During the dissection the forearm should be pronated and supinated as occasion arises. The muscle will be best displayed with the forearm strongly pronated and resting upon its inner border.
1. Transversely from the upper border of the Olecranon process of the Ulna outwards over the external Condyle of the Humerus to the centre of the bend of the elbow in front.
2. Transversely from the posterior border of the Ulna at its centre round the outer border of the forearm to the median line in front.
3. Join the anterior extremities of the preceding by a longitudinal incision along the median line. Reflect the flap backwards and outwards, and expose the superficial fascia containing—
1. The Median vein, its deep communicating branch with the venæ comites of the Brachial artery at its point of bifurcation, and the lower part of the Median-cephalic branch. The Radial vein joining the latter.
2. Cutaneous offsets of the Radial and Interosseous Recurrent arteries in front of and behind the external Condyle respectively. A transverse branch of the latter is found behind the Olecranon, and cutaneous twigs of the Superior Profunda artery behind the external Condyle.
3. A few of the lowest twigs of the superior External Cutaneous branch of the Musculo-spiral nerve in front of the elbow, and the inferior External Cutaneous branch of the same nerve behind the elbow and the forearm.
4. The External Cutaneous nerve beneath the Median-Cephalic vein running down the front of the forearm, giving off its posterior branch at the lower part of the dissection.
5. Part of the Olecranon process, and the external Condyle. The former covered by its bursa.
Remove the preceding cutaneous structures and expose the fascia lata. Clean this away, and the following structures will be displayed, commencing from the front and passing backwards—
1. The outer part of the insertion of the Brachialis anticus muscle.
2. The tendon of the Biceps muscle running down to the Tuberosity of the Radius.
3. The lower part of the Pronator radii teres muscle in its whole width below, but only its outer border above.
4. The Supinator longus muscle descending from the arm, on the outer side of the forearm. This should be hooked outwards.
5. A small portion of the Supinator brevis muscle deep down between the two immediately preceding muscles.
6. The Extensor carpi radialis longior muscle outside the Supinator longus muscle.
7. The common tendon and upper fleshy portions of the following muscles in their order from without inwards—viz., Extensor carpi radialis brevior, Extensor, communis digitorum, Extensor minimi digiti, Extensor carpi ulnaris, and a portion of the origin of the latter from the posterior border of the Ulna.
8. The Anconeus muscle running from the outer Condyle to the outer side of the Olecranon.
9. In front, internal to the Biceps tendon, the lower part of the Brachial artery and its division into Radial and Ulnar branches. The Radial Be current branch passing up beneath the Supinator longus muscle. These arteries are accompanied by their veins.
10. The anterior terminal branch of the Superior Profunda artery above, beneath the Supinator longus muscle, anastomosing with the Radial Recurrent.
11. The Median nerve internal to the Brachial artery, and its branch to the Pronator radii teres muscle.
12. Beneath the Supinator longus muscle, but seen by pulling it outwards, the Musculo-spiral nerve dividing opposite the external Condyle into Radial and Posterior Interosseous branches.
Divide now— a. The Supinator longus and Extensor carpi radialis longior muscles, and throw the ends upwards and downwards.
b. The common Extensor tendon, and pull the fleshy bellies of the muscles taking origin from it downwards. Their nerves from the Posterior Interosseous will be cut.
c. The Pronator teres muscle, and hook the ends upwards and downwards. The upper portion will carry with it the Median nerve. There will now be exposed—
1. The upper and outer portion of the Flexor profundus digitorum muscle.
2. The Radial artery, its Recurrent branch, and the anterior branch of the Superior Profunda artery more fully.
3. Posteriorly, the upper part of the Posterior Interosseous artery at its point of emergence below the Supinator brevis muscle. The Recurrent branch will be seen passing up upon this muscle and beneath the Anconeus. Sometimes this offset pierces the Supinator brevis.
4. The Posterior Interosseous nerve, emerging from the Supinator brevis is seen, giving branches to the Extensor muscles, which however are divided now, and should have been traced when these muscles were reflected.
a. Remove now the Radial vessels and nerve, the Interosseous Recurrent artery, and the Posterior Interosseous nerve.
ft. Pull inwards or remove some of the outer fibres of the Anconeus muscle which slightly overlap the Supinator brevis muscle.
c. Take away that portion of the Flexor profundus digitorum muscle which runs up to the Coronoid process externally to the Brachialis anticus insertion.
The Supinator brevis muscle will now be fully exposed.