ORIGIN OF THE SPINAL NERVES. 



275 



THE ROOTS OF THE SPINAL NERVES. 



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

 another as they quit the spinal canal, and join in the corresponding intervertebral 



Fig. 185. DIAGRAMMATIC OUTLINE OP THE ROOTS AND FIRST 



PART OF THE SPINAL NERVES, TOGETHER WITH THE SYMPA- 

 THETIC TRUNK OF ONE SIDE. (Allen Thomson.) 



The view is taken from before. In the upper part of the 

 figure the pons Varolii and medulla oblongata are represented, 

 and from V to XII, the roots of the several cranial nerves from 

 the trifacial to the hypoglossal are indicated. On thf left side 

 1, is placed opposite the first cervical nerve ; and the numbers 

 2 to 8, following below indicate the corresponding cervical 

 nerves ; Br, brachial plexus ; D 1 , is placed opposite the inter- 

 costal part of the first dorsal nerve, and the numbers 2, to 12, 

 following mark the corresponding dorsal nerves ; L 1, the first 

 lumbar nerve, and the numbers 2, to 5, following the remaining 

 lumbar nevves ; Cr, the anterior crural, and o, the obturator 

 nerve ; S 1, the first sacral, and the following numbers 2, to 5, 

 the remaining sacral nerves ; 6, the coccygeal nerve ; Sc, great 

 sciatic nerve ; x x , the filum terminale of the cord. 



On the left side of the figure the following letters indicate 

 parts of the sympathetic nerve, viz., a, the superior cervical 

 ganglion communicating with the upper cervical spinal nerves 

 and continued below into the sympathetic cord ; b, the middle 

 cervical ganglion ; c, d, the lower cervical ganglion united with 

 the first dorsal ; d', the eleventh dorsal ganglion ; from the 

 sixth to the ninth dorsal ganglion the origins of the great 

 splanchnic nerve are shown ; I, the highest lumbar ganglion ; 

 s s, the upper sacral ganglion. In the whole extent of the 

 sympathetic cord, the twigs of union with the spinal nerves are 

 shown. 



foramen into a single cord ; and each cord so 

 formed separates immediately into two divisions, 

 one of which is destined for parts in front of the 

 spine, the other for parts behind it. 



General arrangement. The roots of the 

 nerves arise on each side of the spinal cord by two 

 nearly continuous series of filaments (flla radicu- 

 laria), the anterior of which emerge from the 

 antero-lateral column opposite the anterior cornu 

 of the grey matter, while the posterior are attached 

 in a narrower line along the postero-lateral sulcus 

 of the cord. The filaments composing a single 

 nerve-root vary in number from five to ten, and 

 converge from above downwards as they pass from 

 the cord to the dura mater, where each root enters 

 a special aperture. 



The posterior roots of the nerves are dis- 

 tinguished from the anterior roots by their greater 

 size, which is due to their constituent filaments 

 being both more numerous and individually larger 

 than those of the anterior roots. Each posterior 

 root is farther marked by a ganglion (interverte- 

 bral or spinal), of a size proportionate to that of 

 the nerve on which it is formed. The cells of 

 these ganglia are unipolar, and give origin to the 

 afferent fibres of the spinal nerves, both central and 

 peripheral. -- s c 



