This muscle is the main agent in effecting the supination of the hand.
2. Extensor ossis metacarpi polaris, (ab ductor limps polaris moots, Scemm. cubito radi sus-metaeorpien, Dumas,) arises from the ulna, interosseous ligament, and the back part of the radius, opposite the insertion of the pronator radii teres, having to its outer side the supi nator radii brevis, to its inner the extensor primi internodii pollicis. It is covered posteriorly by the extensor communis digitorum and extensor carpi ulnaris. Gliding downwards and out wards from beneath these muscles and becoming tendinous on its under surface, it slips over the lower third of the posterior face of the radius, and then running in a groove on the outer side of that bone, common to it and the next men tioned muscle, is ultimately inserted into the metacarpal bone of the thumb.
The action of this muscle is to extend the metacarpal bone of the thumb, which corre sponds as regards its capacity for motion, with the phalanges of the fingers.
3. .Erteitsor primi internodii (extensor minor pollicis manus, Scemm.; cubito sus phalangettien du police, Chauss.) is a very small muscle compared with the last, though varying much in size in different subjects. It lies between the extensor ossis metacarpi pollicis and the extensor secundi internodii, passing through the same groove in the radius as the extensor ossis metacarpi pollicis, it becomes inserted into the first phalanx of the thumb.
4. The extensor secundi internodii pollicis (extensor major pollicis manus,Scemm.; cubito sus-phalangettien du pouce, Chauss.) —This muscle is usually larger than the former ; it arises fleshy from the ulna and interosseous ligament ; becoming tendinous in its centre, it presents the same penniform appearance referred to above. The groove in the radius which is narrow and deep, this tendon alone being lodged in it. It is situated between that for the two radial extensors and the broad and hollow one for the common extensors and indicator. Crossing on the back of the wrist tire radial extensors it is finally inserted into the second phalanx of the thumb. This mus cle is entirely covered in the fore-arm by the common extensors of the fingers. • 5. The indicator (cubito sus-phalangettien de l'index ) occupies the remaining portion of the posterior interosseal space. Like the three last described muscles It is penniform, and arises fleshy from the ulna and interosseous ligament, it descends to the hand and passes through the same groove at the back of the radius as that of the extensor communis digi torum, it is inserted into the posterior surface of the three phalanges of the index finger. This muscle is entirely concealed by the exten sor carpi ulnaris and extensor communis digi torum.
The names of this and the two preceding muscles indicate their actions.