The =median nerve= (Figs. 441, 442, 447, 520, 521) derives its fibers
chiefly from the first thoracic root of the brachial plexus. It is
usually the largest branch and it accompanies the chief arterial trunks
to the distal part of the limb. It descends over the inner face of the
brachial artery, which it crosses obliquely, and continues down the arm
in front of the artery. Near its origin it is joined by a large branch
with the musculo-cutaneous nerve, thus forming a loop in which the
artery appears to be suspended. Near the elbow it crosses obliquely over
the artery (posterior radial) and lies behind it on the internal lateral
ligament. Below the joint it again crosses the artery and lies behind
the radius and the lower part of the long internal lateral ligament.
[Illustration:
FIG. 522.—DISSECTION OF DISTAL PART OF RIGHT FORE LIMB OF HORSE,
VIEWED FROM BEHIND.
_1_, Lateral cartilage; _2_, _3_, _4_, ligaments from first phalanx to
third sesamoid, bulb of heel, and lateral cartilage. (After
Schmaltz, Atlas d. Anat. d. Pferdes.)
]
At the elbow the nerve is covered by the posterior superficial
pectoral muscle and is crossed by the large oblique connection between
the cephalic and brachial veins. The variable and often plexiform
venous arrangement renders approach to the nerve here undesirable. It
can be reached best by an incision just behind the lower part of the
long internal lateral ligament; here the nerve lies under the deep
fascia in the furrow between the radius and the anterior border of the
flexor carpi internus, overlapped by the latter. In some cases the
nerve retains its position in front of the artery at the elbow, and in
others it crosses the deep face of the artery.
Passing beneath the internal flexor of the carpus, the nerve continues
downward in the forearm with the posterior radial vessels, and divides
at a variable distance above the carpus into two branches; these are the
internal volar or metacarpal nerve and the branch which unites with the
ulnar to form the corresponding external nerve.
At the proximal part of the forearm the nerve runs almost straight
downward along the posterior border of the long internal lateral
ligament, while the artery here inclines somewhat backward. Thus the
nerve is superficial to the artery for a short distance, then lies in
front of the latter to about the middle of the region, where it
inclines a little backward and arrives at the interval between the
internal and middle flexors of the carpus. The division may occur
about the middle of the region, but commonly takes place in the distal
third or fourth.
The collateral branches are as follows:
1. The =musculo-cutaneous branch= is in reality the continuation of the
nerve of that name. It is given off about the middle of the arm, passes
beneath the lower part of the biceps, and divides into muscular and
cutaneous branches. The =muscular branch= enters the brachialis. The
=cutaneous branch= emerges between the mastoido-humeralis and the biceps
and divides into two branches; these descend on the fascia of the
forearm with the cephalic vein and its accessory, and ramify on the
front and inner face of the forearm, carpus, and metacarpus.
2. =Muscular branches= to the internal flexor of the carpus and the
flexors of the digit are given off just as the nerve passes under the
first named muscle.
3. The very small =interosseous nerve= passes through the interosseous
space and is distributed chiefly to the periosteum, but in some cases
twigs may go to the extensor muscles.
The =metacarpal= or =volar nerves=,[209] internal and external, are the
continuations of the median and ulnar nerves in the distal part of the
limb.
The =internal metacarpal= or =volar nerve= (N. volaris medialis) arises
as the inner terminal branch of the median nerve at a variable distance
above the carpus (Fig. 442). It descends through the carpal canal along
the inner border of the superficial flexor tendon, and lies at first in
front of the large metacarpal or common digital artery. It then passes
behind the artery to the distal third of the metacarpus, where it lies
behind the vein, the artery here becoming deeper in position. In
addition to cutaneous twigs, the nerve gives off about the middle of the
metacarpus a considerable =anastomotic branch= which winds obliquely
downward and outward over the flexor tendons and joins the external
nerve at the lower third of the metacarpus (Fig. 447). Near the fetlock
the nerve divides into two digital branches.
1. The =dorsal= or =anterior digital branch= (Ramus dorsalis) descends
at first between the digital artery and vein, then crosses over the vein
and ramifies in the skin and the matrix of the hoof on the dorsal
(anterior) face of the digit.
2. The =volar= or =posterior digital branch= (Ramus volaris) is the
direct continuation of the trunk. It descends behind the digital artery,
which it accompanies in its ramification. A =middle digital branch= is
sometimes described as descending behind the vein. In some cases this
branch is distinct, but usually there are instead several small twigs
derived from the posterior branch, which cross very obliquely over the
artery and anastomose in a variable manner with each other and with the
anterior branch.
The =external metacarpal= or =velar nerve= (N. volaris lateralis) is
formed by the union of the external terminal branch of the median with
the deep branch of the ulnar nerve (Fig. 442). It descends with the
internal volar or small metacarpal artery in the texture of the
posterior annular ligament of the carpus. In the metacarpus it descends
along the outer border of the deep flexor tendon behind the external
metacarpal vein, and is accompanied by a small artery from the volar
subcarpal arch. Toward the distal end of the metacarpus it is joined by
the oblique branch from the internal nerve, and beyond this is arranged
like the latter. Below the carpus it detaches a deep branch to the
suspensory ligament and the interossei, and also supplies twigs to the
skin.
Anastomoses are established between the digital branches, and the
areas innervated by them are not well defined, but really overlap each
other. In certain diseased conditions, however, in which the lesions
are confined to the volar structures, relief from pain may be afforded
by section of the volar branches only.