There are thirty-one pairs of spinal nerves. They are named according to the vertebrae in relation to which they emerge. The thirty-one pairs comprise eight cervical, twelve thoracic, five lumbar, five sacral, and one coccygeal. Each nerve arises by two roots, an anterior or motor, and a posterior or sensory, the latter having a ganglion, which is usually lodged within the intervertebral foramen through which the nerve passes. The anterior fibres issue from the antero-lateral column of the cord, and the posterior fibres from the postero-lateral sulcus. The posterior root is larger than the anterior. Roth nerve roots pierce the dura mater independently. In the case of the roots of the cervical and thoracic nerves the dural orifices lie opposite the intervertebral foramina, while in the lumbar and sacral regions the dural openings are at a higher level than the intervertebral foramina.
The nerve roots vary both in size and in direction. Those of the lower lumbar and upper sacral are the largest; the last sacral and the coccygeal are the smallest. As regards direction, the upper cervical nerves run horizontally, the lower cervical and thoracic obliquely, while the lumbar, sacral, and coccygeal nerve roots are vertical. The lumbar, sacral, and coccygeal nerve roots arise from the cauda equina. Immediately beyond the spinal ganglion the two roots join to form a short trunk or spinal nerve, which divides into anterior and posterior primary divisions. These divisions contain both motor and sensory fibres. Before the division occurs, each spinal nerve gives off a minute recurrent or meningeal branch. This is joined by a twig from the connecting branch between the anterior division and the neighbouring sympathetic ganglion. The meningeal nerve passes inwards through the intervertebral foramen to supply the vertebral column and dura mater.
The anterior divisions are destined for the innervation of the muscles and integument of the upper and lower extremities, together with those of the antero-lateral regions of the neck, thorax, and abdomen.
With the exception of the first cervical, fourth sacral, fifth sacral and coccygeal nerves, the posterior primary divisions separate into lateral and medial branches.
C. 1. The posterior primary division of the first cervical is termed the suboccipital. It enters the suboccipital triangle where it breaks up to supply the superior oblique, inferior oblique, rectus capitis, posticus major and minor, and the complexus. It communicates with the second cervical nerve.
C. 2-8. The lateral brandies are muscular to the splenius, cervicalis ascendens, transversalis cervicis, and trachelo-mastoid. The medial branch of the second is called the great occipital. The medial branches of the third, fourth, and fifth supply the semi-spinalis and complexus, then pierce the splenius and trapezius to become cutaneous near the spinous processes of the vertebrae. Those of the remaining cervical nerves supply the semispinalis. The great occipital passes over the inferior oblique, to penetrate the complexus and trapezius, furnishing twigs to the former, and accompanies the occipital artery to the back of the scalp. It communicates with the small occipital nerve of the cervical plexus.
The lateral branches of the upper six innervate the ileocostalis; those of the lower six however, become cutaneous in a line with the angles of the ribs. The medial branches of the first six, after supplying the transverso-spinalis muscle, become cutaneous near the vertebral spines; that of the second is the longest, extending over the scapula. The corresponding branches of the lower six thoracics are muscular to the multifidus spinae.
The lateral branches supply the erector spinae, those of the upper three terminating as cutaneous nerves. The medial twigs end in the multifidus spinae.
The posterior divisions of the last two sacrals do not subdivide, but are connected to each other by a loop, the fifth sacral being also joined to the coccygeal. They terminate in the skin in the region of the coccyx. The lateral branches of the first three sacrals are cutaneous, while the medial branches supply the multifidus spinae.