The appendicular skeleton

The appendicular skeleton is the bones of the limbs and the girdles that join the limbs to the axial skeleton. Each limb has 30 bones.

• Fore limb (arm): the upper arm bone humerus; the two forearm bones radius and ulna; 8 carpals in the wrist; 5 metacarpals in the palm; 14 phalanges in the fingers. 1 + 2 + 8 + 5 + 14 = 30.
• Hind limb (leg): the thigh bone femur, the longest bone in the body; the two lower leg bones tibia and fibula; 7 tarsals in the ankle; 5 metatarsals; 14 phalanges in the toes; and the cup-shaped knee cap, the patella. 1 + 2 + 7 + 5 + 14 + 1 = 30.

The pectoral girdle (shoulder) holds the arms. Each half has two bones:

• The clavicle (collar bone): a long, slender bone with two curves.
• The scapula (shoulder blade): a large, flat, triangular bone on the back, between the 2nd and 7th ribs. A ridge on it, the spine, ends in a flat tip, the acromion, which the clavicle meets. Just below is a shallow hollow, the glenoid cavity, into which the rounded head of the humerus fits: this is the shoulder joint.

The pelvic girdle (hip) holds the legs. It has two coxal bones, each formed by the fusion of three bones: the ilium, ischium and pubis. Where they meet is a cup-shaped hollow, the acetabulum, into which the head of the femur fits. At the front, the two halves meet at the pubic symphysis, joined by fibrous cartilage.