This muscle is a simple extensor of the fore-arm.
2. •rtensor carpi ulnaris, (ulnnris ester nus, Scemm.; cubit° sus-melacarpien, Chauss.) arises from the back part of the outer con dyle between the anconeus and the extensor communis digitorum, with which latter muscle it is so intimately connected that, more strictly speaking, it ought to be said to arise in a common tendon. Connected by a narrow origin to the humerus it gradually expands, and about the middle of the fore-arm, a tendon being formed in the centre, it exhibits in its further course a well-marked specimen of the double penniform muscle. The tendon of this muscle, in its passage towards the wrist-joint, runs in an especial groove appropriated for its reception in the back part of the ulna; it ter minates by being inserted in the metacarpal bone supporting the little finger. The extensor carpi ulnaris is more or less connected with the fascia throughout the whole of the upper arm.
This muscle extends the first row of carpet bones on the second and the whole hand on the forearm; it is likewise an adductor of the hand.
3. Extensor communis digitorum (epicondylo sus-pholongettien common, Chauss. Dumas) arises from the back part of the outer condyle in common with the extensor carpi ulnaris on its outer side, and the extensor carpi radialis brevior on its inner side. The connexion of this muscle to the os humeri is extremely narrow in comparison with the width of the muscle in the centre of the fore-arm. In its ample attachment to the fascia it resembles the flexor ulnaris, and, like it, is a penniform muscle. We not unfrequenly find a portion of this muscle so entirely distinct from the rest that anatomists have occasionally described it as a separate muscle, under the name of the extensor proprius minimi digiti ; for being inserted into the little finger, it possesses the power of extending that portion of the hand. It passes behind the posterior annular ligament of the wrist-joint, splits into four tendons, which, expanding on the back part of the phalanges of the four fingers, convey the power of the muscle to each phalanx in an equal degree. The tendons of this muscle in their passage behind the annular ligament of the wrist-joint are clothed by a synovial membrane (reflected like all other synovial membranes) so as to form a perfect' purse or bursa. Both these muscles are intimately connected upon their under surface at the upper part of the arm, with the aponeurosis covering the supi nator radii brevis.
This muscle is an extensor of the fingers and the hand on the fore-arm.
4. Ertensor carpi rodialis brevior, (radialis externus brevior, Scemm. Epicondylo sue metacarpien, Chauss. Dumas), with a small portion of the extensor carpi radialis longior, occupies the radial division of the poste rior superficial antibrachial region. This muscle arises from the outer condyle by a flattened narrow origin, in common with the extensor communisdigitorum, being overlapped on its outer side by the extensor carpi radialis longior. This muscle, like most we have described in the fore-arm, swells out towards the centre, where, gradually becoming tendi nous, it again diminishes in size. It passes the same groove in the radius as the extensor carpi radialis longior, and terminates by an insertion into the metacarpal bone of the middle finger. The under surface of this muscle is tendinous at the upper part of the arm, which permits it to play without friction upon the smooth and tendinous face of the supinator radii brevis with which it is in contact.
This muscle acts as an extensor of the band on the fore-arm and an abductor.
b. Deep marks of the posterior antibroehial muscles in this region com mencing above, are the supinolor rodii brevis, the extensor nssis metnearpi polaris, the exten sor priori intertioda pollicis, the extensor secundi internodii, and the indientor.
1. Supinalor radii brevis,(epieundylo-radinl, Chauss.) arises tendinous from that portion of the outer and back part of the ulna, unoccupied by the insertion of the anconcus; it arises also from the back part of the outer condyle, covered at its origin from the outer condyle by the extensor communis digitorum and by the exten sor carpi radialis brevior. Its posterior and external surface is tendinous, its intenial fleshy, and it embraces so much of the upper extremity of the radius, as to form an imperfect tube. Anteriorly we find it partly overlapping the tubercle of that bone, with the tendon of the biceps which is inserted into it. Between these and the under surface of the muscle is a large and distinct bursa mucosa ; it covers rather more than the upper third of the radius by its insertion, extending as low down as the pro nator radii teres.