The Humerus is cylindrical above its centre, then prismatic, and finally flattened antero-posteriorly, presenting marked lateral supracondyloid ridges, which give attachment to the intermuscular septa. The principal nutrient foramen is situated on the inner side, about the centre of the shaft, and is directed downwards for about 2 inches before opening into the medullary cavity. Sometimes, however, the foramen is situated posteriorly in the musculo-spiral groove, the nutrient vessel then coming off the superior profunda. A rough elevation at the middle of the outer side indicates the position of the insertion of the deltoid (deltoid tubercle), and the musculo-spiral groove is evident on the posterior aspect. As already pointed out, the internal condyle indicates the direction of the articular facet of the head, and the external condyle is in the same straight line as the great tuberosity. These points are of importance in relation to dislocation, fractures, and excisions. Above the insertion of the deltoid most of the muscles surrounding the humerus do not possess osseous insertions, and thus retract readily when cut, whereas below that point the triceps and brachialis anticus do not retract much when cut, owing to their osseous attachments. The upper end of the humerus presents the hemispherical head, shallow anatomical neck, the two tuberosities, and the surgical neck. The surgical neck is that portion which lies below the tuberosities, but above the insertions of the latissimus dorsi and teres major muscles.
The circumflex nerve and posterior circumflex artery emerge from between the two teres muscles, and wind horizontally round the bone at the surgical neck. The nerve supplies the joint, the deltoid and teres minor muscles, and the skin over the lower two-thirds of the shoulder and upper part of the triceps, thus securing agreement between the moving force and the parts moved. This distribution of the circumflex nerve is an illustration of Hilton's Law :-A nerve trunk, supplying a given joint, also supplies the muscles moving that joint, and the integument covering their insertions. The nerve may be damaged by injury to the shoulder, and more frequently by fracture of the surgical neck or dislocation, particularly backwards, or by tumours of the upper end of the humerus. It may also be affected by neuritis from affections of the shoulder-joint. In all these cases paralysis of the deltoid is apt to result.