The skeletal support for the arm is the humerus (Fig. 7.60). Most of the large muscles of the arm insert into the proximal ends of the two bones of the forearm, the radius and the ulna, and flex and extend the forearm at the elbow joint. In addition, the muscles predominantly situated in the forearm that move the hand originate at the distal end of the humerus.
Shaft and distal end of the humerus
In cross section, the shaft of the humerus is somewhat triangular with: anterior, lateral, and medial borders, and anterolateral, anteromedial, and posterior surfaces (Fig. 7.60).
The posterior surface of the humerus is marked on its superior aspect by a linear roughening for the attachment of the lateral head of the triceps brachii muscle, beginning just inferior to the surgical neck and passing diagonally across the bone to the deltoid tuberosity.
The middle part of the posterior surface and adjacent part of the anterolateral surface are marked by the shallow radial groove, which passes diagonally down the bone and parallel to the sloping posterior margin of the deltoid tuberosity. The radial nerve and the profunda brachii artery lie in this groove.
Approximately in the middle of the shaft, the medial border is marked by thin elongate roughening for the attachment of the coracobrachialis muscle.
Intermuscular septa, which separate the anterior compartment from the posterior compartment, attach to the medial and lateral borders (Fig. 7.61).
Distally, the bone becomes flattened, and these borders expand as the lateral supraepicondylar ridge (lateral supracondylar ridge) and the medial supraepicondylar ridge (medial supracondylar ridge). The lateral supraepicondylar ridge is more pronounced than the medial ridge and is roughened for the attachment of muscles found in the posterior compartment of the forearm.
The distal end of the humerus, which is flattened in the anteroposterior plane, bears a condyle, two epicondyles, and three fossae, as follows (Fig. 7.61).
The condyle
The two articular parts of the condyle, the capitulum and the trochlea, articulate with the two bones of the forearm.
The capitulum articulates with the radius of the forearm. Lateral in position and hemispherical in shape, it projects anteriorly and somewhat inferiorly and is not visible when the humerus is viewed from the posterior aspect.
The trochlea articulates with the ulna of the forearm. It is pulley shaped and lies medial to the capitulum. Its medial edge is more pronounced than its lateral edge and, unlike the capitulum, it extends onto the posterior surface of the bone.
The two epicondyles
The two epicondyles lie adjacent, and somewhat superior, to the trochlea and capitulum (Fig. 7.61).
The medial epicondyle, a large bony protuberance, is the major palpable landmark on the medial side of the elbow, and projects medially from the distal end of the humerus. On its surface, it bears a large oval impression for the attachment of muscles in the anterior compartment of the forearm. The ulnar nerve passes from the arm into the forearm around the posterior surface of the medial epicondyle and can be palpated against the bone in this location.
The lateral epicondyle is much less pronounced than the medial epicondyle. It is lateral to the capitulum and has a large irregular impression for the attachment of muscles in the posterior compartment of the forearm.
The three fossae
Three fossae occur superior to the trochlea and capitulum on the distal end of the humerus (Fig. 7.61).
The radial fossa is the least distinct of the fossae and occurs immediately superior to the capitulum on the anterior surface of the humerus.
The coronoid fossa is adjacent to the radial fossa and is superior to the trochlea.