Fig. 71.
Fig. 71.—Superficial muscles of shoulder and arm (from behind): 1, Trapezius; 2, deltoid; 3, rhomboideus major; 4, infraspinatus; 5, teres minor; 6, teres major; 7, latissimus dorsi; 8, triceps; 9, anconeus; 10, brachialis anticus; 11, supinator longus; 12, extensor carpi radialis longior; 13, extensor carpi radialis brevior; 14, extensor communis digitorum; 15, extensor carpi ulnaris; 16, flexor carpi ulnaris; 17, extensor ossis metacarpi pollicis; 18, extensor brevis pollicis; 19, tendon of extensor longus pollicis. (Dorland’s Dictionary.)
Upper Arm Muscles.—The [biceps] is the most important arm muscle. It rises by a short head from the coracoid process of the scapula and by a long head from a tubercle on the upper margin of the glenoid cavity, the tendon arching over the head of the humerus and descending in the bicipital groove. It is inserted into the back of the tuberosity of the radius and by a broad aponeurosis into the fascia of the forearm. It flexes and supinates the forearm and renders the fascia tense. Its inner border forms a guide in tying the brachial artery, as this artery runs along its inner side.
The [brachialis anticus] rises from the lower half of the outer and inner surfaces of the humerus and is inserted into the coronoid process of the ulna, thus covering and projecting the elbow-joint anteriorly. It is a flexor of the forearm.
Another smaller muscle on the anterior arm, which also aids in flexion, is the coraco-brachialis, which extends from the coracoid process of the scapula to the middle of the inner surface of the humerus.
Extending the entire length of the posterior surface of the humerus is the [triceps], similar to the quadriceps extensor in the thigh and direct antagonist to the biceps and brachialis anticus muscles. It rises by a long head from below the glenoid fossa, by the external head from the upper third of the posterior surface of the humerus, and by the internal head from the middle and lower thirds of the posterior surface. It is inserted in the olecranon process of the ulna and serves to extend the forearm and arm.
The humerus is more often fractured by muscular action than any other bone. Usually the fracture occurs in the lower half of the bone and sometimes the musculo-spiral nerve is involved. There is a great tendency to non-union, probably due to interposition of soft parts. Sometimes the break is across and down between the condyles, T-fracture. Involvement of the elbow-joint is more serious than fracture of the humerus alone. Sarcoma of the humerus does occur and may require the removal of the clavicle and scapula as well as of the arm bone itself. In amputation of the humerus in children a long skin flap is left to allow for growth of the bone, as it is liable to grow again.
The Ulna.—In the forearm there are two bones, the ulna and the radius, of which the former is the longer. [The ulna] is on the inner side of the forearm and its upper end forms the greater part of the articulation with the humerus, as most of the articulation at the wrist is formed by the radius and the inter-articular fibro-cartilage. The head of the ulna is at the lower extremity of the bone and articulates on the outer side with the radius and below with the triangular fibro-cartilage. From its inner side projects the styloid process. The olecranon process forms the upper extremity and presents anteriorly an articular surface, the greater sigmoid cavity, for articulation with the trochlea of the humerus, where it fits into the olecranon fossa during extension. The same articulating surface also covers the coronoid process, a smaller projection below and in front of the olecranon, which fits into the coronoid fossa during flexion. Continuous with the greater sigmoid cavity on the outer side is the lesser sigmoid cavity for articulation with the head of the radius. Under the triceps tendon, which is inserted into the olecranon, is a bursa or sac of synovial membrane, such as occurs in parts where much force is brought to bear.
The Radius.—[The radius], or spoke of the wheel, is on the outer side of the forearm and gets its name from the way it turns upon the ulna in pronation. The shaft is larger below than above and is slightly curved longitudinally for greater strength. The upper extremity or head is small and has a slightly concave upper surface for articulation with the radial head of the humerus. It articulates by its sides with the lesser sigmoid cavity and is bound to the ulna by the orbicular ligament, which runs over a smooth articular surface. Below the head is the constriction of the neck with the [tuberosity] for the biceps tendon to the inner side below. The lower extremity is large and forms the chief part of the wrist-joint, articulating with the semilunar and scaphoid bones of the wrist. From the lower extremity the strong conical styloid process projects externally.