THE SPINAL NERVES 1017 



Structure. The ganglion in an embryo is composed of bipolar nerve cells. In an adult the 

 bipolar nerve cells by fusion of their two poles form unipolar elements. The process of each 

 unipolar cell divides into two a short distance from the cell. One of the processes from each 

 cell passes to the spinal cord, and the other passes into the spinal nerve. Two other forms of 

 cells are, however, present viz. : (a) the cells of Dogiel, whose axones ramify close to the cell 

 (type II of Golgi), and are distributed entirely within the ganglion; and (b) multipolar cells 

 similar to those found in the sympathetic ganglia. On the posterior roots of the lumbar and 

 sacral nerves, between the spinal ganglia and the cord, small cellular masses occasionally exist. 

 They are called accessory or aberrant ganglia ((janylia aberrantia). 



Each typic spinal nerve contains somatic and splanchnic fibre systems, as well as fibres 

 connecting these systems with each other. 



1. The somatic fibres are efferent and afferent. The efferent fibres originate in the cells of 

 the anterior cornu of the spinal gray substance, and run outward through the anterior nerve roots 

 to the spinal nerve. They convey impulses to the voluntary muscles and are continuous from their 

 origin to their peripheral distribution. The afferent fibres (peripheral axones) convey impres- 

 sions from the skin, etc., and originate in the unipolar nerve cells of the spinal ganglia. The 

 central axones enter the spinal cord through the posterior nerve roots, to Jae continued to the 

 medulla oblongata, to end in the nuclei of the gracile and cuneate funiculi (Goll and Burdach), 

 or they may end by arborization around efferent neurones in the same or opposite side of the 

 cord, completing in this way reflex arcs. 



2. The sympathetic fibres are also efferent and afferent. The efferent fibres originate in the 

 visceral motor gray substance of the spinal cord and are conveyed through the anterior nerve root 

 and the white ramus communicans to the corresponding ganglion of the sympathetic chain; here 

 they may end by forming synapses around its cells, or may run through the ganglion to end in 

 another of the chain or in a more distally placed ganglion in one of the sympathetic plexuses, the 

 impulses being relayed in other neurones to be carried to the bloodvessels of the trunk and 

 limbs or to the viscera. The afferent fibres are derived partly from unipolar and partly from 

 the multipolar cells of the spinal ganglia. Their peripheral processes are carried outward 

 through the white rami communicantes, and after passing through one or more sympathetic 

 ganglia without interruption, terminate in the tissues of the viscera. The central processes of 

 the unipolar cells enter the spinal cord through the posterior nerve root and form synapses 

 around either somatic or visceral efferent neurones, thus completing reflex arcs. The dendrites 

 of the multipolar nerve cells form synapses around the cells of Dogiel in the spinal ganglia, and 

 by this path the original impulse is transferred from the sympathetic to the somatic system, 

 through which it is conveyed to the sensorium. 



Points of Emergence of the Spinal Nerves. The roots of the spinal nerves 

 from their origin in the cord run obliquely caudad to their point of exit from the 

 intervertebral foramina, the amount of obliquity varying in different regions of 

 the spine, and being greater in the lower than in the upper part. The level of 

 their emergence from the cord is within certain limits variable, and of course 

 does not correspond to the point of emergence of the nerve from the intervertebral 

 foramina (Fig. 752). 



Each nerve root receives a covering from the pia, and is loosely invested by the 

 arachnoid, the latter being prolonged as far as the points where the roots pierce 

 the dura. The two roots pierce the dura separately, each receiving a sheath 

 from this membrane; this sheath is continuous with the epineurium of the nerve 

 where the roots join to form the spinal nerve. 



Divisions. Immediately beyond the ganglion the two roots coalesce, their 

 fibres intermingle, and the trunk thus formed constitutes the spinal nerve; it 

 passes out of the intervertebral foramen, and divides into a posterior or dorsal 

 primary division for the supply of the dorsal part of the body, and an anterior or 

 ventral primary division for the supply of the ventral part of the body (Fig. 750). 

 Each division contains fibres from both roots. 



Before dividing, each spinal nerve gives off a small recurrent or meningeal 

 branch (ramus meningeus} (Fig. 750) which ree'nters the vertebral canal through 

 the intervertebral foramen and supplies the membranes and bloodvessels of the 

 cord, the vertebrae, and the vertebral ligaments. 



The posterior or dorsal primary divisions (rami posteriores) (Fig. 750) of the 

 spinal nerves are generally smaller than the anterior; they arise from the trunk 



