This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
Caries or necrosis of the humerus may necessitate operative interference at almost any part of the arm. The same may be said of wounds. In operative procedures it is sometimes desirable to avoid important structures and at others to find them.
Fig. 291. - The radial (muscalospiral) nerve and outer side of arm.
The important structures run lengthwise, hence transverse incisions are not to be used. Most of the large vessels and nerves pass down the inner side of the arm, hence this region is usually avoided. The bone can readily be reached by an incision downward from the insertion of the deltoid, but no operation is to be done in this region without a thorough familiarity with the course of the musculospiral nerve,
A line drawn on the posterior surface of the arm from behind and above the insertion of the deltoid to the groove on the anterior surface between the brachialis anticus and brachioradialis (supinator longus) just above and to the inner side of the external condyle will indicate its course. If exposed during an operation bleeding from the accompanying (superior) profunda artery may be expected. The median and ulnar nerves give off no branches in the upper arm. The median can be readily located by its relation to the artery. It lies to the outer and anterior side of the brachial artery above, then in front, and then to its inner side below. The ulnar nerve lies to the inner side of the artery and between it and the vein posteriorly. In the middle of the arm, it leaves it to pierce and pass beneath the internal intermuscular septum and thence behind the medial (internal) condyle. Operations involving it would be accompanied by bleeding from its companion the superior ulnar collateral artery (inferior profunda).
In operations on the lower portion of the bone the position of the inferior ulnar collateral (anastomotica magna), 5 cm. (2 in.) above the elbow, should be borne in mind. It runs on the brachialis anticus muscle and towards the inner and not the outer side. Incisions on the outer side will encounter the cephalic vein in the external bicipital furrow. Incisions on the inner side will encounter the basilic vein; at the junction of the lower and middle thirds of the arm it pierces the deep fascia.