NERVOUS CENTRES. (Human Anatomy. Tae Sprnat Corp.) 
tomoses of the two anterior spinal branches of 
_ the vertebral. In one case there was no artery 
_ on the left side, but the right was twice as large 
_ as usual. The vessel is of considerable size 
until it has passed below the cervical enlarge- 
: ment of the cord, from which point down 
nearly to the lumbar enlargement it becomes 
_ exceedingly delicate; a little above the last- 
named enlargement it suddenly increases in 
_ size, again gradually diminishes as it ap- 
. sae the lower end of the spinal cord, and 
ming capillary is prolonged down to the 
sacrum, together with the fibrous string in 
_ which the spinal cord terminates.” 
“ During its course this artery receives la- 
_teral branches from the ascending cervical and 
_ the vertebral in the neck, and from the spinal 
“Dranches of the intercostal and lumbar arteries 
in the back and loins. Those branches pene- 
trate the fibrous canal formed by the dura 
‘mater around each of the spinal nerves; be- 
come applied to the nervous ganglia to which 
they supply branches, yet intermixed with and 
follow the course of the corresponding nerves ; 
send small twigs backwards to the posterior 
‘spinal arteries, and terminate in the anterior 
Eical trunk at variable angles, similar to those 
at which the nerves are attached to the cord.”’* 
_ The posterior spinal arteries arise from the 
vertebral or from the inferior cerebellar artery: 
they incline backwards to the posterior surface 
of the spinal marrow, along which they descend 
in a tortuous manner, anatomosing freely with 
each other and with the small arteries which 
accompany the nerves in the intercostal fora- 
mina. A network of vessels surrounds each 
posterior root of a spinal nerve, derived from 
ramifications of those arteries. We can trace 
n agi spinal arteries as low down as the 
region, distinct throughout their entire 
_ Veins—The blood is returned from the 
_ Spinal cord by a venous plexus which emerges 
the pia mater and is spread over its 
| whole surface: opposite the roots of each nerve 
a small vein is formed, which passes outwards 
with the nerve in the same sheath, and empties 
itself into the large vein which is situate in the 
intervertebral foramen. Veins accompany the 
‘anterior and posterior spinal arteries in the 
“upper part of their course. Branches from 
this plexus frequently pass to the dura mater 
volved in a fold of arachnoid, and thus com- 
unicate with the general plexus which sur- 
Tounds the sheath. 
We observe that the arteries of the spinal 
rd are reduced to a very minute size before 
etrate the substance of that organ. 
e t vessels are therefore found on its 
or in its fissures. And it may be 
further remarked that when vessels of a size to 
| be readily detected by the naked eye penetrate 
| the substance, numerous foramina, produced by 
the Separation of the nervous fibres, become 
distinetly visible. This is very obvious in the 
white commissure. 
The purpose of such a minute subdivision of 
’ * Cruveilhier, Anat, Descr. 
VOL. IIT, 
657 
bloodvessels, prior to their entrance into the 
substance of the cord, must evidently be to 
guard the nervous substance agaipst the impulse 
of several columns of blood of large size. A 
similar provision, made in a more conspicuous 
manner, is manifest in the brain, and will be 
noticed by-and-bye. 
Of the spinal nerves.—There is a pair of 
Spinal nerves for each intercostal foramen, and 
for that between the atlas and occiput. We 
can thus enumerate in all thirty-one pair of 
nerves having their origin from the spinal cord, 
and this number is exclusive of the spinal ac- 
cessory nerve which is connected with the 
upper part of the cervical region. 
he spinal nerves have the following very 
constant characters. Each has its origin by 
two roots, of which the anterior is distinct- 
ly inferior in size to the posterior. The 
ligamentum denticulatum is placed between 
these roots. Each root passes out through a 
distinct opening in the dura mater. Imme- 
diately after its emergence a ganglion is 
formed on each posterior root, and the ante- 
rior root lies embedded in the anterior surface 
of the ganglion and involved in the same 
sheath (fig 378), but without mingling its 
fibres with those of the ganglion. Beyond it, 
Origin of a spinal nerve. 
( After Beil. ) 
A, A, anterior root. 
P, posterior ditto. 
G, ganglion on the posterior root. 
C, compound nerve resulting from the commin- 
gling of the fibres of both roots. 
the nervous fibres of both roots intermingle, 
and a compound spinal nerve results. The 
trunk thus formed passes immediately through 
the intervertebra! tube and divides into an an- 
terior ‘and posterior branch, which are distri- 
buted to the muscles and integument of the 
20 
