The anterior branches of the dorsal (inter costal) nerves are twelve in number, the first escaping between the first and second dorsal vertebrm, and the last between the last dor sal and first lumbar. They run more or less parallel to each other without forming plexuses, and are destined to supply the tho racic and abdominal parietes, and the skin about the arrn and axilla. They present general and special characters. Each branch runs outwards, from its origin, being sepa rated from the posterior root by the in tervention of the anterior costo-transverse ligament, to reach the intercostal space, be tween the pleura and external layer of the in tercostal muscles, and below the intercostal vessels. Having comtnunicated by one or two filaments with the thoracic ganglia of the sympathetic, these nerves are continued be tween the two layers of the intercostals, to about midway between the spine and the sternum, and here they divide into cutaneous and intercostal branches. The cutaneous branches perforate, in a very oblique manner, the external layer of intercostals; and, after a short course, forwards and outwards, between them and the serratus magnus, either escape between the digitations of the serratus mag nus and external oblique, or perforate their fibres, and divide into anterior and posterior branches. This division takes place some times when the trunks of the cutaneous nerves are covered by the serratus and oblique. 7%e posterior branches are reflected backwards and upwards, and, after a course of an inch or two between the latissimus dorsi and the skin, terminate in the latter. The anterior branches are directed downwards and forwards, or horizontally, and, after a longer course than the posterior branches, terminate, like them, in the skin.
The intercostal branches, though somewhat smaller than the cutaneous, represent the con tinuation of the anterior branches of the dorsal nerves. They continue in the original course of the latter, below the lower edge of the ribs on the one hand, and the costal cud lages on the other ; and near the border of the sternum above, and the linea alba below, perforate the muscular fibres, and become cutaneous. The trunks of the intercostal nerves and their continuation give off numer ous filaments to the supply of the intercostal muscles, and several extremely delicate twig,s, which frequently pass over the inner aspect of the ribs, to communicate above and below with each other in the intercostal spaces.
The special characters of the intercostal nerves are as follow : — The first dorsal nerve, ascending in front, and across the neck, of the first rib, to assist in the formation of the brachial plexus, gives off only a small intercostal nerve. This comes away soon after the nerve has left the intervertebral foramen, and is directed aloug the inner surface of the first rib, to the first intercostal space, without giving off a middle cutaneous branch, and passes along the lower edge of the cartilage to the sternum, by the side of which it perforates the intercostal muscles, and terminates on the skin, at the upper and fore part of the thorax.
The second dorsal nerve crosses obliquely over the second rib, external to its neck, to gain the lower part of the first intercostal space, and again crosses the second rib, to reach the second intercostal space on a level with the middle of the former. Its cutaneous
branch is of large size, and, supplying the arm with cutaneous branches, is named the inter costa-humeral, which perforates the second intercostal space. In traversing the axilla it gives off a branch of communication to the accessory internal cutaneous, and one to com municate with the second intercosto-humeral ; the latter united nerve sending filaments to the skin at the upper and anterior part of the arm. TWO or three filaments represent the termination of the nerve, cross the lower part of the posterior boundary of the axilla, and terminate in the skin, at the upper and back part of the arm.
The cutaneous branch of the third dorsal (the second intercosto-humeral) is smaller than the second, and passes through the third in tercostal space : it divides into an anterior and postertor branch ; the former winds up wards, forwards, and inwards, over the lower border of the pectoralis major, to terminate in the mamma and integument ; the latter, having communicated with the second intercostal, sends filaments to the axilla, and terminal branches, which are directed to the outer and anterior part of the axilla to supply the skin, at the upper and back part of the arm.
The cutaneous brandzes of the foztrth and fifth dorsal nerves send filaments inwards, to supply the mamma ; and filaments backwards, over the superficial surface of the latissimus dorsi, to supply the sldn over the anterior and outer part of the scapula. The intercostal nerves of the eighth, ninth, tenth, and eleventh dorsal nerves perforate the intercostal spaces of the false ribs, pass through the costal attachments of the diaphragm, to get between the external and internal oblique, as far as the border of the rectus, where they give off small cuta neous branches. Entering the sheath of the rectus, they proceed along the posterior sur face of the muscle, and terminate, by giving off some filaments, which ramify in its inner part ; mid others, which perforate the anterior layer of the sheath, at a variable distance from the linea alba, to supply the skin at the anterior part of the abdomen.
The twelfth dorsal nerve is larger than those that have preceded it, and gives a filament of communication to the anterior branch of the first lumbar nerve. It is directed ob liquely downwards and outwards, following the course of the last rib, along the lower border of which it runs, passes behind the anterior layer of the transversalis fascia be tween it and the quadratus lumborum, and, on a level with the apex of the rib, divides into two branches. The cutaneous branch, larger than the abdominal, or continuation of the trunk, perforates, obliquely, the external and internal oblique, gives them some small branches, and then becomes superficial, crosses over the crest of the ilium, and divides into a series of divergent filaments, which lose themselves in the skin of the middle of the glutmal region. The abdominal branch or con tinuation of the nerve passes between the in ternal oblique and transversalis, supplies these muscles, communicates with the first branch of the lumbar plexus, and terminates in the rectus and pyramidalis; and the skin over them.