[Illustration: FIG. 91.--To illustrate the Loss of Sensation produced by
Division of the Median Nerve. The area of complete cutaneous
insensibility is shaded black. The parts insensitive to light touch and
to intermediate degrees of temperature are enclosed within the dotted
line.
(After Head and Sherren.)]
#The Median Nerve# is most frequently injured in wounds made by broken
glass in the region of the wrist. It may also be injured in fractures of
the lower end of the humerus, in fractures of both bones of the forearm,
and as a result of pressure by splints. After _division at the elbow_,
there is impairment of mobility which affects the thumb, and to a less
extent the index finger: the terminal phalanx of the thumb cannot be
flexed owing to the paralysis of the flexor pollicis longus, and the
index can only be flexed at its metacarpo-phalangeal joint by the
interosseous muscles attached to it. Pronation of the forearm is feeble,
and is completed by the weight of the hand. After _division at the
wrist_, the abductor-opponens group of muscles and the two lateral
lumbricals only are affected; the abduction of the thumb can be feebly
imitated by the short extensor and the long abductor (ext. ossis
metacarpi pollicis), while opposition may be simulated by contraction of
the long flexor and the short abductor of the thumb; the paralysis of
the two medial lumbricals produces no symptoms that can be recognised.
It is important to remember that when the median nerve is divided at the
wrist, deep touch can be appreciated over the whole of the area
supplied by the nerve; the injury, therefore, is liable to be over
looked. If, however, the tendons are divided as well as the nerve, there
is insensibility to deep touch. The areas of epicritic and of
protopathic insensibility are illustrated in Fig. 91. The division of
the nerve at the elbow, or even at the axilla, does not increase the
extent of the loss of epicritic or protopathic sensibility, but usually
affects deep sensibility.
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