(4.01)

Now we’ll go back up to the top, and look at the nerves. Four nerves surround the axillary artery as it emerges from beneath pectoralis minor. They’re the musculocutaneous, the median, the ulnar, and the radial. We’ll look at them in that order.

Here's the musculocutaneous nerve. It supplies three flexor muscles in the arm, the first of which is a shoulder flexor, coracobrachialis. The musculocutaneous nerve runs right through coracobrachialis, and emerges here, deep to the biceps. It runs down the arm between biceps and brachialis, supplying both muscles. It emerges here, to become the lateral cutaneous nerve of the forearm.

The median nerve and the ulnar nerve both run all the way down to the elbow without supplying anything.

They start out close together. Halfway down the arm they diverge. The median nerve stays close to the brachial artery, crossing in front of it. At the elbow it lies medial to the artery. It dives down between the brachialis tendon and pronator teres, and passes between the two heads of pronator teres to enter the forearm.

The ulnar nerve slants backwards. It runs down just medial to the triceps tendon, and behind the medial epicondyle. It turns a sharp corner round the underside of the medial epicondyle, where there’s a fibrous tunnel for it. It passes between the two heads of flexor carpi ulnaris to enter the forearm.

Once they get below the elbow, the median and ulnar nerves get busy. Between them they supply all the flexor and pronator muscles of the forearm. Of the muscles that we’ve seen already, the median nerve supplies four, pronator teres, flexor carpi radialis, palmaris longus, and pronator quadratus. The ulnar nerve supplies one muscle that we’ve seen so far, flexor carpi ulnaris.

Lastly, let’s look at the radial nerve. It has a long spiral course, from here, round to here. Up here, the radial nerve lies behind all the other nerves and vessels. Just below the latissimus tendon it runs back between the long head and the medial head of triceps.

To follow its course, we need to go right round to the back, and find the same spot again from behind. Here’s the long head of triceps, here’s the medial head, and here’s the radial nerve. To see where it goes next, we’ll remove the long head of triceps.

As the radial nerve passes round the humerus, it lies right on the bone. It runs between the medial and lateral heads of triceps, then runs beneath the lateral head, to emerge here, still right on the bone, just above brachioradialis.

Under cover of brachioradialis it reaches the lateral epicondyle, where it divides into a deep, or motor branch and a superficial, or sensory branch. That’s as far as we’ll follow the radial nerve for now. Of the muscles that we’ve seen, the radial nerve supplies the triceps, anconeus, brachioradialis, all three wrist extensors, and supinator.