Chapter 12 - ARM: HUMERUS, RADIUS, & ULNA

Abstract

Each vertebrate limb has a base and three segments. The bases (the limb girdles) are the old basal fin plates of fish, which evolved to take on the function of transferring the weight of the body to the limbs of the terrestrial tetra pod. The proximal vertebrate limb segments constitute the upper arm and thigh. The intermediate limb segments, the forearm and foreleg, each bear two bones in humans, the radius and ulna in the arm, and their serial homologs, the tibia and fibula in the leg. This chapter considers the three bones of the two uppermost segments of the forelimb: the humerus, radius, and the ulna. The upper arm bone or humerus is the largest bone in the upper limb (arm). It comprises a proximal end with a round articular head, a shaft, and an irregular distal end, the radius is the shortest of the three arm bones. It is named for its action, a turning movement about the capitulum of the humerus, which allows the bone to rotate relative to the more fixed ulna, and the ulna is the longest, thinnest bone of the forearm. It articulates proximally with the trochlea of the humerus and the head of the radius.