The brachial plexus (axillary) is situated at the inferior and lateral part of the neck, in the posterior inferior cervical triangle, where it is covered in by a considerable quantity of fat, cellular membrane, and lymphatic glands, which separate it from the external jugular vein. The sealenus anticus hounds it in front and internally; the scalenus posticus in the oppo site direction ; and in its course from between these muscles to the clavicle, it is crossed by the omo-hyoid muscle, transversalis coli, and humeral vessels, and more superficially, by the supra-clavicular and aeromial branches of the cervical plexus. Having passed from beneath the clavicle, it becomes placed be tween the coracoid process of the scapula and tbe first digitation of the serratus magnus, and anterior and external to the first rib, and there divides into its terminal branches. In the neck, the plexus is situated superior, pos terior, and external to the artery; but as the trunks gradually converge towards the axilla, and the terminal branches again diverge, the artery conies to be bounded by some inter nally, by others externally. The plexus is broad above, where it represents the base of a triangle, and narrow below at its termina tion at the upper part of the axilla.
The different branches of the plexus may be divided into those given off above, and those below the clavicle : — the former, for the levator anguli scapula, subclavius, rhom boid, and serratus magnus; the latter, for the upper extremity and its muscles.
Muscular supra-scapular.
Supra-clavicular { Subscapular.
Internal cutaneous. External cutaneous.
Median.
In fra-clavicular Ulnar.
Musculo-spiral.
Circumflex.
Of the muscular branches. The nerve for the rhomboideus takes its origin from the an terior branch of the fifth cervical, immediately after it has quitted the intervertebral foramen ; but is frequently given off from the cervical plexus : it is, consequently, deeply seated. It either perforates the scalenus porticus or winds round it, to get between it and the levator anguli scapulm ; continues along the costal surface of the latter muscle, and then passes to the same surface of the rhornboi dens, as far as its lower part, frequently sup plying, in its course, the levator anguli sca pulw, which, in many cases, however, receives filaments from a distinct nerve arising above it, and taking a similar course.
The nerve to the serratus magnus (external respiratory, posterior thoracic), situate at the posterior and upper part of the plexus, arises from it by two delicate roots, which come off from the lower edge of the fifth and sixth cervical, immediately after they have passed the intervertebral foramina. It receives, sometimes, a twig from the seventh. It is directed downwards and outwards, and reaches the thorax between the subscapularis and ser ratus magnus, passing behind the axillary vessels. It passes along this muscle inferior,
to the long thoracic artery, and terminates in its lower part, by numerous filaments.
The nerve for the subclavius is very small, but always present, and is given off from the anterior part of the united trunk of the fifth and sixth cervical. It passes down anterior to the subelavian artery, and enters the mid dle of the muscle.
The remainder of the muscular branches are very small, and come off from the lower and anterior part of the plexus, being princi pally derived from the seventh cervical : some pass behind, and others, in front of the ax illary artery, enter the axilla, and are distri buted to the posterior surfaces of the pecto rails major and minor. They are known under the collective name of anterior or short thoracic.
The supra-scapular nerve, larger than the long thoracic, issues from the upper and back part of the plexus, from the united root of the fifth and sixth cervical at their angle of union. It is directed downwards, outwards, and backwards in company with the supra scapular vessels, passes behind the trapezius and coracoid process to the notch in the upper edge of the scapula, beneath the ligament which converts this notch into a foramen, and which separates it from the supra-scapular vessels. Having reached the supra-spinal fossa, and supplied the supra-spinatus 'muscle, it winds along the concave external border of the spine, and reaches the infra-spinal fossa, supplying the infra-spinatus. From the in ferior filaments one or two twigs can be traced to the teres minor.
The subscapular nerves are intended for the latissimus dorsi, teres major, and subscapularis. That for the first muscle is the largest and longest. it arises from the plexus above, and internal to the circumflex nerve, passes down in the axilla between the subscapularis and serratus magnus, parallel, but posterior to the long thoracic, and terminates by reach ing the lower border of the latissimus dorsi, where it enters its substance. It gives off occasionally in its course the branch from the teres major, which usually, however, arises from the plexus below it. This nerve passes downwards and outwards at the subscapularis, and enters the anterior surface of the teres major.
The nerves for the subscapularis are : a small one, generally constant as to its origin arising high up freer' the same source of origin as the circumflex, passing behind the axillary artery to the upper part of the superficial surface of the subscapularis ; the other larger, and frequently derived from the circumflex, to be distributed to the middle of the muscle.