Divide the fleshy belly of the Flexor sublimis digitorum muscle by a longitudinal incision along the middle line, and hook the halves inwards and outwards. The Median nerve has already been cut, but branches from it to this muscle and arterial twigs from the Ulnar will be divided. There will now be exposed—
1. The Flexor profundus digitorum muscle, lying upon the Ulna and the corresponding half of the Interosseous membrane.
2. The Flexor longus pollicis muscle, concealing the anterior surface of the Radius and the outer part of the Interosseous membrane. Its occasional head from the inner part of the Coronoid process will be seen if present.
3. A further portion of the Ulnar artery is now seen lying upon the Brachialis anticus and Flexor profundus digitorum muscles, and the following branches: the Anterior and Posterior Ulnar Recurrent branches, the former being cut, the Common Interosseous trunk giving off Anterior and Posterior Interosseous branches, the former seen passing between the Flexor profundus and longus pollicis muscles, and the latter going to the back of the forearm.
4. All these arteries have corresponding veins.
5. The Median nerve lying upon the Flexor profundus muscle, accompanied by the C. Comes nervi Mediani artery at the lower part.
Separate now the Flexor longus pollicis muscle from the Flexor profundus digitorum muscle, dissect off their origins from the Interosseous membrane, and pull the former well outwards and the latter inwards. The occasional head of origin of the Flexor longus pollicis must be cut. through if present.
There will now be exposed—
1. The Interosseous membrane.
2. The Pronator quadratus muscle in the greater part of its extent.
3. The Anterior Interosseous artery lying upon the membrane, and disappearing below beneath the Pronator quadratus muscle. Its twigs to the muscles between which it lies, the origin of the Comes nervi Mediani artery, and the nutrient arteries to the Radius and Ulna may now be traced from it. The Posterior Interosseous artery may be traced above, passing to the back of the forearm between the Oblique ligament and the upper margin of the Interosseous membrane.
4. The Anterior Interosseous nerve is found with the artery of the same name, and its several branches to the Flexor profundus and longus pollicis muscles are traceable, if they have not been torn through in separating these muscles.
To expose the artery beneath the Pronator quadratus muscle, one of two courses may be taken, viz., A or B.
(A) a. Separate the tendons of the Flexor profundus muscle well from that of the Flexor longus pollicis muscle as far as the upper border of the Anterior Annular ligament, and draw them by hooks on either side.
b. Divide the Pronator quadratus muscle longitudinally, and reflect the halves inwards and outwards.
In this way there will be exposed—
1. The anterior surface of the wrist-joint and its ligaments.
2. The Anterior Carpal arch, derived from the Radial and Ulnar arteries.
3. The Anterior Interosseous artery passing to the back of the forearm beneath the Interosseous membrane, but sending down a branch to anastomose with the Anterior Carpal arch. (B) a. Divide the tendons of the superficial and deep Flexor muscles of the fingers at the Annular ligament, dissect them up for some distance and hook them inwards.
The same parts enumerated under A will be exposed, but more completely. The Anterior Interosseous artery is now fully exposed except at its ending. To see this, and to trace the Posterior artery, the forearm should now be supinated. The structures exposed by the removal of the fascia lata have been enumerated under Section II. Behind:
a. Remove the Posterior Annular ligament.
b. Separate the Extensor communis digitorum muscle from the Extensor carpi radialis brevior muscle as high as the external Condyle, cut through its Condyloid origin and reflect the muscle downwards as far as the bases of the Metacarpal bones.
c. Cut through the origin of the Extensor minimi digiti, and reflect it down to the same extent.
d. Divide the origin of the Extensor carpi ulnaris from the external Condyle and the deep fascia as low down as its attachment to the posterior margin of the Ulna, and hook the muscle inwards. Branches of the Posterior Interosseous nerve will be cut through in the division of these muscles, together with twigs from the artery of the same name.
e. Remove the Anconeus muscle, taking care of the artery which will be found beneath it. There will now be exposed—
1. The posterior surface of the Supinator brevis muscle, highest of all.
2. Next below, the Extensor ossis metacarpi pollicis muscle, arising from the Radius and Ulna.
3. Next, the Extensor primi internodii pollicis muscle, arising from the Radius.
4. Still lower, the Extensor secundi internodii pollicis muscle, arising from the Ulna.
5. Lowest of all, the Extensor indicis muscle, from the Ulna.
6. The Posterior Interosseous artery, passing from the front, between the Supinator brevis and Extensor ossis metacarpi pollicis muscles, and running down the forearm along the outer side of the tendon of the Extensor carpi ulnaris, lying upon the other Extensors of the thumb, and that of the index finger. The Recurrent branch uncovered by the removal of the Anconeus muscle is seen passing upwards, lying first upon the Supinator brevis muscle and then upon the Ulna, anastomosing above with the posterior branch of the Superior Profunda artery, and communicating across the Olecranon with the Posterior Ulnar Recurrent artery. The corresponding veins are seen. 7. The Posterior Interosseous nerve is displayed, coming through the Supinator brevis muscle, dividing into branches for the Extensor muscles, and disappearing below beneath the Extensor secundi internodii pollicis.
Divide now the Extensor secundi internodii pollicis and Extensor indicis muscles. Remove the fleshy portions and pull the tendons outwards. This will expose—
1. The termination of the Anterior Interosseous artery.
2. The Posterior Carpal arch.
3. The communication between the two preceding, and the branch from the Posterior Interosseous artery, which joins the Anterior, and the Carpal arch.
4. The further portion of the Posterior Interosseous nerve, and its gangliform enlargement.
To see the origin of the Posterior Interosseous artery from the common trunk, remove, if necessary, a portion of the lower margin of the Supinator brevis muscle.