POSTERIOR PRIMARY DIVISIONS OF SPINAL NERVES. 1279 



groove as a continuous series, called the posterior root zone (Fig. 884). These 

 rootlets are sometimes so numerous and so crowded, that those of adjacent nerves 

 overlap and adhere to one another. Where more typically disposed, as in the 

 thoracic region, the cord-segments (page 1024) are distinct. The fasciculi for any 

 one nerve usually collect into two bundles which pass to the proximal aspect of the 

 spinal ganglion. 



The spinal ganglia (gg. spinalia) are aggregations of nerve-cells found on the 

 posterior roots of all the spinal nerves (Fig. 852). They are usually ovoid in shape, 

 from 4-6 mm. in length, and are occasionally bifid at their proximal ends. They 

 consist of a cluster of unipolar neurones, whose centrally directed axones form the 

 sensory root of the spinal nerve and whose dendrites extend peripherally as the 

 sensory distribution. The ganglia are usually situated in the intervertebral foramina, 

 but exceptions to this rule are presented by the ganglia of the first and second cervi- 

 cal nerves, which lie upon the neural arches of the atlas and axis respectively, and 

 by those of the sacral and coccygeal nerves, which are lodged within the vertebral 

 canal. Although situated beyond the dural sheath of the cord, with the exception 

 of the ganglion of the coccygeal nerve, they are invested by a prolongation of it. 



Variations. — The first cervical nerve may either have no posterior root or may derive it from 

 or share it with the eleventh cranial nerve. Its ganglion may be very rudimentary or entirely 

 absent. Considerable variation is found in the thoracic region, where either the anterior or pos- 

 terior or both roots of one of the nerves may seemingly be absent. In the lumbar and upper 

 sacral nerVes the ganglion may be double, each bundle of the posterior root having its own. 



Ganglia aberrantia are small detached portions of the spinal ganglia occasionally found 

 along the posterior roots of the upper cervical, the lumbar and the sacral nerves. 



The motor, anterior or ventral roots (radices anteriores) are smaller than 

 the posterior and have no ganglia. They emerge from the anterior surface of the 

 cord in a series of fasciculi (fila radicularia), the ayiterior 7'oot-zone^ with a tendency 

 to form two groups which unite in the completed root (Fig. 878). As in the pos- 

 terior roots, the fasciculi of origin may overlap one another or fuse with those of 

 adjoining nerves. 



Number. — As usually found the thirty-one pairs are grouped as follows: — eight 

 cervical, twelve thoracic, five lumbar, five sacral and one coccygeal. 



Variations. — Should there be any anomaly in the number or arrangement of the vertebrae, 

 there is a corresponding modification of the nerves. The greatest variation occurs in the 

 coccygeal region. There may be none at all in this situation, or one or two additional ones may 

 be found. Traces of two extra ones, which are rudimentary caudal nerves, may be found in the 

 filum terminale. 



Size. — The largest spinal nerves are those which are concerned in the forma- 

 tion of the limb plexuses — brachial, lumbar and sacral — and are, tljierefore, the lower 

 cervical, the first thoracic, the lower lumbar and the upper sacral. The largest nerves 

 in the entire series are the lower lumbar and upper sacral. The smallest are the lower 

 sacral and the coccygeal. Those of the upper cervical region are smaller than those 

 of the lower, the sixth being the largest of those in the neck. With the exception of 

 the first, the thoracic nerves are comparatively small. 



Divisions. — The common trunk formed by the union of the two roots emerges 

 from its intervertebral foramen and almost immediately gives of? a meningeal or 

 recurrent braiich (r. meningeus). This tiny nerve is joined by a filament from a 

 gray ramus communicans and enters the vertebral canal through the foramen to be 

 distributed to the vertebrae and their ligaments, and to the blood-vessels of the 

 vertebral canal and of the spinal cord and its membranes. After giving ofT the 

 recurrent twig, each trunk soon splits into two branches, called the anterior and 

 posterior prit?iary divisions (rr. anterior et posterior), each of which is composed 

 of fibres from both roots (Fig. 1085), as well as of sympathetic filaments. 



THE POSTERIOR PRIMARY DIVISIONS OF THE SPINAL NERVES. 



The posterior primary divisions (rr. posteriores) of the spinal nerves are as a 

 rule smaller than the anterior (rr. anteriores). They arise either as a single cord 

 from the trunk formed by the union of the two roots, or as two separate strands 



