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Muscles of the Upper Extremity

fingers, arm, finger, fig, hand, outer, shoulder, radius and front

MUSCLES OF THE UPPER EXTREMITY.

The muscles that attach the upper extremity to the trunk in front and behind have already been sufficiently mentioned.

Muscles of the Shoulder. chief muscle of the shoulder is the deltoid (Pl. X., fig. 1, a). It springs in front from the outer portion of the collar-bone, from the tip of the shoulder—the acromion process of the shoulder blade,—as well as from the spine of the shoulder blade, thus overlapping the most prominent part of the shoulder. From this it converges downwards over a third of the upper arm, till it is inserted into a rough surface on the outer side, and about the middle of the upper arm bone. By its contraction the arm is raised from the side till it is at right angles with the trunk. For further elevation the action of the trapezius is necessary. Several other muscles pass to the shoulder from the upper and under surfaces of the shoulder-blade.

Muscles of the Arm.—They are classed to gether as either flexors, that is muscles that bend the arm through the medium of the elbow joint, or extensors, muscles that straighten the elbow-joint. The main flexor is the biceps (Fig. 71, a), so called because it has two heads, one from the coracoid process of the shoulder blade, the other from the upper edge of the socket in the shoulder-blade for the reception of the head of the arm-bone. Its tendon passes over the elbow-joint, and is inserted into a rough prominence near the head of the radius —the outer of the forearm bones. When this muscle contracts, the forearm is bent up on the upper arm, and if anything be held in the hand, heavy enough to offer resistance to this move ment, then the muscle is seen standing out in front of the upper arm. (See P1. I.) The chief opponent of the biceps is the triceps (Fig. 71, b), a muscle which arises by three heads, one of them from the shoulder-blade, the others from the arm-bone itself, whose tendon is inserted behind into the tip of the elbow, and whose action is to straighten or extend the elbow joint.

Muscles of the muscles of the forearm are divided into four classes: I. Flexors, those which bend the hand at the wrist-joint and also bend the fingers, and II. their opponents the extensors, which straighten the hand and fingers, III. the pronators, which turn the radius (p. 62) and with it the hand, which it carries, palm downwards, and IV. their opponents, the supinators, which rotate the radius back again, and so turn the palm up wards.

I. All the flexors spring from the inner pro jection of the lower end of the upper arm-bone, and pass down the front of the arm. One set is devoted to bending the wrist-joint, one on the outer the other on the inner side, another set to bending the fingers. Of those that bend the fingers there is a superficial and a deep set, the superficial having four tendons, one for each finger, which are inserted into the second pha lanx of each of the four fingers. The deep set

has also four tendons which pass tip the front of the fingers, pierce the superficial tendons, and pass on to be inserted in the base of the last phalanx of each finger. So that the tendons of the deep set bend the fingers at the first joint, those of the superficial set bend them at the second joint. The thumb has a separate flexor muscle. Fig. 72 (b) shows these arrangements.

II. On the back of the forearm and arising from the outer prominence or condyle of the upper arm-bone are the extensor muscles. There are extensors of the wrist and extensors of the fingers. The extensors of the wrist are three in number, two on the outer and one on the inner side. There is an extensor common to the four fingers having four tendons, one to each finger, which are attached to the first and second phalanges, and end upon the las/ phalanx, of each finger. The tendon of the ring finger is attached on one side to that of the little finger, and on the other to that of the middle finger, so that if the middle and little fingers are bent, the ring finger cannot be straightened, being held down by theni. Fig. 72 (a). The thumb has three extensor muscles of its own, one attached to the base of its meta carpal bone, one to the first, and the other to the second phalanx. The first and second fingers have also special extensors of their own.

The tendons of the flexor and extensor muscles are bound down as they pass over the wrist into the hand by a strong band of fibres called the annular ligament, because it surrounds the wrist like a ring (see Fig. 72, 1).

III. The chief pronator muscle is called pronator radii teres, and is in front of the arm and at the upper part (Pl. X., fig. 2, 7). It rises from the inner prominence of the arm bone, and passes obliquely to be attached to the outer side of the radius about its middle. When it contracts it causes the radius to roll over the ulna, and as it carries the hand the palm is turned down.

IV. The supinator muscles are on the back of the forearm; and are so attached to the radius as to turn it back to its former position, making the hand turn palm upwards, Muscles of the lie in the spaces between the bones of the palm called interossei muscles. The thumb has a series of muscles, one for bending it, another for draw ing it from the middle line, a third for ap proaching it to the middle line, and still another for opposing it to the other fingers. Similarly the little finger has its special flexor, its abduc tor and adductor muscles, as well as one for opposing it to the thumb.