SPINAL CORD. 421 
of the coccyx. This prolongation receives the name of the filum terminale. The 
diameter of the cord is very much shorter than that of the spinal canal within 
which it lies. A wide interval is left between its surface and the walls of its 
canal, and this excess of space is clearly a provision for allowing free movement 
of the vertebral column without producing any jarring contact between the delicate 
spinal cord and the surrounding bones. 
Three protective membranes are wrapped around the cord. From within 
outwards these are termed (1) the pia mater; (2) the arachnoid mater, and (3) the 
dura mater. The pia mater is a fibrous mem- 
brane which forms the immediate investment. 
It is closely applied to the cord, and from its 
deep surface numerous fine septa penetrate into 
the substance of the cord. The arachnoid mater 
is an exceedingly delicate transparent membrane 
which is loosely wrapped around the cord so 
as to leave a considerable interval, termed the 
subarachnoid space, between itself and the pia 
mater, in which there is always a varying 
amount of cerebrospinal fluid. Outside the 
arachnoid mater, the dura mater forms a wide, 
dense, fibrous, tubular sheath, which extends 
downwards within the spinal canal for a con- 
siderable distance beyond the conical extremity 
of the cord. The spinal cord is suspended 
within its sheath or theca of dura mater by two 
lateral wing-lhke ligaments, termed the ligamenta 
denticulata. These extend outwards from the 
sides of the cord and are attached by a series 
of pointed or tooth-like processes to the inner 
surface of the theca of dura mater. Between 
the wall of the spinal canal and the dura mater 
there is a narrow interval, which is filled up 
by soft areolo-fatty tissue and numerous thin- 
walled veins arranged in a plexiform manner. 
Thirty-one pairs of spinal nerves arise from 
the sides of the spinal cord. These are classified ¥ se Sd: alae CONUS MebuLbAHs AND Tia 
. . : > ILUM TERMINALE EXPOSED WITHIN THE 
into eight cervical, twelve dorsal, five lumbar, — gpiwar, Canal, 
five sacral, and one coccygeal; and according 
to the attachments of these groups of nerves the spinal cord is arbitrarily sub- 
divided into a cervical, dorsal, lumbar, and sacral region. In employing these 
terms, therefore, to different districts of the cord, it must be understood that the 
regions are determined by. the nerve attachments, and not by any direct relation- 
ship between these parts of the cord and the sections of the vertebral column 
which bear the same names. 
Each spinal nerve is attached to the cord by a ventral and a dorsal root, and as 
these are traced to their central attachments they are seen to break up into a 
number of separate nerve fascicles or bundles, which spread out, in some cases very 
widely from each other, as they approach the side of the cord (Fig. 316). Each 
pair of nerves is therefore attached to a portion of spinal cord of some length, and 
such a portion, with its pair of nerves, receives the name of a “segment of the 
spinal cord.” It must be clearly understood, however, that, in so far as the surface 
of the cord is concerned, there is absolutely no means of marking off one segment 
from another, except by the nerve attachments. 
Conus medullaris 
First lumbar vertebra 
Filum terminale 
internum 
Roots of third lumbar 
nerve 
Dura mater 
Termination of 
theca of dura 
mater 
First sacral vertebra 
Filum 
terminale 
externum 
First 
coceygeal 
vertebra 
In the cervical and lumbar regions of the cord the nerve-roots are somewhat crowded 
together, so that little or no interv Ale is left between the adjoining root fascicles of neighbouring 
nerves. In the dorsal region, however, distinct intervals may be “observed, and the root fascicles 
are more loosely arranged. From this, it will be evident that the cord s segments in different parts 
of the cord are not of equal length. a the cervical region the segments measure about 12 mm, 
in length, in the dorsal region from 20 to 24 mm., and ‘in the lumbar region about 10 min. 
