The Ulnar Nerve

The ulnar nerve passes downward in the groove on the back of the medial (internal) condyle and between the condyle and olecranon process. It passes between the two heads of the flexor carpi ulnaris muscle and is covered by it, lying on the flexor profundus digitorum; when half way down the forearm it becomes superficial and lies under or at the edge of the flexor carpi ulnaris muscle with the ulnar artery and flexor sublimis muscle to its outer or radial side. The ulnar artery joins the nerve just above the middle of the forearm. Just below the elbow the artery gives off the posterior ulnar recurrent branch, which passes up with the nerve behind the medial condyle. From the middle of the forearm to the wrist the ulnar nerve lies behind and to the ulnar side of the artery.

Branches

It gives muscular branches in the upper third of the forearm to the flexor carpi ulnaris and ulnar half of the flexor profundus digitorum muscles. It gives small articular branches to both the elbow-joint and wrist-joint.

It also gives off anterior and posterior cutaneous branches. The anterior, one or two, come off about the middle of the forearm; one supplies the anterior surface of the ulnar side of the forearm, while another, called the palmar cutaneous, runs down the front of the artery to be distributed to the palm.

The dorsal or posterior cutaneous branch is given off about 5 cm. (2 in.) above the wrist and passes downward and backward beneath the tendon of the flexor carpi ulnaris, across the interval between the pisiform bone and styloid process of the ulna, over the tendon of the extensor carpi ulnaris, and thence to the fingers.

Wounds

This nerve in the forearm is not infrequently wounded. It is especially liable to injury in resecting the elbow-joint. From what has been said of its course and branches it will be seen that in order for paralysis of any of the muscles of the forearm to be produced it must be injured high up in its upper third. Then the flexor carpi ulnaris and inner half of the flexor profundus digitorum will be paralyzed. If injured lower down the only muscular paralysis which will ensue is that of the short muscles of the hand which it supplies.

If the nerve is divided above the middle of the forearm the anterior cutaneous nerves will be involved. If divided between that point and 5 cm. above the wrist the anterior cutaneous escapes but the dorsal cutaneous branch is paralyzed. Below this latter point the dorsal cutaneous branch escapes and the muscular and sensory disturbances produced are on the palmar surface (except the dorsal interossei muscles).

Operations

In all operations on the nerve it should be remembered that its course is a straight line from the medial condyle to the radial edge of the pisiform bone. In the lower half of its course it lies along the outer (radial) edge of the flexor carpi ulnaris and this tendon will serve as a guide to it. It is here covered only by skin and superficial and deep fasciae, though it may be overlapped by either the artery or the edge of the tendon. If it is desired to reach the nerve in its upper half it can be followed either from above downward or from below upward, the fibres of the flexor carpi ulnaris muscle which cover it being split to the extent necessary for proper exposure. Below the middle of the forearm the ulnar artery lies to its radial side. Near the elbow the posterior ulnar recurrent artery accompanies it upward, but the nerve is far removed from the ulnar artery in this part of its course.

The Volar Interosseous Nerve and the Superficial and Deep Branches of the Radial (Musculospiral). - In addition to the large nerve-trunks of the median and ulnar the forearm contains the volar (anterior) interosseous, and the deep and superficial branches of the radial (musculospiral) nerve.

The volar {anterior) interosseous nerve leaves the median opposite to or below the bicipital tubercle of the radius; it lies on the interosseous membrane to the ulnar side of the accompanying volar interosseous artery. It supplies the outer half of the flexor profundus digitorum and the flexor longus pollicis muscles, between which it lies, and the pronator quadratus muscle. It is rarely wounded alone.

The deep and superficial branches are the continuation of the radial (musculo-spiral) which divides in the groove between the brachioradialis (supinator longus) and brachialis anticus muscles just above the elbow.

The deep branch (posterior interosseous) is the larger and is a muscular nerve; the superficial branch (radial) is smaller and is solely sensory. The deep branch passes down under the brachioradialis and extensor carpi radialis longior and brevior muscles and then enters the substance of the supinator (brevis) through which it passes to supply the extensor muscles on the back of the forearm and terminates in a gangliform enlargement on the back of the wrist. It supplies all the muscles on the back of the forearm except the anconeus, brachioradialis, and extensor carpi radialis longior, which are supplied directly from the radial (musculospinal) nerve. In removing the head of the radius, in resection of the elbow, the supinator (brevis) is to be carefully raised from the bone so as to carry the nerve with it and avoid injuring it. Injury to this nerve causes paralysis of the extensors, and wrist-drop follows.

The superficial branch {radial) is purely a nerve of sensation. It passes down almost in a straight line and lies to the outer side of the radial artery at the junction of its upper and middle thirds. It lies alongside of the artery to its outer side in its middle third and then, about 7 or 8 cm. (3 in.) above the wrist, quits the artery, passes beneath the tendon of the brachioradialis, and divides into two branches which supply sensation to the dorsal (radial) side of the hand and fingers (Fig. 374, p. 361).

In operating on the radial artery in the middle third of the forearm care should be exercised not to include the nerve in the ligature with the artery.