The internal cutaneous, the smallest of the terminal branches of the brachial plexus, and situated most internally, takes its origin princi pally from the last cervical and first dorsal. It descends, covered in by the brachial aponeuro sis, along the inner aspect of the arm, between the median and the ulnar, and concealed above by the axillary artery.. Deeply seated in the axilla, in leaving this cavity it inclines slightly forwards and outwards in company with, but anterior to, the basilic vein ; and at a variable distance from the elbow joint, generally a little below the middle of the arm, divides into external and internal cutaneous branches : both of which perforate the fitscia. In this part of its course the internal cutaneous gives off in the axilla a small cutaneous filament, which, having communicated with the second or third intercostal nerve, perforates the fascia, and supplies the skin on the inner part of the arm as far as the internal condyle.
The external terminal branch, the continua tion of the trunk in the arm, and the larger of the two, divides into two or three twigs, which pass either in front or_ behind the median basilic vein, some occasionally pass ing in front, and some behind. The exter nal filaments course down the anterior and inner part of the fore-arm, following the di rection of the median vein, and communica ting with branches of the external cutaneous : the internal follows the course of the ulnar vein, communicating with a twig of the ulnar nerve at the lower part of the fore-arm. Both terminate in the integument over the annular ligament.
The internal branch, frequently perforatin, the fascia lower down than the external, passes behind and then below the median basilic vein, to the inner and back part of the fore-arm, and having communicated a little below the elbow with the accessory internal cutaneous, continues its course, and supplies the integument along the inner and back part of the fore-arm as far as the inner edge of' the hand, communicating, in its course, with the innermost filaments of the external branch.
Placed behind and internal to the internal cutaneous nerve, is the cutaneous nerve of Wrisberg (the accessory nerve of the internal cutaneous), considerably smaller than it. It arises from the united chord formed by the seventh cervical and first dorsal. It descends along the inner part of the axilla, and com municates with the cutaneous branch of the second intercostal. Coursing down the arm on a plane behind the ulnar and internal to the basilic vein, it perforates the fascia about the lower third, and, becoming cutaneous, di vides into anterior filaments, communicating with the internal cutaneous : and posterior, communicating with the internal cutaneous branch of the musculo-spiral.
The external cutaneous neons perforans casserii), larger than the preceding, but smaller than all the other nerves, and most external, ts formed by the fifth and sixth cervical. It is directed ob liquely downwards and outwards in front of the tendon of the subscapularis to the inner aspect of the-coraco-brachialis, perforates this muscle (occasionally, however, passes behind it without perforating), and then becomes situated obliquely between the biceps and bra chialis anticus. At a short distance from the
elbow it emerges from beneath the outer border of the biceps, and internal to the supinator longlis ; and at the bend of the elbow, after passing behind the median ce phalic vein, bcomes subcutaneous. In this part of its course the external cutaneous nerve gives off a series of muscular branches. Of the two branches to the the upper, having perforated it, terminates in the short head of the biceps.
The branches to the biceps unite separately or by a common trunk, and one of them per fbrates the biceps, and supplies the elbow joint, being here situated to the outside of the superficial flexor tendons.
The branches for the brachialis anticus are several, and penetrate the muscle by its superfi cial surface. The continuation of the external cutaneous nerve in the fore-arm is represented by a series of internal and external cutaneous branches, which pass down along either side of the radial vein. The fornzer near the wrist joins with a branch from the radial nerve, and gives off a filament which perforates the fascia, and accompanies the radial artery to the outer and back part of the wrist, where it supplies small twigs to the front and back of the radio ulnar articulation. The latter gives filaments to the outer and back part of the fore-arm, as far as the wrist.
The median The largest of the bra chial plexus, and situated between the external cutaneous and the ulnar, arises by two roots, the external common to the median, and the external cutaneous : the internal common to the median, the internal cutaneous, and the ulnar. The fifth, sixth, seventh, and eight cervical and first dorsal nerves consequently concur to form it. Between its two roots is placed the axillary artery. It passes along the inner side of the arm in company with the axillary artery to the bend of the elbow, 13 ing at first to the outside of the vessels, and then a little above the middle of the arm, crosses to its inner side, occasionally, however, con tinuing all along to its outside. It is slightly overlapped by the inner border of the biceps, having the brachialis anticus to its outside : the latter muscle separates it inferiorly from the ulnar nerve. The upper part of the in ternal cutaneous nerve runs alongitsinner side. It sinks into the bend of the elbow behind the semilunar fascia, and in front of the brachialis anticus, passes between the tvvo heads of the pronator radii teres, and is then conducted along the forearm between the flexor digi torum sublimis and profundus to the annular ligament, behind which it passes; and at the lower border of this becomes expanded, and di vides into a series of terminal digital branches.