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.