SPINAL NERVES (Les Nelfs rachidiens, Fr.; Die Rzieckennzarksnerven, Germ. ; I Nervi Spinali, Ital.) are thirty-one pairs, and are dis tributed to the neck, and the upper extremi ties, the trunk and lower extremities. They are divided into Cervical, Dorsal, Lumbar and Sacral : the first division comprising eight ; the second, twelve ; the third, five ; and the fourth, six. Their general and special cha racters, and their apparent and absolute con nexion with the spinal chord having been already described*, vve shall limit the details of this article to their ultimate distribution.
Each spinal nerve, after the union of its roots, divides into an anterior and posterior branch, the former having generally a much more complicated and extensive distribution than the latter. It will be convenient there fore for the purpose of description to enter first into a consideration of the posterior branches.
The posterior branch of the first cerrical or sub-occipital nerve is larger than the anterior, and passes internal to and below the vertebral artery, between the arch of the atlas and the occipital bone, to gain the triangular space be tween the rectus capitis posticus major, the su perior and inferior oblique muscles. It is here imbedded in a considerable quantity of fat and dense cellular membrane, and having directed itself from before, backwards, and slightly from below upwards, dividesinto a series of branches. Two external branches are sent to the two oblique muscles : an internal ascends to the rectus capitis posticus major, and which having supplied this, terminates in the minor : another fildment is directed to the anterior aspect of the complexus near to its occipital attachment: and the terminal branch descends, generally perforating the inferior obliquely, and anastomoses with the posterior branch of the second cervical nerve.
The posterior brazzch of the second cervi cal nerve emerges from between the lower border of the posterior arch of the atlas, and the lamina of the axis, and is larger than any of the posterior branches of the cervical nerves, and three or four times greater than the anterior branch of the same nerve. It
appears at the lower border of the inferior oblique, and having passed a short distance horizontally inwards, winds round this muscle to the anterior aspect of the outer part of the complexus, which it perforates. It inclines outward and upwards between it and the trapezius, passes through the latter, and ter minates in the skin in the occipital region as the great occipital nerve, coursing along with the occipital artery but lying internal to it. Before becoming great occipital, it gives off at the lower border of the inferior oblique a branch to supply this muscle and a superior and inferior anastomotic branch to commu nicate with the first and third cervical. When passing along the anterior surface of the com plexus, numerous branches are given off to this muscle, the trapezius, and splenius. Those for the last muscle are more numerous and larger than the branches, for the two others are directed to the anterior aspect of the muscle, and one or more of them perforate the complexus before reaching it.
The posterior branch of the third cervical is smaller than the second, but larger than the fourth, and situated more externally, emerg ing from between the transverse processes of the second and third cervical vertebra. It is directed inwards, between the opposed surfaces of the complexus and semispinalis coIli towards the median line, and haying reached the sides of the spinous processes of the vertebrm, di vides into ascending and horizontal cutaneous branches. The ascending branch, after a short course, perforates the inner border of the complexus and trapezius, and becomes cuta neous. It continues its course close to the median line, as far as the region of the oc ciput, the inner and lower part of which it supplies on the internal side of the great occi pital nerve.