Last's Anatomy: Regional and Applied

Part five. Anterior compartment of the arm

Coracobrachialis

Functionally unimportant, the muscle arises from the apex of the coracoid process, where it is fused with the medial side of the short head of biceps. The muscle is inserted midway along the medial border of the humerus.

Nerve supply. By the musculocutaneous nerve (C5, 6).

Action. It is a weak flexor and adductor of the shoulder joint.

Biceps

The long head of this muscle arises from the supraglenoid tubercle and adjoining part of the glenoid labrum of the scapula (Fig. 2.8). The rounded tendon passes through the synovial cavity of the shoulder joint, surrounded by a sheath of synovial membrane, and emerges beneath the transverse ligament at the upper end of the intertubercular groove. The synovial sheath pouts out below the ligament to an extent which varies with the position of the arm, being greatest in full abduction (Fig. 2.12).

The short head arises from the apex of the coracoid process together with and to the lateral side of coracobrachialis. The tendinous origin of each head expands into a fleshy belly; the two bellies lie side by side, loosely connected by areolar tissue, but do not merge until just above the elbow joint, below the main convexity of the muscle bellies. The flattened tendon at the lower end rotates (anterior surface turning laterally) as it passes through the cubital fossa to its insertion into the posterior border of the tuberosity of the radius (Fig. 2.25). A bursa separates the tendon from the anterior part of the tuberosity. At the level of the elbow joint, the tendon has a broad medial expansion, the bicipital aponeurosis (Fig. 2.29), which is inserted by way of the deep fascia of the forearm into the subcutaneous border of the upper end of the ulna.

Nerve supply. By the musculocutaneous nerve (C5, 6) with one branch to each belly.

Action. The biceps is a powerful flexor of the elbow and supinator of the forearm. During supination the bicipital aponeurosis draws the distal end of the ulna slightly anteromedially. The biceps is a weak flexor of the shoulder, where the tendon of the long head helps to stabilize the joint as it runs over the top of the head of the humerus.

Test. With the forearm supinated the elbow is flexed against resistance. The contracted muscle in the arm, and the tendon and aponeurosis at the elbow are easily palpable.

Brachialis

The muscle arises from the front of the lower half of the humerus and the medial intermuscular septum. Its upper fibres clasp the deltoid insertion and some fibres arise from the lower part of the radial groove. The broad muscle flattens to cover the anterior part of the elbow joint and is inserted by mixed tendon and muscle fibres into the anterior surface of coronoid process and the tuberosity of the ulna (Fig. 2.30).

Nerve supply. By the musculocutaneous nerve (C5, 6). A small lateral part of the muscle is innervated by a branch of the radial nerve (C7).

Action. Brachialis is a flexor of the elbow joint.

Medial intermuscular septum

This fibrous septum is attached along the medial supracondylar ridge, extends proximally behind the coracobrachialis insertion and fades out above, between that muscle and the long head of triceps. It gives origin to the most medial fibres of brachialis and the medial head of triceps, and is pierced by the ulnar nerve, the superior ulnar collateral artery and the axillary branch of the radial nerve to the medial head of triceps.

Lateral intermuscular septum

This is attached along the lateral supracondylar ridge and fades out behind and above the insertion of deltoid. Both brachioradialis and extensor carpi radialis longus gain attachment to the septum in front, and posteriorly the medial head of triceps arises from it. It is pierced by the radial nerve and profunda brachii artery (radial collateral branch).

Vessels and nerves of the arm

Brachial artery

This is the continuation of the axillary artery. The brachial artery has the median nerve lateral to it above (Fig. 2.14), but the nerve crosses obliquely in front of the artery at about the middle of the arm and lies on its medial side below. The ulnar nerve, posterior to the artery above, leaves it in the lower part of the arm and slopes backwards through the medial intermuscular septum. The artery is superficial in its course in the arm, lying immediately deep to the deep fascia of the anteromedial aspect of the arm (Fig. 2.21). It passes deeply into the cubital fossa before dividing into the radial and ulnar arteries, usually at the level of the neck of the radius.

The surface marking of the brachial artery, with the arm abducted to a right angle, is along a line from the middle of the clavicle to the midpoint between the humeral epicondyles, where it is readily palpable. To palpate the artery in the upper arm, the finger pressure must be directed laterally, not backwards, as the vessel here lies medial to the humerus.

Surgical approach. The artery can be exposed at the medial border of biceps, in the groove between biceps and triceps. The deep fascia is incised and the groove opened up to display the neurovascular bundle embedded in connective tissue.

Branches. Apart from the terminal radial and ulnar arteries, the largest branch is the profunda brachii artery (Fig. 2.31). It leaves through the lower triangular space to run in the radial groove with the radial nerve. It supplies triceps, sometimes gives a nutrient artery to the humerus, and divides into two terminal branches which participate in an anastomosis around the elbow; the middle collateral descends in the medial head of triceps, while the radial collateralcontinues the course of the artery through the lateral intermuscular septum accompanying the radial nerve.

Other branches are the superior ulnar collateral, which accompanies the ulnar nerve, and the inferior ulnar collateral, which divides into anterior and posterior branches; all take part in the cubital anastomosis. There are also muscular branches to flexor muscles, and a nutrient artery to the humerus which enters the bone near the coracobrachialis attachment directed distally.

Veins of the arm

Venae comitantes accompany the brachial artery and all its branches. In addition, the basilic and cephalic veins course upwards through the subcutaneous tissue (Fig. 2.22). The former perforates the deep fascia in the middle of the arm and ascends to become the axillary vein; the latter lies in the groove between deltoid and pectoralis major and ends by piercing the clavipectoral fascia to enter the axillary vein. The venae comitantes of the brachial artery join the axillary vein.

Median nerve

The nerve (Fig. 2.14) is formed at the lower border of the axilla by the union of its medial and lateral roots, from the corresponding cords of the brachial plexus. The axillary artery is clasped between the two roots, the medial root crossing in front of the vessel. The commencement of the nerve is lateral to the artery. Passing distally through the arm the nerve lies in front of the brachial artery and at the elbow is found on its medial side. The nerve gives vascular (sympathetic) branches to the brachial artery and may give a branch to pronator teres above the elbow joint.

The surface marking of the nerve is along a line from lateral to the brachial artery in the proximal arm to medial to the artery in the cubital fossa.

Musculocutaneous nerve

The nerve gives a branch to and then pierces coracobrachialis. It comes to lie between biceps and brachialis (Fig. 2.29) and supplies both muscles. The remaining fibres appear at the lateral margin of the biceps tendon as the lateral cutaneous nerve of the forearm. The musculocutaneous is the nerve of the flexor compartment of the arm, supplying all three muscles therein. The branch to brachialis supplies the elbow joint.

Ulnar nerve

Lying posterior to the vessels this nerve inclines backwards away from them and pierces the medial intermuscular septum in the lower third of the arm, accompanied by the superior ulnar collateral artery and a branch of the radial nerve to the medial head of triceps. It gives no branch in the arm; its branch to the elbow joint comes off as it lies in the groove behind the medial epicondyle of the humerus, where it is readily palpable.

Medial cutaneous nerve of the arm

Lying medial to the vessels this small nerve pierces the deep fascia in the middle of the arm and supplies the skin on the medial side of the arm (Fig. 2.49).

Medial cutaneous nerve of the forearm

Commencing between the axillary artery and vein, this large nerve descends medial to the brachial artery and pierces the deep fascia with the basilic vein. It divides into anterior and posterior branches which descend to the forearm, the former passing in front of the median cubital vein (Fig. 2.22). The nerve supplies skin over the lower part of the front of the arm and over the medial part of the forearm (Fig. 2.49). The part of the nerve that lies in the upper arm can be used as a graft as this part has a long length without branches.

Intercostobrachial nerve

This nerve is the lateral cutaneous branch of the second intercostal nerve. It emerges from the second intercostal space anterior to the long thoracic nerve and crosses the axilla. It supplies the skin of the axilla and over a variable extent on the medial side of the upper arm, often communicating with the medial cutaneous nerve of the arm (Fig. 2.49). It may be in contact with level I lymph nodes and be at risk during node excision. The thoracoepigastric vein (see p. 179) crosses the nerve vertically on its posterior aspect and aids identification. Not infrequently the lateral cutaneous branch of the third intercostal nerve also extends outwards to supply the skin of the axilla.

Lymph nodes

Two groups of one or two lymph nodes each (not part of the axillary group) are found in the arm. The infraclavicular group lie along the cephalic vein in the upper part of the deltopectoral groove and drain through the clavipectoral fascia into the apical axillary nodes. They receive afferents from the superficial tissues of the thumb and lateral side of forearm and arm. The supratrochlear group lie in the subcutaneous fat just above the medial epicondyle. They drain the superficial tissues of the medial part of the forearm and hand, the afferent lymphatics running with the basilic vein and its tributaries. Their efferent vessels pass to the lateral group of axillary nodes.