Topic Overview

Compartments of the Arm

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Overview

• The arm (brachium) is divided by the lateral and medial intermuscular septa into:

  • Anterior (flexor) compartment

  • Posterior (extensor) compartment

• The deep fascia of arm (brachial fascia) encloses both compartments and provides attachment for muscles and septa.

• Neurovascular arrangement:

  • Anterior compartment → supplied mainly by musculocutaneous nerve; blood supply by brachial artery.

  • Posterior compartment → supplied by radial nerve; blood supply by profunda brachii artery.

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Anterior Compartment of the Arm

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Muscles

The anterior compartment contains three muscles:

1. Biceps brachii

2. Coracobrachialis

3. Brachialis

All three muscles are supplied by the musculocutaneous nerve (C5–C7).

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1. Biceps Brachii

Origin

• Long head: Supraglenoid tubercle of scapula (tendon passes through shoulder joint capsule and intertubercular sulcus).

• Short head: Tip of coracoid process of scapula.

Insertion

• Radial tuberosity and bicipital aponeurosis (which blends with deep fascia of forearm).

Nerve Supply

• Musculocutaneous nerve (C5, C6).

Actions

• Flexes elbow joint.

• Supinates forearm (especially when flexed).

• Assists in shoulder flexion (weakly).

Clinical Anatomy

• Biceps reflex (C5, C6) used for neurological testing.

• Biceps tendon rupture → “Popeye deformity” (bulging of muscle belly).

• Tenosynovitis of long head → shoulder pain.

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2. Coracobrachialis

Origin

• Tip of coracoid process of scapula (with short head of biceps).

Insertion

• Middle of medial border of humerus.

Nerve Supply

• Musculocutaneous nerve (C5–C7).

Actions

• Flexes and adducts arm at shoulder joint.

Clinical Anatomy

• Musculocutaneous nerve pierces coracobrachialis — a useful identification landmark during dissection.

• Hypertrophy or fibrosis may compress the nerve.

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3. Brachialis

Origin

• Lower half of anterior surface of humerus.

Insertion

• Coronoid process and tuberosity of ulna.

Nerve Supply

• Musculocutaneous nerve (C5, C6).

• Radial nerve gives small branch to lateral part.

Actions

• Chief flexor of elbow joint (acts in all positions of pronation/supination).

Clinical Anatomy

• Important in testing C6 segment of spinal cord.

• Deep to biceps — easily injured in fractures of lower humerus.

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Nerves and Vessels in Anterior Compartment

• Musculocutaneous nerve: motor to all three flexor muscles; continues as lateral cutaneous nerve of forearm.

• Median nerve: runs medially to brachial artery but gives no branches in arm.

• Ulnar nerve: passes medial to brachial artery → pierces medial intermuscular septum → behind medial epicondyle.

• Brachial artery: main arterial trunk; gives off profunda brachii, superior and inferior ulnar collateral arteries.