Arm anatomy

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Arm Anatomy

The "arm" in strict anatomical terminology refers to the upper arm (shoulder to elbow), but clinically it encompasses the entire upper limb: upper arm, forearm, and hand. Here is a region-by-region breakdown.

1. Bones

Region	Bones
Upper arm	Humerus
Forearm	Radius (lateral), Ulna (medial)
Wrist	8 carpal bones (scaphoid, lunate, triquetrum, pisiform, trapezium, trapezoid, capitate, hamate)
Hand	5 metacarpals + 14 phalanges

The radius and ulna are connected by the interosseous membrane, which serves as both a structural stabilizer and an origin for several forearm muscles.

2. Compartments and Muscles

Upper Arm

The humerus is surrounded by two fascial compartments divided by the medial and lateral intermuscular septa:

Anterior (flexor) compartment:

Biceps brachii (long + short heads) - flexes elbow and supinates forearm
Brachialis - primary elbow flexor
Coracobrachialis - shoulder flexion/adduction

Posterior (extensor) compartment:

Triceps brachii (long, lateral, and medial heads) - extends elbow

The cross section below (right arm, proximal view) shows this arrangement clearly:

Cross section through the right arm - showing biceps brachii, brachialis anteriorly and triceps brachii (long, lateral, medial heads) posteriorly, with the humerus centrally, and key neurovascular structures

Forearm

The forearm contains two bones (radius and ulna) and is divided into two compartments:

Anterior (flexor/pronator) compartment - 8 muscles in 3 layers:

Superficial: Pronator teres, Flexor carpi radialis, Palmaris longus, Flexor carpi ulnaris
Intermediate: Flexor digitorum superficialis
Deep: Flexor digitorum profundus, Flexor pollicis longus, Pronator quadratus

Posterior (extensor/supinator) compartment - 12 muscles:

Superficial: Brachioradialis, ECRL, ECRB, Extensor digitorum, Extensor digiti minimi, Extensor carpi ulnaris
Deep: Supinator, Abductor pollicis longus, Extensor pollicis brevis, Extensor pollicis longus, Extensor indicis

The cross-section below shows the full forearm arrangement with both radius and ulna, the interosseous membrane, and all muscles + neurovascular bundles:

Cross section through the right forearm - showing radius and ulna, interosseous membrane, anterior flexor muscles and posterior extensor muscles, with median nerve, radial nerve (superficial branch), ulnar nerve, radial artery, and ulnar artery

3. Innervation (Brachial Plexus - C5 to T1)

Almost all arm muscles are innervated by the brachial plexus (C5-T1). The key principle: extensor muscles are supplied by posterior divisions (radial, axillary nerves), while flexor muscles are supplied by anterior divisions (musculocutaneous, median, ulnar nerves).

Nerve	Key muscles innervated
Musculocutaneous (C5-C7)	Coracobrachialis, Biceps brachii, Brachialis
Axillary (C5-C6)	Deltoid, Teres minor
Radial (C5-T1)	Triceps brachii, Anconeus, Brachioradialis, all wrist/finger extensors, Abductor pollicis longus
Median (C6-T1)	Pronator teres, Pronator quadratus, FCR, Palmaris longus, FDS, FDP (lateral half), FPL, thenar muscles, 1st & 2nd lumbricals
Ulnar (C8-T1)	FCU, FDP (medial half), hypothenar muscles, interossei, 3rd & 4th lumbricals, Adductor pollicis

Source: General Anatomy and Musculoskeletal System (THIEME Atlas of Anatomy) - Classification of upper limb muscles by innervation

4. Vasculature

Arterial supply:

Axillary artery continues as the brachial artery at the lower border of teres major
The brachial artery runs in the anteromedial arm, adjacent to the median nerve
At the elbow (cubital fossa), it divides into the radial artery (lateral) and ulnar artery (medial)
The radial and ulnar arteries anastomose at the superficial and deep palmar arches in the hand

Venous drainage:

Deep veins follow the arteries (brachial veins paired with the brachial artery)
Superficial: Cephalic vein (lateral, drains into axillary), Basilic vein (medial, pierces fascia at mid-arm), Median cubital vein (connects the two at the elbow - used for venipuncture)

5. Key Anatomical Spaces

Space	Location	Contents
Axilla	Between arm and chest wall	Brachial plexus, axillary artery/vein, lymph nodes
Cubital fossa	Anterior elbow	Brachial artery, median nerve, biceps tendon, radial nerve (lateral)
Carpal tunnel	Wrist, deep to flexor retinaculum	Median nerve + 9 flexor tendons
Guyon's canal	Medial wrist	Ulnar nerve + ulnar artery

6. Quick Clinical Memory Aids

"Saturday night palsy" = radial nerve compression (wrist drop, loss of finger extension)
Carpal tunnel syndrome = median nerve compression (thenar wasting, loss of thumb opposition, 1st-3rd finger numbness)
Cubital tunnel syndrome = ulnar nerve compression at elbow ("ring and little finger" numbness, interosseous wasting)
"Carrying angle" = normal valgus angle at elbow (~5-15° in males, up to 20° in females)
Volkmann's contracture = compartment syndrome of forearm leading to ischemic contracture of flexors

Source: THIEME Atlas of Anatomy - General Anatomy and Musculoskeletal System, Sections 2 and 3.7

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