THE SPINAL NEKVES. 



685 



DESCRIPTION OF THE PERIPHEEAL NEKVES AND 

 SYMPATHETIC SYSTEM. 



I. THE SPINAL NERVES. 



Origin of the Spinal Nerves. Each spinal nerve is attached to the 

 medulla by two roots, called respectively posterior 

 (dorsal, afferent) and anterior (ventral, efferent). 



The posterior root is larger than the anterior 

 root ; it contains a larger number of radicular fibres, 

 and the individual fibres are of larger size than in 

 the anterior root. It has a vertical linear attach- 

 ment to the postero- lateral sulcus of the spinal 

 medulla. The fibres of contiguous posterior roots are 

 in close relation, and, in some instances, overlap. 

 The posterior root separates, as it passes away from 

 the spinal medulla, into two bundles, both of which 

 become connected with the proximal end of a spinal 

 ganglion. From the distal end of this ganglion the 

 posterior root proceeds to its junction with the 

 anterior root in the intervertebral foramen. 



The spinal ganglia are found on the posterior 

 roots of all the spinal nerves. (In the case of the 

 first cervical or sub-occipital nerve, the spinal ganglion 

 may be rudimentary or absent ; and the posterior root 

 itself may be wanting, or derived from the accessory 

 nerve.) They occupy the intervertebral foramina, 

 except in the case of the sacral and coccygeal nerves, 

 the ganglia of which lie within the vertebral canal ; 

 and the first and second cervical nerves, the ganglia 

 of which lie upon the vertebral arches of the atlas 

 and epistropheus respectively. With the exception 

 of the coccygeal ganglia they are outside the cavity 

 of the dura mater, but are invested by the mem- 

 brane. The ganglia are of ovoid form, bifurcated in 

 some cases at their proximal ends. They consist of 

 unipolar nerve-cells, whose axons, after a very short 

 course, divide into central (root) and peripheral 

 (trunk) fibres. The central fibres form the portion 

 of the root entering the spinal medulla ; the peri- 

 pheral fibres are continued in a lateral direction 

 from the ganglion into the spinal nerve. 



Ganglia Aberrantia (aberrant spinal ganglia). Between 



spinal 



the spinal ganglion and the spinal medulla small collections 

 of cells are occasionally found on the posterior roots, either as 

 scattered cells or distinct ganglia. They are most frequently 

 met with on the posterior roots of the lumbar and sacral 

 nerves. 



The anterior root is smaller than the posterior 

 root. It arises from the anterior surface of the spinal 

 medulla (anterior root zone} by means of scattered 

 bundles of nerve-fibres, which occupy a greater hori- 

 zontal area and are more irregular in their arrange- 

 ment than the radicular fibres of the posterior root. 

 It possesses no ganglion in its course. The rootlets 

 sometimes overlap, and are not infrequently con- 

 nected with neighbouring radicular fibres above and 

 below. 



The dorsal and ventral roots, from their attachment to the spinal medulla, proceed 



Co 



FIG. 605. DIAGRAMMATIC REPRE- 

 SENTATION OF THE ORIGIN OF THE 

 SPINAL NERVES, showing the posi- 

 tion of their roots and ganglia re- 

 spectively in relation to the vertebral 

 column. The nerves are shown as 

 thick black lines on the left side. 



