‘—_ = 
428 THE NERVOUS SYSTEM. 
Central Canal (canalis centralis)—As previously stated, the central canal is 
found in the gray commissure. It is a very minute tunnel, barely visible to the 
naked eye when seen in transverse section, and it traverses the entire length of the 
cord. Above, it passes into the medulla oblongata, and finally opens into the fourth 
ventricle of the brain; below, it is continued for a variable distance into the filum 
terminale, and in this it ends blindly. Only in the lumbar region does the central 
canal occupy the centre of the cord. Above this level, in the dorsal and cervical 
regions, it lies very much nearer the anterior than the posterior aspect of the cord; 
whilst below the lumbar region, as it is traced down into the conus medullaris, it 
inclines backwards and approaches the posterior aspect of the cord. The calibre of 
the canal likewise varies somewhat in different parts of the cord. It is narrowest 
in the dorsal region; and in the lower part of the conus medullaris it expands into 
a distinct fusiform dilatation (very nearly 1 mm. in transverse diameter), which is 
termed the ventriculus terminalis (Krause). 
The central canal is lined by a layer of ciliated columnar cells, the deep taper- 
ing ends of which are prolonged into slender processes, which penetrate into the 
substance of the cord. These cells constitute the ling ependymal cells of the 
canal. 
The central canal is of interest because it represents in the adult the relatively 
wide lumen of the early ectodermal neural tube from which the spinal cord is 
developed. 
Filum Terminale—The delicate thread to which this name is applied is con- 
tinuous with the lower tapered end of the conus medullaris. It is easily distin- 
guished by its silvery and glistening appearance from the numerous long nerve-roots 
(cauda equina) amidst which it lies. It is about six inches long, and down to the 
level of the second sacral vertebra it is inclosed with the surrounding nerve-roots 
within the theca of dura mater. Piercing the tapered and closed end of the theca 
at this point, and receiving an investment from it, the filam terminale proceeds 
downwards in the sacral canal, and finally receives attachment to the periosteum on 
the dorsal aspect of the coccyx (Fig. 515, p. 421). It is customary to speak of the 
filum as consisting of two parts, viz. the filam terminale internum and the filum 
terminale externum, or the part inside and the part outside the theca of dura 
mater. 
The filum terminale externum is simply a fibrous thread, strengthened by the 
prolongation it receives as it pierces the dura mater. The filum terminale internum 
is largely composed of pia mater; but in its upper half it incloses the terminal part 
of the central canal, and around this a variable amount of the gray substance of the 
cord is prolonged downwards into the filum. When transverse sections are made 
through the upper part of the filum terminale internum some bundles of medul- 
lated nerve-fibres are observed clinging to its sides, and with these are associated 
some nerve-cells identical with those in the spinal ganglia. These represent rudi- 
mentary or aborted caudal nerves (Rauber). 
COMPONENT PARTS OF THE GRAY MATTER OF THE SPINAL CORD 
Neuroglia enters largely into the constitution of the gray matter of the cord. 
It forms a bed within which the nervous elements are distributed. These nervous 
elements consist of (1) nerve-cells and (2) nerve-fibres—both medullated and non- 
medullated. The nerve-cells le in small spaces within the neuroglia, whilst the 
nerve-fibres traverse fine passages the walls of which are formed of the same sub- 
stance. The neuroglia is thus an all-pervading basis substance, which isolates more 
or less completely the nervous elements from each other, and at the same time 
binds them together into a consistent solid mass. In the gray matter of the cord 
we distinguish two different kinds of material, viz. the substantia gelatinosa and the 
substantia spongiosa. The substantia gelatinosa, as already mentioned, forms the 
apex of the posterior horn of gray matter, and in this situation it is called the 
substantia gelatinosa Rolandi. It is also present in the form of a thick ring around 
the central canal, and here it receives the name of the substantia gelatinosa centralis. 
The remainder of the gray matter constitutes the substantia spongiosa. The sub- 
a 
