Etiology and Occurrence.--Nothing definite is known about the cause of some forms of radial paralysis. However, radial paralysis is encountered following injury to the nerve occasioned by its being stretched, as in cases where the triceps brachii is unduly extended in restraining subjects by means of a casting harness. Berns[10] states that in confining horses on an old operating table where it was necessary to draw the affected foot forward twenty-four to thirty-six inches in advance of its fellow, which was secured in a natural vertical position, radial paralysis of a mild form was of frequent occurrence. Country practitioners, in restraining colts by casting with harness or ropes, occasionally observe a form of paresis wherein the radial nerve suffers sufficient injury that there is caused a temporary loss of function of the triceps brachii. Such cases recover within three or four days and are not a true paralysis, but nevertheless constitute conditions wherein normal nerve function is temporarily suspended. [Illustration: Fig. 8--Radial paralysis.] Symptoms.--Immediately subsequent to injuries which involve the radial nerve, there is manifested more or less impairment of function. Remembering the structures supplied by the radial nerve and its branches, one can readily understand that there should occur as Cadiot[11] has stated: In complete paralysis, the joints of the affected limb with the exception of the shoulder are usually flexed when the horse is resting. In consequence of loss of power in the triceps and anterior brachial muscles, the arm is extended and straightened on the shoulder, the scapulohumeral angle is open, and the elbow depressed. The forearm is flexed on the arm by the contraction of the coracoradialis (biceps brachii), while the metacarpus and phalanges are bent by the action of the posterior antibrachial muscles. The knee is carried in advance, level with, or in front of, a vertical line dropped from the point of the shoulder. The hoof is usually rested on the toe, but when advanced beyond the above mentioned vertical line, it may be placed flat on the ground, the joints then being less markedly bent. When the limb as a whole is flexed, it may be brought into normal position by thrusting back the knee with sufficient force to counteract the action of the flexor muscles. [Illustration: Fig. 9--Merillat's method of fixing carpus in radial paralysis. Courtesy, Alex. Eger.]