Anterior Compartment of the Arm Anatomy
The anterior compartment of the arm contains three muscles (biceps brachii, coracobrachialis and brachialis), the brachial artery with its venae comitantes and three nerves (the median, ulnar and musculocutaneous nerves).
The three muscles of the compartment are supplied by the muscu- locutaneous nerve. The two heads of biceps separate proximally (Fig. 3.21) and have tendinous attachments to the scapula. The short head lies medially and attaches to the tip of the coracoid process. The tendon of the long head attaches to the supraglenoid tubercle, leaves the shoulder joint deep to the transverse humeral ligament and continues distally in the intertubercular sulcus deep to the tendon of pectoralis major. The muscle bellies fuse and are attached by a tendon (Fig. 3.27) to the tuberosity of the radius and by the bicipital aponeurosis, which fuses with deep fascia on the medial side of the forearm. Biceps is a strong flexor of the elbow and supinator of the forearm at the radioulnar joints, and a weak flexor of the shoulder joint.
Coracobrachialis attaches to the coracoid process with the short head of biceps. The muscle attaches distally to the medial side of the shaft of the humerus near its midpoint (Fig. 3.23). Coracobrachialis is pierced by the musculocutaneous nerve (Fig. 3.22) and functions as a weak flexor and adductor of the shoulder joint.
Brachialis lies deeply and has an extensive attachment to the anterior surface of the distal half of the shaft of the humerus (Fig. 3.23) and adjacent intermuscular septa. The muscle passes to the coronoid process of the ulna (Fig. 3.34) and acts as a powerful flexor of the elbow joint.
Fig. 3.24 Brachialis, the brachial artery and the nerves of the compartment. Biceps brachii and most veins have been excised.
The brachial artery is the continuation of the axillary artery distal to teres major. In the upper part of the arm, the brachial artery with its venae comitantes is accompanied by the median and ulnar nerves (Fig. 3.24) and the medial cutaneous nerve of the forearm. The artery passes distally and laterally, lying medial to biceps and anterior to coracobrachialis and brachialis. An important branch, the profunda brachii artery (p. 107), supplies the posterior compartment of the arm. Division of the bra- chial artery into its terminal radial and ulnar branches usually occurs in the cubital fossa but may occur more proximally. The brachial artery is vulnerable to injury in fractures just proximal to the humeral epicondyles, with risk of subsequent fibrosis in forearm muscles (Volkmann’s ischaemic contracture).
Venae comitantes, ascending from the cubital fossa, accompany the brachial artery and are joined by the basilic vein after it has pierced the deep fascia at about midarm level. At the lower border of the axilla the venous channels usually combine to form a single axillary vein.
The musculocutaneous nerve (Fig. 3.22) is a terminal branch of the lateral cord. It pierces coracobrachialis and lies between biceps and brachialis, supplying each of these muscles. The nerve continues distally as the lateral cutaneous nerve of the forearm, which pierces the deep fascia between biceps and brachioradialis to lie superficially over the cubital fossa (Fig. 3.26).
The median and ulnar nerves traverse the entire length of the arm, but neither gives any branches above the elbow joint. The median nerve arises by lateral and medial heads, which are terminal branches of the lateral and medial cords. In the upper part of the arm the nerve lies lateral to the brachial artery but at midarm level it crosses anterior to the vessels and finally lies medial to the artery (Fig. 3.24), a position retained in the cubital fossa. The ulnar nerve is a terminal branch of the medial cord and, together with the medial cutaneous nerve of forearm, initially lies medial to the brachial artery but leaves the artery at midarm level (Fig. 3.24). It then pierces the medial intermuscular septum and enters the posterior compartment to lie between the septum and the medial head of triceps.