Whereas the axial skeleton lies along the body’s central axis, the appendicular skeleton’s 126 bones include those in all four appendages — arms and legs — plus the two primary girdles to which the appendages attach: the pectoral (chest) girdle and the pelvic (hip) girdle.
The pectoral girdle is made up of a pair of clavicles, or collarbones, which attach to the sternum medially and to the scapula laterally articulating with the acromion process, a bony prominence at the top of each of the pair of scapulae, better known as shoulder blades. Each scapula has a depression in it called the glenoid fossa where the head of the humerus (upper arm bone) is attached.
The capitulum of the humerus articulates with the trochlea of the forearm’s long ulna bone to form the elbow joint. The process called the olecranon forms the elbow and is also referred to as the funny bone, although banging it into something usually feels anything but funny.
The forearm also contains a bone called the radius. The biceps muscle attaches to the radial tuberosity, flexing the elbow. The radius, together with the ulna, articulates with the eight small carpal bones that form the wrist. The carpals articulate with the five metacarpals that form the hand, which in turn connect with the phalanges (finger bones), which are found as a pair in the thumb and as triplets in each of the fingers.
The pelvic girdle consists of two hipbones, called os coxae, as well as the sacrum and coccyx, more commonly referred to as the tailbone. During early developmental years, the os coxa consists of three separate bones — the ilium, the ischium, and the pubis — that later fuse into one bone sometime between the ages of 16 and 20.
Posteriorly, the os coxa articulates with the sacrum, forming the sacroiliac joint, the source of much lower back pain; it’s formed by the connection of the hipbones at the sacrum. Toward the front of the pelvic girdle, the two os coxae join to form the symphysis pubis, which is made up of fibrocartilage. A cuplike socket called the acetabulum articulates with the ball-shaped head of the femur (thigh bone).
The femur is the longest bone in the body. The femur articulates with the tibia (shin bone) at the knee, which is covered by the patella (kneecap). Also inside each lower leg is the fibula bone, which joins with the tibia to connect with the seven tarsal bones that make up the ankle. These are the calcaneus (heel bone), talus, navicular, cuboid, lateral cuneiform, intermediate cuneiform, and medial curneiform. The largest of these bones is the calcaneus.
The tarsals join with the five metatarsals that form the foot, which in turn connect to the phalanges of the toes — a pair of phalanges in the big toe and triplets in each of the other toes.
