J. he muscles of the forearm move the wrist, the hand and fingers. They are muscular above and tendinous below. These tendons are strapped down to pass over and under the wrist and fingers. There is a great variety of formation and shape to the muscles of the forearm. There are muscles with tendons that are single and again double as they pass to the wrist and hand. The muscles act separately or in groups with quickness and precision as the occasion requires.
1 The front and inner side of the forearm is composed of muscles that arise from the internal condyle of the humerus by common tendons and terminate below by tendons that are two-thirds the length of the muscle. These tendons separate to be inserted into the wrist and fingers and are known as flexors.
2 The muscles of the back and outer side of the forearm as a group arise from the external condyle and adjacent ridge of the humerus. As a mass they are on a higher level than those of the inner side of the forearm. As to these muscles in general: they pass down the back of the forearm and divide into tendons as they approach the wrist where they are held in place by a band called the annular wrist ligament.
3 When the arm is bent to a right angle and the hand is directed toward the shoulder, the flexor muscles are set in motion by contraction. They swell to their muscular centers and their tendons pull the hand downward. When the hand is bent at the wrist in the direction toward the front of the forearm, it is flexion. The reverse is called extension.
4 The extension of the hand on the forearm shows the muscles and the tendons lying on the outer side and back of the forearm. These are held in place by the annular ligament. The rounded forearm is made up of the fleshy bodies of muscle that terminate mostly in long tendons that pass to and over the wrist and hand. Some of these muscles move the hand on the forearm or the different finger joints on each other. There are also deep muscles of the forearm from which the tendons emerge though the muscles are hidden.
I. bones: (1) The upper arm bone, the humerus, is the longest bone of the upper limb. (2) At the lower end of the humerus there are two projections. The inner projection (the inner condyle) is quite prominent, always in evidence, and is used as a point of measurement. (3) The ulna hinges at the elbow, and articulates with the bone above by a beak-like process. It descends toward the little finger side of the hand, where it is seen as a knob-like eminence at the wrist. (4) The radius carries the thumb side of the wrist and hand at its lower extremity. At the upper end, the head is hollowed out to play freely on the radial head of the humerus.
11. muscles: (1) The pronator teres. From its origin on the internal condyle of the humerus, it is directed downward and outward and inserted into the outside of the radius about half way down the shaft. In contraction it turns the forearm and thumb side of the hand inward causing pronation. (2) There are four flexor muscles that arise from the internal condyle of the humerus. Their bodies are mostly fleshy, terminating at their lower half in long tendons. (3) The palmaris longus, also a flexor, shows a long slender tendon directed toward the middle of the wrist. It is inserted into the palmaris facia that stretches across the palm of the hand. (4) Flexor carpi ulnaris.
III. Muscles must lie above and below the joint they move. Muscles that bulge the forearm in front are flexors. They terminate as wires or strings that pull the wrist, hand and fingers together as they contract.
IV. The inner condyle of the humerus is a landmark when the forearm is seen from the front and the bones are parallel. In this position, the muscles and their tendons are directed downward to the wrist and hand.
The first, the pronator teres, passes obliquely to the middle of the radius. The second, the flexor carpi, radiates toward the outer side of the hand. The third, the palmaris longus, is toward the middle. The fourth, the flexor carpi ulnaris, is toward the inner border of the hand. The muscles just named are situated on the front and inner side of the forearm and all arise from the inner condyle of the humerus.
I. bones: (1) The humerus of the arm presents a shaft and two extremities. (2) Olecranon process of the ulna, elbow. (3) The ulna, from the elbow to the little finger side of wrist. (4) Radius, the thumb side of the forearm at the wrist. (5) The styloid process of the radius.
II. muscles: (1) The supinator longus arises from the outer border of the humerus about a third of the way up its shaft. It then enlarges as it descends to its greatest size at about the level of the external condyle. Below, its fibres are replaced by a long tendon that is inserted into the styloid process of the radius. (2) On the humerus, just below the supinator, arises the long extensor of the wrist. This muscle descends by a slender tendon to the index.finger and is named the extensor carpi radialis longus. (3) Anconeus, a small triangular muscle attached to the external condyle of the humerus and inserted into the ulna just below the elbow. (4) There are four extensors including the long extensor of the wrist just mentioned. Three of these arise from the external condyle of the humerus, descend as muscles about half way down and end as tendons that extend the wrist, the hand and the fingers. The fourth arises from the shaft of the humerus just above the external condyle. (5) Extensors of the thumb.
III. The muscles of the forearm are placed just below the elbow, moving the hand, the wrist and fingers by long slim tendons that are securely strapped down as they pass under or over the wrist. It is a fixed law that a muscle contracts toward its center. Its quickness and precision of movement depends upon its length and bulk. If the muscles of the forearm had been placed lower down, the beauty of the arm would have been destroyed.
IV. The muscles that lies on the outer side and back of the forearm are known as the supinator and the extensor group. They emerge from between the biceps and the triceps at about a third of the distance up the arm as a fleshy mass. These wedge-shaped muscles are placed on a higher level than the pronator or flexor group, as they arise some distance above the outer condyle of the humerus. The extensor group take their origin from the condyle below. The extensor tendons are on the back of the arm and always point to the outer condyle of the humerus. The extensor muscles are the direct antagonists of the pronators and flexors in front. The chief action of the supinator longus is that of a flexor but acts as in supination as well.
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