The Appendicular Skeleton

THE APPENDICULAR SKELETON

The appendicular skeleton includes the bones of the shoulder girdle, upper limbs, pelvic girdle and lower limbs. It is specialised for movement and interaction with the environment.

1) Shoulder Girdle

The shoulder girdle connects each upper limb to the trunk. It consists of two clavicles (collarbones) and two scapulae (shoulder blades).

Clavicle (Collar Bone)

• An S-shaped bone that links the upper limb to the axial skeleton at the sternoclavicular joint.
• Medially articulates with the manubrium of the sternum and laterally with the acromion of the scapula (acromioclavicular joint).
• Acts as a strut to keep the shoulder away from the thorax, allowing a wide range of movement.

Scapula (Shoulder Blade)

• A flat triangular bone on the posterior chest wall.
• The glenoid cavity at its lateral angle forms the socket of the shoulder joint (glenohumeral joint).
• Important landmarks: spine of scapula, acromion, coracoid process — muscle attachments and joint formation.

2) The Upper Limb

Humerus

• Long bone of the upper arm.
• Proximal end: head articulates with glenoid cavity to form the shoulder joint. Greater and lesser tubercles serve as muscle attachment points.
• Distal end: articulates with radius and ulna to form the elbow joint (trochlea and capitulum visible on distal humerus).

Radius and Ulna (Forearm)

• Two parallel bones.
• Ulna: medial in anatomical position (toward little finger), longer, forms major part of the elbow joint (trochlear notch).
• Radius: lateral (thumb side); its head articulates with the capitulum and participates in wrist joint.
• Interosseous membrane between them stabilizes the forearm and transmits forces.

Carpal (Wrist) Bones

• Eight small bones arranged in two rows (proximal: scaphoid, lunate, triquetrum, pisiform; distal: trapezium, trapezoid, capitate, hamate).
• Form flexible wrist and small gliding joints; ligaments and retinacula hold tendons in place.

Metacarpals and Phalanges

• Five metacarpals form the palm (numbered I–V from thumb to little finger).
• Phalanges: 14 finger bones — three in each finger (proximal, middle, distal) and two in the thumb.

3) Pelvic Girdle

The pelvic girdle is formed by two innominate (hip) bones, each made by fusion of ilium, ischium and pubis. Together with the sacrum they make the pelvis.

Innominate Bone

• Ilium: large, wing-like superior portion (iliac crest and anterior superior iliac spine are palpable landmarks).
• Ischium: posterior inferior part — ischial tuberosity bears weight when sitting.
• Pubis: anterior part; the two pubic bones meet at the pubic symphysis.

The Pelvis

• The pelvis is divided by the pelvic brim into the greater (false) pelvis above and the lesser (true) pelvis below.
• Male and female pelvises differ: female pelvis is generally broader with a wider pelvic inlet/outlet to facilitate childbirth.

4) Lower Limb

Femur

• The longest and strongest bone in the body (thigh bone).
• Proximal end: head (articulates with acetabulum), neck, greater and lesser trochanters (muscle attachments).
• Distal end: two condyles form part of the knee joint.

Tibia (Shin Bone)

• Larger medial bone of lower leg; proximal end articulates with femur (knee), distal with talus (ankle).
• Tibial tuberosity is an important landmark for quadriceps tendon attachment.

Fibula

• Thin lateral bone; does not bear major weight but provides muscle attachments and forms the lateral malleolus which stabilises the ankle.

Patella (Kneecap)

• Sesamoid bone embedded in the quadriceps tendon; improves mechanical advantage of knee extension and protects the knee joint.

Tarsals, Metatarsals, Phalanges

• Tarsals: seven bones form the ankle and posterior foot (talus, calcaneus (heel bone), navicular, cuboid, and three cuneiforms).
• Metatarsals: five long bones forming the foot arch and dorsum.
• Phalanges: 14 toe bones (two in big toe, three in each other toe).