ROOTS OF THE STINAL NERVES. 5G9 



and, according to tlie region in which they issue from the spinal canal, 

 they are named cervical, dorsal, lumbar, sacral, and coccygeal. 



By universal'usage each pair of nerves in the dorsal, lumbar, and sacral 

 regions is named in correspondence with the vertebra beneath which it 

 emerges. Of the remaining eight pairs of nerves between the cranium 

 and the first dorsal 'nerve, the uppermost is placed above the atlas, and 

 the second and following nerves below the seven cervical vertebrae in 

 succession. These eight pairs are usually reckoned as eight cervical 

 nerves, but the first is also distinguished by the name of sub-occipital 

 nsrve. The nerves of the thirty-first pair emerge from the lower end 

 of the sacral canal, pass below the first vertebra of the coccyx, and are 

 named coccygeal. 



Although the plan of counting eight cervical nerves is continued in this work 

 for the sake of convenience, it being that which is most frequently followed, it is 

 by no means intended to represent this method as the best or as scientifically cor- 

 rect. The plan of Willis, who reckoned the sub-occipital as a cranial nerve, had 

 at least the advantage that it made the numbers of the remaining seven cer- 

 vical nerves correspond each with the vertebra beneath which it emerged, as do 

 the dorsal, lumbar and sacral nerves ; and if the sub-occipital nerve, while 

 recognised as the fii-sc spinal nerve, were to be kept distinct from the seven 

 which succeed, a more uniform system of nomenclature would be arrived at than 

 that wliich is at present in most common use. 



Varieties. — Sometimes an additional coccygeal nerve exists. Among seven 

 cases which appear to have been examined with great care, Professor Schlemm 

 (■' Observat. Neurologicce," Berolini. 1834) found two coccygeal nei-ves on each 

 side in one instance, and on one side in another case. In all the rest there was 

 only a single coccygeal nerve on each side. 



THE ROOTS OF THE SPINAL XERYES. 



Each spinal nerve springs from the spinal cord by two roots which 

 approach one another as they quit the spuial canal, and join in the 

 corresponding intervertebral foramen into a single cord ; and each cord 

 so formed separates immediately into two divisions, one of which is 

 destined for parts m front of the spine, the others for parts behind it. 



Structure. — The posterior roots of the nerves are distinguished from 

 the anterior roots by their greater size, as well as by the greater thick- 

 ness of the fasciculi of which they are composed. Each spinal nerve 

 is furnished with a ganglion situated on the posterior root : but the 

 first cervical or sub-occipital nerve is in some cases without one. The 

 size of the ganglion is in proportion to that of the nerve on which 

 it is formed. 



The ganglia are in general placed in the intervertebral foramina, 

 immediately beyond the points at which the roots perforate the dura 

 mater lining the spinal canal. The first and second cervical nerves, how- 

 ever, which leave the spinal canal, over the laminai of the vertebrge, have 

 their ganglia opposite those parts. The ganglia of the sacral nerves are 

 contained in the spinal canal, that of the last nerve being occasionally 

 at some distance from the point at which the nerve issues. The gan- 

 glion of the coccygeal nerve is placed within the canal in the sac of 

 dura mater, and at a variable distance from the origin of the nerve. 



The fibres of the posterior root of the nerve divide into two bundles 

 as they approach the ganglion, and the inner extremity of the oval- 

 shaped ganglion is sometimes bilobate, the lobes corresponding to the 

 two bundles of fibres. 



