Cross Section of the Arm

Cross Section of the Arm

Connective tissues that envelope the arm:

• The skin
• The brachial fascia, which is a thick band of connective tissue that envelops the muscles of the arm.

Connective tissues that separates the arm into posterior and anterior compartments:

• Lateral intermuscular septum separates anterior and posterior compartments
• Medial intermuscular septum separates anterior and posterior compartments

Muscles visible at mid-humeral shaft

Anterior Upper Arm

• Biceps brachii, which lies superficial to brachialis Its two heads have merged.
• Brachialis
• Coracobrachialis, the third flexor of the anterior compartment, is not drawn here because it terminates proximally.

Posterior Upper Arm

• Triceps brachii, which has three heads: long, medial, and lateral.
• Medial head lies against the humerus.
• Lateral head lies superficially and laterally
• Anconeus, which is often considered a muscle of the posterior arm, lies distal to our cross-section drawing.

Key neurovascular structures of the arm at mid-humeral shaft

Mixed motor and sensory nerves:

• Musculocutaneous
• Median
• Ulnar
• Radial

Cutaneous nerves (sensory)

• Medial brachial
• Medial antebrachial
• Posterior antebrachial

Major arteries:

• Brachial
• Superior ulnar collateral
• Middle collateral
• Radial collateral

Major veins:

• Cephalic
• Brachial
• Basilic

Regional descriptions

Deeper structures in the medial side of the arm:

• Ulnar nerve (which serves the anterolateral forearm) is posterior to the medial intermuscular septum.
• Superior ulnar collateral artery, which is a branch of the brachial artery.

Anterior to the medial intermuscular septum:

• Median nerve.
• Musculocutaneous nerve between brachialis and biceps brachii. As its name implies, this nerve innervates both muscles and skin.
• Brachial artery and vein are adjacent to the median nerve.
  The brachial artery and vein are the primary vessels of the arm; they are continuations of the axillary artery and vein.
• Medial antebrachial cutaneous nerve runs superficially.
• Basilic vein, which will eventually perforate the medial intermuscular septum to become superficial.

Posterior:

• Radial nerve, which serves the posterior compartment of the arm.
• Posterior antebrachial cutaneous nerve, which is a branch of the radial nerve.
• Radial collateral artery, and,
• Middle collateral artery.

Superficial structures of the arm:

• Medial brachial cutaneous nerve; this nerve innervates the skin of the arm;
• Cephalic vein continues into the hand.

Key points:

• Anterior compartment muscles are innervated by the musculocutaneous nerve; they are primarily responsible for flexion of the arm and forearm.
• Posterior compartment muscles are innervated by the radial nerve; they are responsible for extension of the forearm, and, in the case of the long head of triceps brachii, for extension and adduction of the arm.
• The ulnar and median nerves do not innervate structures in the arm; their targets lie in the forearm and hand.