The posterior bratzches of the sacral nerves exist as distinct branches within the spinal canal, and consequently differ from the cer vical, dorsal, and lumbar, which become dis tinct trunks after the main trunks have issued from the spinal or intervertebral foramina. They decrease in size from above downwards, being extremely small, and passing out of the posterior sacral foramina, the fifth coming out between the sacrum and coccyx. They form a minute anastomosis with each other, and with the corresponding branch of the last lumbar, and after having given filaments to the lower part of the erector spinm, per forate the tendon of that muscle, and are distributed to the skin over the sacrum and coccyx, and immediately mound the anus.
7'he anterior branches of the spinal nerves are much larger than the posterior branches, the two upper cervical forming the only ex ception. They form intricate plexuses in the neck, the lower part of the spine and sacrum, the nerves given off fronii those in the first situation being principally intended for the neck and upper extremities ; in the two last for the lower extrernities. The intervening series represented by the thoracic nerves, being comparatively simple in their distribution, do not form plexuses.
The Anterior Branches of the Cervical Nerves.
The anterior branch of the first cervical nerve, smaller than the posterior, is di rected between the occipital bone and the transverse process of the atlas, passes over the outer edge of the vertebral artery, and appears at the inner side of the rectus pins lateralis. It then descends, and forms an anastornotic arch with the anterior branch of the second, in front of the transverse process. In its course the rectus capitis lateralis, and rectus capitis anticus minor receive one or more filaments, and it also sends a filament into the canal for the vertebral artery, and which communicates with the trunk of the second cervical between the transverse pro cesses of the atlas and axis. Frotn this ana stomotic arch are given off filaments which communicate with the lingual and par vagum and superior cervical ganglion of the sym pathetic.
The anterior branch of the second cervical nerve, also tnuch smaller than the posterior passes forwards between the transverse pro cesses of the atlas and the axis, being con cealed by the levator anguli scapulm, splenius, and first inter-transverse muscle, and divides into an ascending branch, passing in front of the transverse process of the atlas, to com municate with the first cervical; and a de scending branch.
The descending branch soon subdivides, and gives several filaments of communication with the superior cervical ganglion ; one small filament to communicate w ith the par vagum, another enters the rectus capitis anticus major, and the last concurs to form the cer vical plexus.
Anterior branch of the third cervical nerve, larger than the posterior, and twice as large as the preceding, passes between the verte bral artery and inter-transverse muscles, and having given branches to the levator anguli scapulre and rectus capitis anticus major, com municates above with the descending branch of the second, below with that of the fourth, and in the interval with the superior cervical ganglion, and then again bifurcates to enter into the formation of the cervical plexus.
The anterior branch of the fourth cervical nerve, of the same size as the preceding, communicates above with the third, below with the fifth cervical, in the middle with the superior cervica/ ganglion, and then enters into the formation of the lower part of the cervical plexus.
The cervical plexus (the deep cervical plexus) is composed of the primary and secondary anastomosing arches of the anterior branches of the four upper cervical nerves. These anastomosing arches are subject to consider able variation, though generally formed by each nerve bifurcating, and, after having com muuicated with the nerve above and .below, again reuniting in a more or less uniform manner prior I to giving off their terminal branches. The plexus is situated deeply at the upper anterior and outer part of the neck behind the posterior edge of the sterno-mastoid, in front of the scalenus posticus, external to the rectus capitis anticus major, the carotid artery, jugular vein, and par vagum. It con stitutes the chief contents of the posterior superior cervical triangle, and is surrounded by a large quantity of loose cellular membrane, absorbent glands, and fat, and immediately in vested with a prolongation of the deep cervical fascia, which renders the dissection of' the nu merous branches as they immediately proceed from it, difficult. It communicates internally by several delicate filaments with the superior and middle cervical ganglia of the sympathetic; below with the upper part of the brachial plexus, and externally with the spinal acces sory, giving several filaments to the muscles with which it is in immediate relation. The branches given off from the cervical plexus may be divided as follows, into The svperflcialis eolli (superficial cervical nerve) takes its origin from the middle of the plexus in company with, but anterior to the great auricular, the anastomosing branches of the second and third cervical nerves concurring to form it. It emerges from behind the pos terior border of the sterno-mastoid about the middle of the neck, and is directed horizontally forwards and inwards, behind the external jugular vein, and between the sterno-mastoid and platysma, and at a variable point divides into two branches, an ascending and descend ing, the former larger than the latter.