To flex the fingers on the hand, commencing with the flexion of the last phalanx on the others, and the whole hand on the fore-arm, constitutes the principal action of this muscle.
3. Pronator quadratus, Scemm. ( Cubito radial, Chauss.) This muscle, entirely covered by those mentioned above, presents a beauti ful appearance on their removal, from the ten dinous surface admitting by its transparency the colour of the muscle to shine, as it were, through it.
It arises from the ulna about an inch and a half above the wrist-joint, occupying exactly that extent of the surface of the bone with its attachments : it is inserted fleshy into the lower part of the radius.
This muscle, simple as its action appears, that of rolling the radius over the ulna, per forms a most important part in those easy mo tions of the hand which the artist uncon sciously produces when he is engaged sketching in bold and flowing lines the subject of his picture.
To the surgeon a knowledge of the attach ments of this muscle is peculiarly important, for in those cases in which the radius is frac tured near its lower extremity it draws the injured bone into the field of the flexor tendons, and by bringing it into close contact with the ulna, produces a deformity which great care will alone obviate.
Posterior antibrachial region.—If we now look to the posterior part of the fore-arm, we shall find that though it may be divided into radial and ulnar sections like the anterior, the propor tions between them will be very different; for one fifth of the transverse diameter of the arm alone can be correctly allotted to the radial region in the upper part, and two-fifths close to the wrist joint. The line of demarcation between these two regions is accurately formed in the dissected arm by the radial edge of the extensor com munis digitorum. This muscle, like those on the anterior surface of the arm, is wide and muscular above, tendinous and comparatively narrow below; and hence we find the radial section wider below than it is above. In the ulnar section, we have the extensor communis digitorum to the outer side ; in contact with this muscle, on its ulnar side, is the extensor carpi ulnaris. This muscle, at its origin at
the upper part of the arm, is narrow, and the space, thus yielded as it were by its form, is occupied by the anconeus, which forms the boundary of this region on the ulnar side. The space left at the lower part of the arm, from the divergence of the tendons of the extensor carpi ulnaris and extensor communis digitorum, permits a view of the indicator. The radial section contains at its upper part solely the extensor carpi radialis brevior ; but at the upper part of the middle of the arm, we have sliding into it from behind the extensor communis digitorum, the extensor ossis meta carpi pollicis, and extensor primi internodii pollicis. These pursue their course obliquely across the radial section till they reach the outer edge of the arm. Lower down than these muscles and scarcely in contact with their inferior edges, we discover the tendon of the extensor secundi internodii pollicis likewise emerging from beneath the extensor communis digitorum.
a. Superficial muscles of the posterior anti brachial region. —1. Anconeus (epicondylo cubital, Chauss.) though usually described as a distinct muscle, is, in reality, a continuation of the triceps extensor cubiti: the fibres of each are perfectly continuous, and there is no line of demarcation between them. An artificial boundary may be made by drawing a line horizontally inwards when the fore-arm is ex tended on the upper arm, between the outer condyle of the os humeri and the olecranon process of the ulna. With this view of the limit of the upper edge of the anconeus, it may be described as a triangular muscle, the base above and the apex below. This muscle arises ten dinous from the back part of the outer condyle, its external and anterior edge continuing ten dinous almost to its insertion ; its superior fleshy fibres pass transversely inwards and backwards, to be inserted into the fascia of the fore-arm and also into the olecranon ; the middle and inferior fibres pass backwards to the ulna with various degrees of obliquity, and occupy by their insertion about one-third of the bone from its superior extremity.