The superficial veins which are subject to the greatest variety, are usually more distinct and numerous on the dorsal aspect, particularly at the lower part.
The subcutaneous nerves, which are very numerous, are derived from the following sources : 1st, the internal cutaneous nerve, which is one of the divisions of the axillary plexus; 2dly, the cutaneous branch of the radial ; 3dly, the musetilo-cutaneous.
The internal cutaneous nerve divides into two branches in the upper arm, in which region it accompanies the basilic vein. These two branches penetrate the fascia separately above the elbow joint, and the one, the an terior branch, descends on the front of the fore-arm, the other, the posterior, on the back of it. The anterior branch usually passes behind the basilic vein, sending a small twig or two anterior to it. Its course is continued to the wrist joint, supplying the skin on the anterior and inner side throughout; the pos terior division accompanies the basilic vein, and may be always traced to the back part of the wrist.
The small branches of this nerve, which cross in front of the basilic or median basilic vein, are occasionally wounded in the opera tion of venesectionan accident which gene rally excites considerable inflammation, with severe constitutional irritation, symptoms which are sometimes erroneously attributed to the action of a foul lancet.
The skin on the anterior surface of the outer half of the fore-arm is supplied with nerves by the external cutaneous nerve, a division of the axillary plexus : it is a deep-seated mus cular nerve in the upper arm and penetrates the fascia, becoming subcutaneous anterior and a little below the elbow joint. In this situation it is posterior to the median cephalic vein ; its branches are numerous throughout its course, which terminates at the wrist-joint in the supply of branches to the skin on the dorsum and ball of the thumb, which inosculate with the cutaneous of the radial.
This last-mentioned branch, the cutaneous of the radial, is derived from its trunk on the outer side of the middle of the arm ; imme diately after that nerve has emerged from between the triceps extensor and the hone, a series of branches is distributed to the skin of the arm. The remainder of the nerve,
which is a descending branch of some size, passes down behind the elbow-joint, and be coming subcutaneous supplies the skin of the posterior surface of the outer half of the fore arm, and corresponds to the musculo-cutaneons on the anterior. Thus it will be seen that the skin on the inner side of the fore-arm, both anteriorly and posteriorly, is supplied by the internal cutaneous, while that on the outer side is supplied in front by the musculo-cu taneous, and behind by the radial nerve.
The superficial veins of the fore-arm, though subject to the greatest variety, are usually distinguished by the names of the cephalic, basilic, and median. The two first commence on the back of the hand ; the cephalic on the external, and the basilic on the internal side. They freely anastomose at the lower part of the fore-arm, after which they separate, and reaching the anterior surface below the elbow, are joined by the median vein, as described in the article ELBOW.
The superficial absorbents take nearly the same course as the veins, though they are far more numerous, and on the whole pursue a straighter direction. The course of these vessels is occasionally demonstrated in the living subject by active inflammation of their coats following the absorption of irritating matter.
.Aponcurosis.—The aponeurosis of the fore arm is simply a continuation of the same structure, which surrounds and supports the muscles of the upper arm ; it varies very much in density and appearance iu different situa tions; this difference arises from the fact that both the triceps extensor and biceps flexor cubiti send to it many fibres, which not merely give additional strength to its texture, but also act as a medium through which these muscles possess the power of making tense the fascia. .1:Iiis provision for tightening and supporting the fascia of the fore-arm is analogous to those arrangements which we meet with in the thigh and leg.