Arm - Cross Section
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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.
- 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:
Cutaneous nerves (sensory)
- Medial brachial
- Medial antebrachial
- Posterior antebrachial
- Brachial
- Superior ulnar collateral
- Middle collateral
- Radial collateral
- 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.
- 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.