Page:Popular Science Monthly Volume 42.djvu/784

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the musculo-cutaneous nerve from the outer cord. In the movements of the wrist and fingers the fifth and sixth nerves appear to co-operate, and those of the fingers are chiefly due to the sixth. The supinators and extensors of the wrist, fingers, and thumb get their nerves from the musculo-spiral or its interosseous branch. To resume, the fifth and sixth cervical nerves raise the shoulder, flex the forearm, and extend the wrist. The nervous energy passes from them along the upper trunk and outer cord of the brachial plexus to the flexors of the forearm, while the impulses to raise the shoulder, rotate the humerus, and extend the wrist and fingers travel chiefly through the posterior cord by the musculo-spiral nerve and its interosseous branch to the extensors of the wrist and digits. From the fifth and sixth cervical nerves we make a jump to the first dorsal, which has an exactly opposite action. The movements it produces are that the hand closes firmly upon the apple, the wrist is twisted round into the prone position and flexed to the ulnar side. The forearm is extended, and the upper arm is retracted in the manner required to pull the apple from the tree. In these movements, if you put your hand upon your chest, you will find that the pectoral muscles are largely engaged, and they receive their nerves partly from the internal cord of the brachial plexus. Flexion to the ulnar side is produced by the ulnar nerve, and the dragging of the arm down is effected by the subscapular, teres major, and latissimus dorsi muscles, which are supplied by the subscapular nerves, and the triceps by which the arm is extended gets its nerve supply from the musculo-spiral. But these movements, especially if executed forcibly, as they would be if the apple were firmly attached, would bring the hand below the level of the mouth, and the prone position would keep the apple away.

We must now go back to the sixth cervical nerve, which we find will rectify this action, for it raises the arm inward and upward with the forearm flexed, so as to bring the hand to the mouth, supinated, and with the wrist and basal phalanges extended, so as to present the apple comfortably for eating. In effecting this movement the nervous impulses travel by the posterior thoracic, circumflex, musculo-cutaneous, musculo-spiral, and median nerves to the serratus magnus, deltoid, biceps, brachialis anticus, supinator longus, and extensors of the wrist and basal phalanges. The position of Adam's left hand in Raphael's picture shows this action in its middle stage, before it has carried the hand to the mouth. The few last phalanges of his fingers are flexed, and we may suppose that the flexion is effected by means of the median nerve, but it is just possible that their flexion may be due to mechanical pulling on the tendons by the extension of the wrist and basal phalanges just as the hand is opened in the well-known