566 THE NERVOUS SYSTEM. 
spinal nerves ; the thoracic nerves (except the first) are much more slender than the limb 
nerves ; and the cervical nerves diminish in size from below upwards. 
Origin of the Spinal Nerves.—Kach spinal nerve is attached to the spinal 
cord by two roots, called respectively dorsal (posterior), and ventral (anterior). 
Fie. 420.— DIAGRAMMATIC REPRE- 
SENTATION OF THE ROOTS AND 
GANGLIA OF THE SPINAL NERVES, 
showing their position in relation 
to the spinal column. The nerves 
are shown as thick black lines on 
the left side. 
The dorsal root is larger than the ventral root ; 
it contains a larger number of rootlets, and the in- 
dividual rootlets are of larger size than in the ventral 
root. It has a vertical linear attachment to the 
postero-lateral sulcus of the spinal cord. The root- 
lets of contiguous dorsal roots are in close relation, 
and, in some instances, overlap. The dorsal root 
separates as it passes away from the cord into two 
bundles, both of which become connected with the 
inner end of a spinal ganglion. From the outer end of 
this ganglion the dorsal root proceeds to its junction 
with the ventral root in the inter-vertebral foramen. 
The spinal ganglia are found on the dorsal roots 
of all the spinal nerves. (In the case of the first 
cervical (sub-occipital) nerve, the spinal ganglion may 
be rudimentary or absent; and the dorsal root itself 
may be wanting, or derived from the spinal accessory 
nerve.) They occupy the inter-vertebral foramina, 
except in the case of the sacral and coccygeal ganglia, 
which he within the vertebral canal, anid the first 
and second cervical nerves, the gangha of which he 
upon the neural arches of the atlas and axis respec- 
tively. With the exception of the coccygeal ganglion 
they are outside the cavity of the dura mater, but 
are invested by the membrane. The ganglia are of . 
ovoid form, bifurcated in some cases at their inner 
ends. They consist of unipolar nerve-cells, whose 
processes, after a very short course, divide into central 
and peripheral fibres: the central fibres form the 
portion of the root entering the spinal cord; the 
peripheral fibres are continued in an outward direc- 
tion from the ganglion into the spinal nerve. 
Accessory spinal ganglia (ganglia aberrantia)—Between 
the spinal ganglion and the spinal cord small collections of 
cells are occasionally found on the dorsal roots, either as 
scattered cells or distinct ganglia. They are most frequently 
met with on the dorsal roots of the lumbar and sacral nerves. 
The ventral root is smaller than the dorsal root. 
It arises from the anterior surface of the spinal cord 
(anterior root zone) by means of scattered bundles of 
nerve fibres, which occupy a greater horizontal area 
and are more irregular in their arrangement than the 
fascicles of the dorsal root. It possesses no ganglion 
in its course. The rootlets sometimes overlap, and 
are not unfrequently connected with neighbouring 
rootlets above and below. 
The dorsal and ventral roots, from their attach- 
ment to the spinal cord, proceed outwards in the 
spinal canal towards the intervertebral foramina, 
where they unite to form the spinal nerve. The 
direction of the roots of the first two nerves is 
upwards and outwards; the roots of the remaining nerves course obliquely down- 
wards and outwards, the obliquity gradually increasing until, in the case of the 
lower lumbar, the sacral and coccygeal nerve-roots, their course is vertically down- 
