When one considers the anatomy of the brachial nerve plexus and the distribution of its various branches, the location of this plexus and its proximity to the first rib, and the inevitable injury it must suffer in fracture of this bone, together with the inaccessibility of the plexus, it is not strange that a correct diagnosis of the various affections of the brachial plexus and the radial nerve is often impossible until several days or weeks have passed.  And, in some instances, diagnosis is not established until an autopsy has been performed.  Here, too, we fail to find cause for paralysis in some rare instances.

Anatomy.—­The radial nerve is a large branch of the brachial plexus and is chiefly derived from the first thoracic root of the plexus and is here situated posterior to the deep brachial artery.  It is directed downward and backward under the subscapularis and teres major muscles, rounding the posterior part of the humerus, and passing to the anterior and distal end of the humerus, it finally terminates in the anterior carpal region.  The radial nerve supplies branches to the three heads of the triceps brachii, to the common and lateral extensors of the digit and also to the skin covering the forearm.

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