DIVISIONS OF A SPINAL NERVE. 567 
wards in the spinal canal. The collection of nerve-roots which occupies the lower 
part of the canal below the first lumbar vertebra, and comprises all the nerve- 
roots below those of the first lumbar nerve, is designated the cauda equina. They 
arise from the lumbar enlargement and conus medullaris, and surround the filum 
terminale of the spinal cord. 
Within the spinal canal the nerve-roots are in relation with the meninges of 
the cord, and are separated from one another by the ligamentum denticulatum, and, 
in the neck, by the spinal part of the spinal accessory nerve. Each receives a 
covering of pia mater, continuous with the neurilemma; the arachnoid invests each 
root as far as the point where it meets with the dura mater; the two roots, after 
piercing the dura ‘mater separately, are enclosed by it in a single tubular sheath, in 
which is included the spinal ganglion of the dorsal root. 
Divisions of a Spinal Nerve.—The spinal nerve thus formed lies in the 
intervertebral foramen, except in the case of the first two cervical and the sacral 
and coceygeal nerves. It immediately ied ea pay My 
divides into two primary divisions, named BAO BRANCH 
respectively the posterior and anterior \ 
primary divisions. Just before its divi- 
sion each nerve gives off a minute re- 
current branch, which re-enters the 
vertebral canal after effecting a junction 
with a branch from the sympathetic M 
cord, and is distributed to the spinal AUER 
cord and its membranes. Ae 
The posterior and anterior primary 
divisions of the spinal nerve are respon- 
sible for the innervation of the skeletal 
POSTERIOR 
PRIMARY 
DIVISION 
POSTERIOR 
ROOT 
MIXED 
SPIMAL 
ANTERIOR 
RAMUS PRIMARY 
sy & COMMUNICANS \\wision 
. . POSTERIOR 
muscles and the skin covering the trunk BRANCH 
and limbs. 
In relation to certain spinal nerves, a series 
of much smaller branches exist which are con- eles 
nected with the sympathetic system (Fig. 421, 
SY), in a way to be described later. These con- 
stitute the white rami communicantes, and may sg Ales J ANE, 
be termed the visceral divisions of the spinal 
nerves. They are derived from the anterior 
primary divisions of the nerves, and receive 
air fihres a] r fr > . rs 0 a ANTERIOR 
their fibres mainly from the ventral roots, pileliioey 
though, at least in the case of certain nerves, 
from the dorsal roots as well. These nerves the. flere Gin GaSe IDR GTw 
are directed inwards from the intervertebral TyprcaL SPINAL NERVE. 
foramen over the vertebral column, and, be- 
coming connected with the sympathetic cord, convey spinal fibres to the organs and tissues in 
the splanchnic area. 
The posterior and anterior primary divisions of the nerves contain fibres from 
both dorsal and ventral roots. Indeed each root can be seen on removal of its 
sheath to divide into two portions, of which one portion enters into the formation 
of the posterior, the other into the formation of the anterior primary division. 
The posterior primary divisions, with the exception of the first two, are smaller 
than the anterior primary divisions. They supply in general the skin and muscles 
of the back. They do not supply muscles. of the limbs; although in their 
cutaneous distribution they are prolonged on to the back of the head, the shoulder, 
and the buttock. They form two small plexuses, the posterior cervical and the 
posterior sacral plexuses. The anterior primary divisions are, with the exception 
of the first two cervical nerves, much larger than the posterior primary divisions. 
They supply the sides and fore- -parts of the body, the perineum, and the limbs. 
For the most part they have a complicated arrangement. The thoracic or inter- 
costal nerves alone have a simple mode of distribution; the other nerves give rise 
to the three great plexuses—cervical, brachial, and lumbo-sacral. 
Distribution of the Spinal Nerves.—tThe distribution, like the origin of the 
