medial and long heads of the triceps brachii, principal extensor of the arm,
arises from the radial nerve (in the axilla) as the medial muscular branches.
The innervation to the lateral head, and to a smaller portion of the medial
head, arises from the radial nerve as it passes along the musculospiral groove
at mid-humerus. The supinator muscle is innervated by muscular twigs from
the deep branch of the radial nerve in the forearm, just before the radial nerve
reaches the supinator muscle. Thus, paralysis of the supinator muscle, but not
of the triceps brachii (thus not answers dande), localizes the fracture to the
distal third of the humeral shaft between the elbow and musculospiral groove.
Damage to the posterior cord (answer a)or division (answer b)of the
brachial plexus would also affect the axillary nerve that innervates the deltoid
which is notaffected.
457.The answer is a.(Moore and Dalley, pp 827–828.)Muscles are most
powerful (disregarding leverage factors) when stretched by extension of the
joint(s) over which they pass, because this places the sarcomeres at the
optimum tension-producing length in the length-tension relationship.
Thus, hand grasp is strongest when the wrist joint and metacarpopha-
langeal joints are extended, which stretches the digitorum superficialis and
profundus flexors to their optimum position [thus not (answer d)]. Paralysis
of the radial nerve with subsequent wrist-drop will weaken hand grasp
because the extrinsic flexor muscles are compelled to operate in a nonopti-
mum region. The lever arms of the lumbricals (answer c)and interossei
(answer b)are greatest when the metacarpophalangeal joints are flexed, a
consideration that does notapply to the patient presented in the question.
The median nerve innervates the radial side of the flexor digitorum pro-
fundus [the (answer e)is irrelevant to the question].
458.The answer is c.(Moore and Dalley, pp 823–824.)Radial nerve palsy
produces an inability to extend the metacarpophalangeal joints, owing to
paralysis of the extensor digitorum communis muscle. However, the lum-
brical and interossei muscles, which are served by the median and ulnar
nerves and insert into the dorsal expansions (extensor hoods) of the proxi-
mal phalanges, are able simultaneously to flex the metacarpophalangeal
joints and to extend the interphalangeal joints [thus not (answers d and e)].
Also, abduction of the digits, a function of the dorsal interossei, and adduc-
tion, a function of the palmar interossei, are both mediated by the ulnar
nerve and, therefore, unaffected [thus not (answers a and b)].
590 Anatomy, Histology, and Cell Biology