The long head of this muscle arises from the supraglenoid tubercle and adjoining part of the glenoid labrum of the scapula. The rounded tendon passes through the synovial cavity of the shoulder joint, surrounded by synovial membrane, and emerges beneath the transverse ligament at the upper end of the intertubercular groove. The membrane pouts out below the ligament to an extent which varies with the position of the arm, being greatest in full abduction. The range of mobility of the tendon under the transverse ligament between full adduction and abduction of the shoulder joint is 6 cm. The long tendon develops outside the capsule, then sinks through it, then hangs on a mesotendon which subsequently breaks down. This embryological progression may become arrested at any stage.
The short head arises from the apex of the coracoid process 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 belly. The tendon at the lower end, flat from side to side, lies across the elbow joint and converges into a flattened cord which 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. A bursa separates the tendon from the anterior part of the tuberosity. From the medial border of the tendon, at the level of the elbow joint, the bicipital aponeurosis is inserted by way of the deep fascia of the forearm into the subcutaneous border of the upper end of the ulna. This aponeurosis has a sharp concave upper margin which can be felt tensed when the supinated forearm is flexed to a right angle. The main tendon, sliding in and out of the cubital fossa during pronation and supination, has a total range of movement of 6 cm.
Nerve supply of biceps brachii:
By the musculocutaneous nerve (C5, 6).
Action of biceps brachii:
If the elbow is extended the muscle is a simple flexor thereof, but in any position of the elbow short of full extension the biceps is a powerful supinator of the forearm. In ordinary supination the triceps contracts to prevent flexion at the elbow joint. In full supination the bicipital aponeurosis is pulled taut and the biceps then pulls on both bones of the forearm to flex the elbow. Unopposed by triceps the biceps, while supinating the forearm, flexes the elbow and shoulder joints. It ‘puts in the corkscrew then pulls out the cork’.