The Musculo-Spiral nerve is the continuation of the posterior cord of the brachial plexus. It passes through the axilla behind the axillary artery, and down the arm behind the brachial artery on the long head of triceps. It then courses downwards and outwards in the musculo-spiral groove with the superior profunda artery, separating the heads of the triceps. The nerve then pierces the external intermuscular septum, and lies in front of the external condyle of the humerus between brachialis anticus and supinator longus, where it ends by dividing into the radial and posterior interosseous nerves.

Branches

On the inner side of the humerus :

1. Internal Cutaneous supplies the skin of the upper third of the inner side of the arm.

2. Muscular to the three heads of triceps. At the back of the humerus:

Muscular to the three heads of triceps and anconeus.

On the outer side of the humerus:

1. Cutaneous. - A superior branch supplies the skin on the outer side and back of the lower third of the arm and the upper half of the back of the forearm. An inferior branch supplies the skin in the upper two-thirds of the back of the forearm on the inner side of the area supplied by the musculo-cutaneous nerve.

2. Muscular to brachialis anticus, supinator longus, and extensor carpi radialis longior.

3. Radial nerve passes down the upper two-thirds of the forearm external to the radial artery and covered by supinator longus. It pierces the deep fascia on the outer side of the lower third of the forearm, and passes to the back of the wrist. It supplies the skin on the back of the wrist, the radial side of the back of the hand and the back of the thumb, first and second fingers, and radial side of the third finger as far as the second phalanges, the rest being supplied by the median nerve.

4. Posterior Interosseous nerve is entirely muscular and articular in its distribution. It reaches the back of the forearm by passing from under supinator longus round the outer side of the radius, and pierces supinator brevis. On the back of the forearm it passes beneath the extensor muscles with the posterior interosseous artery, then gets on the interosseous membrane by passing beneath extensor longus pollicis, and terminates in a gangliform enlargement on the back of the wrist.

Branches

Articular to the carpal joints.

Muscular to extensor carpi radialis brevis, supinator brevis, and all the extensor muscles on the back of the forearm.

Subscapular Nerves

There are three subscapular nerves arising from the posterior cord of the brachial plexus. They all pass down behind the axillary artery, and are all purely muscular :

First or short subscapular nerve to subscapularis.

Second or lower subscapular nerve to teres major.

Third or long subscapular nerve to latissimus dorsi.