Attachments

Coracobrachialis arises from the apex of the coracoid process, together with the tendon of the short head of biceps, and also by muscular fibres from the proximal 10 cm of this tendon; it ends on an impression, 3–5 cm in length, midway along the medial border of the humeral shaft between the attachments of triceps and brachialis (Fig. 46.20, Fig. 47.4). Accessory slips may be attached to the lesser tubercle, medial epicondyle or medial intermuscular septum.

Fig. 47.4 A, Transverse section through the left arm a little below the middle of the shaft of the humerus. B, MRI scan at similar level.

Relations

Coracobrachialis forms an inconspicuous rounded ridge on the upper medial side of the arm; pulsation of the brachial artery can be felt and often seen in the depression behind it. The muscle is perforated by the musculocutaneous nerve. Anteriorly it is related to pectoralis major above and, at its humeral insertion, to the brachial vessels and median nerve, which cross it. The tendons of subscapularis, latissimus dorsi, and teres major, the medial head of triceps, the humerus and the anterior circumflex humeral vessels all lie posterior. The axillary artery (third part) and proximal parts of the median and musculocutaneous nerves lie medial. Biceps and brachialis lie lateral.

Vascular supply

One or more branches from the axillary artery pass deep to the lateral root of the median nerve, and the musculocutaneous nerve, to reach the deep surface of the muscle. Branches from the anterior circumflex humeral artery also supply the deep surface of the muscle. The accompanying artery of the musculocutaneous nerve sends a recurrent branch to the coracoid attachment and gives off a series of branches to the muscle during its intramuscular course. Accessory branches from the thoracoacromial artery provide additional supply to the superficial part of coracobrachialis.

Innervation

Coracobrachialis is innervated by the musculocutaneous nerve, C5, 6 and 7.

Actions

Coracobrachialis flexes the arm forward and medially, especially from a position of brachial extension. In abduction it acts with anterior fibres of deltoid to resist departure from the plane of motion.

Clinical anatomy: testing

Coracobrachialis can be tested by palpating its fibres during shoulder flexion against resistance.

Attachments

Biceps brachii derives its name from its two proximally attached parts or ‘heads’ (Fig. 46.20). The short head arises by a thick flattened tendon from the coracoid apex, together with coracobrachialis. The long head starts within the capsule of the shoulder joint as a long narrow tendon, running from the supraglenoid tubercle of the scapula at the apex of the glenoidal cavity, where it is continuous with the glenoidal labrum (p. 805). The tendon of the long head, enclosed in a double tubular sheath (an extension of the synovial membrane of the joint capsule), arches over the humeral head, emerges from the joint behind the transverse humeral ligament, and descends in the intertubercular sulcus, where it is retained by the transverse humeral ligament and a fibrous expansion from the tendon of pectoralis major. The two tendons lead into elongated bellies that, although closely applied, can be separated to within 7 cm or so of the elbow joint. At this joint they end in a flattened tendon, which is attached to the rough posterior area of the radial tuberosity; a bursa separates the tendon from the smooth anterior area of the tuberosity. As it approaches the radius, the tendon spirals, its anterior surface becoming lateral before being applied to the tuberosity. The tendon has a broad medial expansion, the bicipital aponeurosis, which descends medially across the brachial artery to fuse with deep fascia over the origins of the flexor muscles of the forearm (Fig. 47.5). The tendon can be split without difficulty as far as the tuberosity, whence it can be confirmed that its anterior and posterior layers receive fibres from the short and long heads, respectively.

Fig. 47.5 Anterior aspect of left upper arm showing arteries and nerves.

(From Sobotta 2006.)

In 10% of cases, a third head arises from the superomedial part of brachialis and is attached to the bicipital aponeurosis and medial side of the tendon of insertion. It usually lies behind the brachial artery, but it may consist of two slips, which descend in front of and behind the artery. Less often, other slips may spring from the lateral aspect of the humerus or intertubercular sulcus.

Relations

Biceps is overlapped proximally by pectoralis major and deltoid; distally it is covered only by fasciae and skin, and it forms a conspicuous elevation on the front of the arm. Its long head passes through the shoulder joint; its short head is anterior to the joint. Distally it lies anterior to brachialis, the musculocutaneous nerve and supinator. Its medial border touches coracobrachialis, and overlaps the brachial vessels and median nerve; its lateral border is related to deltoid and brachioradialis.

Vascular supply

Biceps brachii is typically supplied by up to eight vessels originating from the brachial artery in the middle third of the arm. These vessels pass laterally, posterior to the median nerve and divide into ascending and descending branches just before reaching the deep surface of the muscle. Smaller branches arise from the anterior circumflex humeral artery and the deltoid branch of the acromial division of the thoracoacromial axis.

There is great variation in the arterial supply to the muscle. The main arterial supply may originate from the superior or inferior ulnar collateral artery, subscapular artery, axillary artery, ulnar or radial arteries in cases of proximal bifurcation of the brachial artery, or the profunda brachii artery.

Innervation

Biceps brachii is innervated by the musculocutaneous nerve, C5 and 6, with separate branches passing to each belly.

Actions

Biceps brachii is a powerful supinator, especially in rapid or resisted movements. It flexes the elbow, most effectively with the forearm supinated, and acts to a slight extent as a flexor of the shoulder joint. It is attached, via the bicipital aponeurosis, to the posterior border of the ulna, the distal end of which is drawn medially in supination. The long head helps to check upward translation of the humeral head during contraction of deltoid. When the elbow is flexed against resistance, the tendon of insertion and bicipital aponeurosis become conspicuous.

Lowering the hand under the influence of gravity by extension at the elbow calls for controlled lengthening of biceps. This is an example of a habitual movement in which muscle tension increases despite increasing length. As the hand descends and the elbow extends, the vertical through the centre of gravity of the forearm is carried further from the fulcrum of movement; the turning moment exerted by the load therefore increases and must be matched by an increase in the moment exerted by the muscle.

Clinical anatomy: testing

With the forearm supinated, biceps brachii can be tested by palpating its fibres during elbow flexion against resistance.

Attachments