well as 
the nervous elements of the cord disappear, so that the 
canalis centralis actually opens by a Avidc ajierture, 
the “terminal neural pore,” into a lymph-space that is 
morphologically continuous with the perineural spaces. The 
jiosterior Avail of the sinus terminalis is thus formed only 
by the fibrous sheath of the spinal cord, which aj)pears to 
consist of united pia and dura mater. From the canalis 
centralis Feissner’s fibre passes through the terminal neural 
pore, behind Avhich point in many cases, if not in all, it expands 
into a conical “terminal plug” (fig. 51, which passes 
into, and blends with, the connective tissue of the meningeal 
])ortion of the wall of the sinus terminalis. 
The actual fibre is an extremely tenuous thread, who.se 
normal diameter in the adult condition in the greater number 
of species Avhich I have examined is between one and three 
micra. In well-hardened material it is almost always dis- 
tinctly brittle,^ but in life or in the fresh state in recently 
' In a sin<>lo caso (Geotria) tlio fihrn appoar.s to have retained, to a 
great extent, its flexihility even in Caimda hal.sani. 
