The arm (proximal to the forearm) is separated into 2 compartments; anterior and posterior. There are three muscles in the anterior compartment; coracobrachialis, biceps brachii and brachialis. The posterior compartment houses the triceps brachii.

The biceps brachii has both a long head and a short head, hence ‘Biceps.’ The long head arises from the supraglenoid tubercle (a small prominence above the glenoid fossa), which runs in the intertubercular sulcus on the anterolateral surface of the superior humerus, and the short head arises from the coracoid process (and hence contributes to shoulder stabilisation during weight lifting). The muscle has a large belly, and inserts onto the radial tuberosity located on the proximal shaft of the radius bone. The tendon of the muscle also gives rise to the biccipital aponeurosis, which inserts onto the ulna, and which is superficially traversed by the median cubital vein.

 

The muscle is innervated by the musculocutaneous nerve (C5-C7, a branch of the lateral cord of the brachial plexus). Biceps is primarily a supinator of the forearm (along with supinator) and also acts as a flexor alongside brachialis and brachioradialis. It does not contribute to flexion when the forearm is in supination, as the muscle is not under tension. The biceps also is a weak flexor and adductor of the shoulder/glenohumeral joint, The blood supply of the biceps comes from the profunda brachii (the deep branch of the brachial artery), and the venous drainage occurs from the deep veins that accompany the profunda. As the brachialis is the main elbow flexor, yet biceps is known far more, the saying is that ‘behind every biceps is a brachialis’ i.e. behind the showman there is always a hard worker who receives little credit.

Brachialis is a large and strong muscle in the flexor compartment of the arm. It is innervated by the musculocutaneous nerve (C5-7), which is a branch of the lateral cord of the brachial plexus. It has been shown that up to 70-80% of the population may also have the radial nerve innervating their brachialis muscle. It originates from the distal half of the anterior surface of the humerus and also takes origin from the medial and lateral intermuscular septa. The muscle inserts onto the tuberosity of the ulna (where the fibers merge with the joint capsule) and coronoid process of the ulna. The muscle acts as the primary flexor of the elbow, and lies deep to the belly of biceps brachii for the majority of its course. It is a stronger flexor than biceps brachii, because it lies closer to the joint axis, and only has to flex one joint (unlike biceps which also flexes and adducts the shoulder joint via its long head). This enables the brachialis muscle to generate more tension more easily and hence flex more effectively. The muscle is not easily palpable, although its large surface area makes the biceps brachii appear larger than it actually is. It receives its blood supply from the profunda brachii. Venous drainage occurs via the deep veins of the arm (venae commitantes) that run in pairs along the profunda brachii artery. 

Coracobrachialis is a further muscle of the anterior compartment of the arm. It is innervated by the musculocutaneous nerve (C5-7), and is the first muscle of the compartment to receive innervation from it. It originates from the tip of the coracoid process (an anterior projection from the superior lateral part of the scapula), and inserts onto the upper part of the humerus. The other two muscles to arise from the coracoid process are the short head of biceps brachii (the long head arises from the supraglenoid tubercle), an elbow flexor, shoulder abductor and forearm supinator, and the pectoralis minor, which inserts onto the upper ribs and acts a scapular stabiliser and accessory muscle of respiration.

 

It functions as a flexor and adductor of the shoulder. Due to its oblique angle of insertion onto the humerus, the muscle also acts as an internal rotator of the joint. It receives its blood supply from the muscular branches of the brachial artery and venous drainage occurs via the deep veins of the arm (venae commitantes) that run in pairs along the muscular branches. The profunda brachii gives off muscular branches close to the insertion of coracobrachialis on the humerus. These vessels can be damaged during shoulder surgery.

Biceps Brachii

Developmental precursor- Hypaxial somatic mesoderm

Blood supply- Profunda brachii artery

Venous drainage- Brachial vein

Nerve: Musculocutaneous nerve (C5–C7)

Lymph drainage- Brachial and delto-pectoral lymph nodes.

Origin- Long head originates from the supraglenoid tubercle, short head originates from the anterior surface of the humerus.

Insertion- Radial tuberosity, ulnar shaft via the biccipital aponeurosis.

Action- Supination and flexion

Antagonistic muscle- Triceps brachii, pronator teres, pronator quadratus.

Brachialis

Developmental precursor- Hypaxial somatic mesoderm

Blood supply- Profunda brachii artery

Venous drainage- Brachial vein

Lymph drainage- Brachial and delto-pectoral lymph nodes.

Nerve supply- Musculocutaneous nerve

Origin- Humeral shaft

Insertion- tuberosity of the ulna, coronoid process

Action- shoulder adduction and flexion

Antagonistic muscle- Deltoid muscle, supraspinatus, latissimus dorsi, posterior deltoid

The word Brachialis has the same origin as the word brachii meaning arm. Remembering the two words will help you place it in the anterior compartment of the arm. 

The ‘coraco’ part of its name states where the muscle originates. The brachialis part of the name states which compartment it lies in i.e. the anterior compartment of the arm.

The biceps brachii is a supinator of the forearm and flexor of the elbow. It lies in the flexor compartment of the arm, and is innervated by the musculocutaneous nerve.

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