The horizontal branch passes between the ligamentum nuchm and the inner border of the complcxus, and after having perforated the tendon of the trapezius, terminates in ex ternal small cutaneous filaments. The nerve prior to division, and at the outer border of the semispinalis colli, communicates by one or more filaments with the posterior branch of the second cervical. From the anastotnosis between the communicating branches of the posterior roots of the three first cervical nerves, results an irregular plexus placed between the complexus and outer part of the semi spinalis colli, and consequently nearly in a line with the transverse processes of the superior cervical vertebrm. From this posterior cervical plexus, eumerous branches arise to supply the complexits, splenius, and semispinalis colli. The anastomosis between these posterior branches is, according to Cruveilhier, sorne tim es deficient.
The posterior root of the fourth cervical nerve varies much as to individual size, but is al ways smaller than the preceding. It passes downwards and inwards between the com plexus and semispinalis colli, and haying reached the side of the median line, perforates the tendons of the splenius and trapezius, and becotnes cutaneous. In its course it supplies these muscles, and occasionally' terminates in the splenius without going to the skin.
The posterior branches of the fifth, sixth, seventh, and eighth cervical nerves have a similar course to the fourth, but decrease in size from above downwards. The fifth and sixth usually pass between the opposed sur faces of the semispinalis colli and complexus, give branches to these muscles, and perforate the inner part of the tendons of the splenius and trapezius, to terminate in the skin at the lower part of the nape of the neck.
The posterior branches of the seventh and eighth cervical nerves pass either through the deep-seated fibres of the semispinalis colli, or between it and the multifidus spinm, give branches to these two muscles, perforate the tendons of the trapezius and splenius, and terminate by ramifying, the one on the skin above the scapula, the other over the integu ment, as far as about the spinous process of the third dorsal vertebra. The inter-trans versales muscles, cervicalis ascendens, tra chelo-mastoid, and transversalis colli, receive numerous small filaments from these nerves almost immediately after their appearance in the neck.
The posterior branches of the dorsal (tho racic) nerves are much smaller than the an terior, and are directed backwards between the ascending costo-transverse ligaments and the sides of the vertebrm. Having reached
the outer border of the semispinalis dorsi and multifidus spinm, they divide into external and internal branches, the latter being muscular and cutaneous in their distribution, the former only muscular in the eight upper. In the first eight the internal branches are larger, in the last four much smaller than the ex ternal.
The external or nzuscular branches of the eight superior pass between the sacro-lum balls and longissimus dorsi, and give off nu merous filaments to supply these muscles, and the levatores costarum ; that of the first send ing a few filaments to the cervicalis ascendens, trachelo-mastoid, transversalis colli, and sca leni muscles.
The internal branches wind either over the posterior aspect of the semispinalis dorsi, or between it and the multifidus spinm, and haying supplied these muscles with numerous filaments, reach the sides of the spinous pro cesses ; here they perforate the rhotnboid, latissimus dorsi, and trapezius, the last muscle very obliquely, and become cutaneous, being principally distributed to the skin at the back part of the scapular region.
The external branches of the four inferior pass obliquely downwards and outwards be tween the sacro-lumbalis and longissimus dorsi, communicating with each other in their course, and at the outer border of the former muscle perforate the tendon of the latis simus dorsi, and become cutaneous, some of the lower filaments being capable of being traced over the glutwal region.
The internal branches of the four inferior are remarkably small, and are lost either in the substance of the multifidus spinm, or semi spinalis dorsi. The cutaneous filaments from the posterior branches of the dorsal nerves given off on the one hand from the internal, and on the other from their external divisions, are situated somewhat in a line with the angles of the ribs, so that they become more external in proportion to their inferior posi tion.
The posterior brandies of the lumbar nerves are analogous in their distribution to the four lower dorsal branches, having an external large musculo-cutaneous, and small internal muscular divisions. The external branches run along the deep surface of the longissimus dorsi, and at its outer edge perforate the tendon of the latissimus dorsi and terminate in cutaneous filaments directed' over the crest of the ileum to the glutmal region, as far as on a level with the great trochanter. The internal branches are lost in the substance of the multifidus spinm.