The Appendicular Skeleton

The humerus is the single bone of the upper arm. It connects the shoulder region to the forearm and helps form the shoulder and elbow joints. In the upper limb, it sits between the arm, forearm, wrist, and hand bones.

At the top of the humerus is a smooth, rounded head. It fits into the glenoid cavity of the scapula, or shoulder blade, to form the glenohumeral joint. The edge of the head is the anatomical neck, which is a leftover epiphyseal plate.

Near the top, the humerus also widens into...

The ulna is a forearm bone that helps support the elbow and wrist. It runs parallel to the radius, and it lies on the medial side of the antebrachium, or forearm. Its shape and joint surfaces help the arm bend, rotate, and stay stable during movement.

At the proximal end, the ulna has the olecranon process. This hook-like tip fits into the olecranon fossa of the humerus and forms the ulnohumeral part of the elbow joint. That hinge-like joint allows flexion and extension at the forearm while...

The radius is the longer bone in the forearm, and it plays a key role in elbow, wrist, and hand movement. It sits beside the ulna in the antebrachium, which is the forearm. At the upper end, the radius meets the capitulum of the humerus and the radial notch of the ulna to help form the elbow joint.

The lower end of the radius connects with the ulna through the ulnar notch. This connection forms the distal radioulnar joint. The radius also joins the scaphoid and lunate carpal bones at the wrist...

The femur and patella help support movement at the hip and knee. The femur is the longest and strongest bone in the body. It spans the thigh region and connects the hip joint to the knee joint.

The head of the femur fits into the acetabulum of the hip bone. A small dip on the medial side of the femoral head is the fovea capitis. This is where the ligament of the head of the femur attaches. That weak ligament runs between the femur and acetabulum and helps support the hip joint. Below the head...

The tibia and fibula are the two bones of the lower leg. The tibia is the main weight-bearing bone. It is larger than the fibula and is the second longest bone in the body. It lies close to the skin, so many of its parts are easy to feel.

The upper end of the tibia helps form the knee joint. Its medial condyle and lateral condyle join with the condyles of the femur. Between them is the intercondylar eminence, an uneven raised area that anchors two supporting ligaments of the knee. The tibial...

The appendicular skeleton begins as limb buds on the side of the early embryo. The upper limb bud appears near the end of the fourth week of development, and the lower limb bud follows soon after. Each limb bud has a core of mesenchyme, which is a loose embryonic tissue, covered by ectoderm, the outer cell layer.

A thickened part of the ectoderm forms the apical ectodermal ridge. This ridge signals the mesenchyme below it to divide quickly and drive limb outgrowth. As the limb bud grows...