The region of the arm extends from the axilla to the elbow. The contour varies according to the muscular development, being cylindrical where the development is poor, but flattened from side to side where the muscles are well developed, and displaying the prominent mass of the biceps in front. On either side of the biceps is a sulcus, that on the inner being the more marked, and extending from the front of the elbow to the posterior axillary fold. It indicates generally the course of the basilic vein and axillary vessels, and separates the biceps and coraco-brachialis from the triceps above, and the biceps from the pronator radii teres below. The external sulcus extends from the bend of the elbow to the insertion of the deltoid, indicates so much of the course of the cephalic vein, and separates biceps and brachialis anticus from triceps above, and biceps from the supinator longus and radial extensors below. The insertion of the deltoid marks the centre of the humeral shaft where the cylindrical upper joins the lower prismatic portion.

and the nutrient artery enters the bone. About this level the superior profunda artery and musculo-spiral nerve cross the back of the bone, and the coraco-brachialis is inserted and the brachialis anticus takes origin. The course of the brachial artery, and also of the median nerve-, is shown by a line drawn from the junction of the anterior and middle third of the outlet of the axilla to the centre of the bend of the elbow, when the limb is extended and supinated. The same line indicates the course of the ulnar nerve in the upper third, after which it runs down and backwards with the inferior profunda artery to the posterior aspect of the internal condyle. The artery is superficial, can be easily felt and compressed by pressure (out and backwards in upper two-thirds and backwards in lower one-third) against the humerus, and when atheromatous, is frequently visible pursuing a tortuous course just under the superficial tissues. The musculo-spiral nerve follows the line of the artery for a short distance, and then descends obliquely outwards across the back of the arm to the external bicipital sulcus, which it reaches about 1 inch below the insertion of the deltoid, and follows to near the external condyle.

The skin of the arm is thin, smooth, and very mobile in front and on the inner side, but thicker and more adherent on the outer and posterior aspects. The skin of the front of the arm is used in Tagliacozzi's plastic operation for restoration of the nose. The looseness of the skin allows of its being easily drawn up by hand in circular amputation, and large flaps may be torn up in various injuries.

The loose fatty subcutaneous tissue is directly continuous with that of the axilla and forearm, and thus inflammatory infections easily spread from one region to the other. In this tissue lie the superficial veins (cephalic on the outer and basilic on inner side), small arterial twigs, superficial lymphatics, and supracondyloid lymphatic gland (the latter about 2 inches above the elbow and immediately behind the internal bicipital sulcus), and superficial nerves-intercosto-humeral, lesser internal cutaneous, branches of the musculo-spiral and internal cutaneous.

The deep fascia is continuous with that of the axilla and forearm, and forms a complete investing sheath, which is thin in front but thick behind, especially over the lower part of the triceps. It sends in outer and inner intermuscular septa to the supracondyloid ridges of the humerus, thus dividing the arm into anterior and posterior compartments, of which the anterior contains all the muscles except the triceps, the brachial vessels, basilic vein, median, internal cutaneous, and musculo-cutaneous nerves. The posterior compartment contains the triceps, while the superior profunda artery and musculo-spiral nerve, the inferior profunda artery and ulnar nerve, and the anastomotica magna are common to both compartments. While various effusions tend to be limited by these compartments, they may pass from one compartment to the other by following these perforating structures.

The biceps muscle is occasionally ruptured, the long head frequently giving way, the muscle then causing a prominent swelling in the middle of the arm when contracted, while the arm is weakened. In such cases the humeral head tends to be displaced upwards toward the coraco-acromial arch. Sometimes the tendon of the long head is displaced, generally inwards, from its groove, the head of the bone again tending to be drawn up. In some cases of rheumatoid arthritis the intracapsular portion of the tendon has been destroyed by friction, the lower portion acquiring an attachment to the bicipital groove. While the biceps muscle is free, the bra-chialis anticus is attached to the bone, and hence in amputating the former retracts more markedly than the latter. Where the muscles are well developed the biceps may considerably overlap the brachial artery.

The brachial artery extends from the lower border of the teres major to the bend of the elbow. At first it lies to the inner side, but ultimately lies in front of the humerus. It is superficial save near its termination, where it dips under the bicipital fascia, between the supinator longus and pronator radii teres. Along with the venae comités and median nerve, it is surrounded by loose connective tissue, in which lie the deep lymphatics of the limb. Externally the vessel is in relation to the coraco-brachialis above and biceps below, while it lies from above downwards on the long and internal heads of the triceps, coraco-brachialis insertion, and brachialis anticus. In its upper part it is surrounded by nerves, the median lying in front and external, the ulnar and internal cutaneous internally, and musculo - spiral behind. The median nerve remains in close relationship, but crosses in front of the artery at the middle of the arm to the inner side. The ulnaf nerve leaves the artery in the middle third of the arm, and piercing the internal intermuscular septum along with the inferior profunda, descends in the triceps to the interval between olecranon and internal condyle posteriorly. The internal cutaneous nerve also accompanies the artery in the upper third, and then, piercing the brachial aponeurosis, divides into superficial, anterior, and posterior branches. The musculo-spiral nerve soon leaves the artery, and inclines back and outwards behind the humerus in the musculo-spiral groove, along with the superior profunda artery. At the outer border of the humerus it pierces the external intermuscular septum, and descends in the anterior compartment between the brachialis anticus and extensor carpi radialis longior. The close relationship of the brachial vessel to various nerves, and particularly the median, explains the pain frequently caused by the application of tourniquets. The musculo-spiral nerve is frequently damaged by contusion and fracture, the latter damaging the nerve either at the time of injury or by subsequent callus It is frequently paralyzed by pressure, as in sleeping with the arm over the back of a chair, and in crutch paralysis, the ulnar suffering next most frequently.

The artery is accompanied by vena? comités, the inner being generally the larger, connected by numerous branches, crossing the vessel both in front and behind. At the elbow the vessel is crossed by the median basilic vein, the bicipital fascia intervening. In the lower part of the arm the basilic lies above the artery, separated from it by the deep fascia, while in the upper part of the arm the basilic vein pierces the deep fascia, and lies internal to the artery. The chief branches of the artery are the two profunda arteries already described, the former arising about 1˝ inches behind the commencement of the artery, and the latter about the middle of the arm ; the nutrient branch, given off about the centre of the arm, but sometimes from the superior profunda ; the anastomotica magna, given off 2 inches above the elbow, which runs in across the brachialis anticus behind the median nerve, and bifurcates into the anterior and posterior branches. There are also numerous small muscular branches.

The vessel is not often wounded, owing to its protected position.

Abnormalities are of sufficient frequency to require attention. Most frequently, when abnormal, the brachial artery divides in the upper third of the arm, the two vessels then running together to the elbow, where they become radial and ulnar, or one may divide into radial and ulnar, and the other form a common interosseous. Frequently one of the two brachial arteries (the vas aberrans) lies superficial to the median nerve, and lower down may pass, along with the nerve, under a process of bone arising from the inner side of the humerus, 2 inches above the epicondyle, called the supracondyloid process.

Ligature of the brachial artery may be required for injury to the vessel itself, or its larger branches in the forearm or hand, and is most frequently performed in the middle of the upper arm. An incision 2˝ inches long is made in the line of the artery, the limb being held abducted, and not supported underneath, as the triceps is then apt to be pushed forward and simulate the biceps. Skin, superficial and deep fascia are cut through, the biceps and coraco-brachialis and median nerve are drawn outwards, the ulnar nerve and basilic vein, if seen, kept to the inside, and the vessel ligatured. A large inferior profunda has been mistaken for the main vessel, and the median nerve lying on the vessel and receiving communicated pulsation from it might similarly be mistaken. Ligature in the upper third is similar, the basilic vein, ulnar and internal cutaneous nerves being displaced inwards, and the coraco-brachialis and median nerve outwards. Ligature at the bend of the elbow is done through an oblique incision along the inner border of the biceps tendon, the superficial tissues being divided, the median basilic vein drawn inwards, the bicipital fascia divided as far as is necessary, and the artery thus isolated and ligatured. The median nerve here lies to the inside, at the junction of the inner and middle third; the biceps tendon lies outside the artery, which lies at the centre of the elbow ; and the musculo-spiral nerve lies outside it, at the junction of the outer and middle third. There is a free anastomosis at the elbow between the profunda vessels and anastomotica magna above, and the anterior and posterior ulnar recurrents, radial recurrent, and interosseous recurrent below. Where ligature is performed above the profunda vessels, the anastomosis occurs between the circumflex vessels above' and the ascending branches of the superior profunda below.