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The position of the upper limb is typical: the arm and forearm hang close to the side, with the forearm extended and pronated; the deltoid, spinati, biceps, brachialis, and supinators are paralysed, and in some cases the radial extensors of the wrist and the pronator teres are also affected. The patient is unable to supinate the forearm or to abduct the arm, and in most cases to flex the forearm.
At the opposite end of the lever, in the, upturned palm of the hand, we shall place a weight of 1 lb. to represent the load to be moved. The power which we are to yoke to the lever is a strong muscular engine we have not mentioned before, called the brachialis anticus, or front brachial muscle. It lies in the upper arm, where it is fixed to the bone of that part the humerus.
A similar ossification has been observed in relation to the insertion of the brachialis muscle as a sequel of dislocation of the elbow. After reduction of the dislocation, the range of movement gradually diminishes and a hard swelling appears in front of the lower end of the humerus. The lump continues to increase in size and in three to four weeks the disability becomes complete.
a, Extensor carpi radialis; g, brachialis; g', anterior superficial pectoral; c, common digital extensor; e, ulnaris lateralis. The fascia and the ulnar head of the flexor carpi ulnaris have been removed. 1, Distal end of humerus; 2, median vessels and nerve. Motion is that of flexion and extension; slight rotation is possible when the position is that of flexion.
The musculo-cutaneous nerve is very rarely injured; when cut across, there is paralysis of the coraco-brachialis, biceps, and part of the brachialis, but no movements are abolished, the forearm being flexed, in the pronated position, by the brachio-radialis and long radial extensor of the wrist; in the supinated position, by that portion of the brachialis supplied by the radial nerve.
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