Anatomy of the upper limb

The left upper limb is the upper appendage on the left side of the body, comprising the pectoral girdle and the free limb. Its bones and joints position the hand, while muscle compartments, nerves and arteries support movement and sensation. The same basic organisation applies to the right upper limb.

What makes up the pectoral girdle?

The left pectoral girdle consists of the clavicle, or collarbone, and the scapula, or shoulder blade. The left free upper limb extends from the shoulder to the fingers and includes the arm, forearm and hand.

The sternoclavicular joint joins the clavicle to the sternum, the breastbone, and is the only direct bony joint between the upper limb and the trunk. The acromioclavicular joint joins the clavicle to the acromion, the projecting upper part of the scapula. Both are synovial joints: joints whose opposing surfaces are separated by a fluid-containing cavity enclosed by a capsule.

The scapula also glides over the chest wall through the scapulothoracic articulation, a functional relationship rather than a true synovial joint. Muscles hold it against the chest and move it during reaching and lifting. The glenohumeral joint, the main shoulder joint, joins the shallow glenoid cavity of the scapula to the rounded head of the upper arm bone.

Which bones and joints form the arm?

The arm is the region between shoulder and elbow; its bone is the humerus. At the shoulder, the head of the humerus forms a ball-and-socket joint with the glenoid cavity. This permits flexion and extension, forward and backward movement; abduction and adduction, movement away from and towards the trunk; and rotation around the bone's long axis.

The rotator cuff is a group of muscles whose tendons blend with the shoulder joint capsule and help hold the humeral head against the glenoid cavity. A tendon is the connective tissue through which a muscle transmits force to its attachment.

At the elbow joint, the lower end of the humerus meets both forearm bones. Its trochlea, a pulley-shaped surface, meets the ulna, while its capitulum, a rounded surface, meets the radius. The elbow mainly permits flexion and extension: bending and straightening the forearm relative to the arm.

How do the forearm bones allow rotation?

The forearm lies between elbow and wrist and contains the radius and ulna. In anatomical position—standing with the palms facing forwards—the radius lies on the thumb side and the ulna on the little-finger side. Anterior means towards the front of the body, and posterior means towards the back.

The proximal and distal radioulnar joints join these bones near the elbow and wrist respectively. Proximal means nearer the limb's attachment to the trunk; distal means farther from it. These joints allow pronation, turning the palm backwards from anatomical position, and supination, returning it forwards.

During pronation, the radius crosses over the ulna; during supination, the bones lie approximately parallel. The interosseous membrane, a fibrous sheet between their shafts, links the bones and provides muscle attachment. At the wrist, the radius and an articular disc, a pad of fibrous cartilage, meet the proximal carpal bones; the ulna does not directly contact them.

Which bones and joints form the hand?

The hand contains carpal bones at the wrist, metacarpal bones in the palm and phalanges in the digits. A phalanx is an individual finger or thumb bone. The thumb has proximal and distal phalanges, while each finger also has a middle phalanx.

The carpal bones form two rows:

• Proximal row, from thumb side to little-finger side: scaphoid, lunate, triquetrum and pisiform.
• Distal row, in the same direction: trapezium, trapezoid, capitate and hamate.

The radiocarpal joint connects the forearm to the scaphoid, lunate and triquetrum of the proximal carpal row; the pisiform does not participate. The midcarpal joint lies between the rows. Together they permit wrist flexion, extension and side-to-side movement. Intercarpal joints lie between neighbouring carpal bones.

Carpometacarpal joints connect the wrist bones to the metacarpals. The thumb carpometacarpal joint is saddle-shaped and permits opposition, bringing the thumb pad towards the finger pads. Metacarpophalangeal joints form the knuckles, while interphalangeal joints lie between adjacent phalanges and mainly permit bending and straightening.

What do the anterior and posterior muscle compartments do?

A muscle compartment is a group of muscles enclosed by fascia, the connective tissue surrounding muscles, and partitions called intermuscular septa. The arm and forearm each have anterior and posterior compartments. Their shared actions and nerve supplies provide a useful framework, although individual muscles have additional roles.

The anterior arm contains biceps brachii, brachialis and coracobrachialis. Brachialis bends the elbow, while coracobrachialis flexes and adducts the arm at the shoulder. Triceps brachii is the principal posterior arm muscle and straightens the elbow.

In the anterior forearm, flexor carpi ulnaris and the portion of flexor digitorum profundus serving the ring and little fingers receive ulnar nerve supply. The remaining muscles receive median nerve supply. Brachioradialis is a notable exception to the posterior forearm's general action: it receives radial nerve supply but flexes the elbow.

How is the brachial plexus organised?

The brachial plexus is a network of nerves supplying the upper limb. It is usually formed by the anterior rami of spinal nerves C5–T1: the anterior branches of the lower four cervical and first thoracic spinal nerves. Its organisation proceeds through roots, trunks, divisions, cords and terminal branches.

The C5 and C6 roots unite into the superior trunk, C7 continues as the middle trunk, and C8 and T1 unite into the inferior trunk. Each trunk divides into anterior and posterior divisions, which regroup into lateral, medial and posterior cords, named by their position around the axillary artery in the armpit.

The five major terminal nerves are:

• Musculocutaneous nerve: supplies the anterior arm muscles and continues as a sensory nerve to the outer forearm.
• Axillary nerve: supplies deltoid and teres minor, shoulder muscles involved in abduction and outward rotation, and skin over the lower deltoid region.
• Radial nerve: supplies the posterior arm and forearm compartments and skin over parts of the posterior limb and back of the hand.
• Median nerve: supplies most anterior forearm muscles and selected thumb and finger muscles within the hand.
• Ulnar nerve: supplies part of the anterior forearm and most muscles located within the hand.

How does arterial blood reach the hand?

The subclavian artery becomes the axillary artery at the outer border of the first rib. The axillary artery becomes the brachial artery at the lower border of teres major, a muscle passing from the scapula to the humerus. The brachial artery descends through the arm and usually divides near the elbow into radial and ulnar arteries.

The radial artery follows the thumb side of the forearm, and the ulnar artery follows the little-finger side. In the hand, their branches contribute to the superficial and deep palmar arches, which usually form communicating arterial loops; the superficial arch may be incomplete. The superficial palmar arch is mainly supplied by the ulnar artery, and the deep palmar arch mainly by the radial artery; branches from these arches supply the digits.

What passes through the carpal tunnel?

The carpal tunnel is a passage on the palm side of the wrist. Its floor and sides are formed by the concave arrangement of carpal bones, and its roof is the flexor retinaculum, a strong fibrous band spanning the wrist.

It contains the median nerve and the tendons of flexor digitorum superficialis, flexor digitorum profundus and flexor pollicis longus, which bend the fingers and thumb. The ulnar nerve and artery pass outside the tunnel, and the flexor carpi radialis tendon occupies a separate passage.

In the adjacent 3D model, rotate the limb, select a structure and isolate it to examine its position relative to the surrounding anatomy.