7. =N. musculocutaneus= (Fig. 160, _d_).--The musculocutaneous nerve or
external cutaneous arises from the ventral surface of the sixth and
seventh nerves (Fig. 159, _f_). It passes toward the shoulder-joint and
supplies the biceps (Fig. 160, 4) and coracobrachialis muscles. It then
passes distad, resting on the dorsal border of the biceps (4), and
supplies the brachialis muscle. Thence it passes dorsad of the biceps to
the lateral side, and reaches the integument by passing between the
clavobrachialis and pectoantibrachialis muscles near the elbow; it is
then distributed to the skin on the radial side of the ventral surface
of the forearm, as far as the wrist (Fig. 130, _h_, page 319).
8. =N. cutaneus medialis= (Fig. 160, _o_).--The medial (or internal)
cutaneous nerve rises from the first thoracic. It passes distad along
the medial side of the biceps, and at the junction of the second and
third thirds of the upper arm reaches the integument by passing between
the epitrochlearis and pectoantibrachialis muscles. It then curves
spirally about the dorsal border of the forearm and is distributed to
the integument of the forearm on the ulnar side, extending nearly to the
wrist.
9. =N. medianus= (Fig. 160, _e_).--The median nerve is formed by the
junction of three branches, one each from the seventh and eighth
cervical with the brachial artery (_g_) passing between them, and one
from the first thoracic nerve. It accompanies the brachial artery (_g_)
lying on its medial side, and passes with it through the supracondyloid
foramen; here it is connected by a branch to the musculocutaneous nerve.
It passes into the forearm (Fig. 161, _b_), lying at first beneath the
pronator teres (5). Here it gives branches to the pronator teres and
flexor muscles (flexor carpi radialis, palmaris longus, five heads of
the flexor profundus digitorum, but _not_ to the flexor carpi ulnaris).
Continuing along the forearm, lying on the flexor carpi radialis, it
sends a =posterior interosseous= nerve to the pronator quadratus muscle
and branches to the deep and superficial divisions of the flexor
sublimis digitorum. It thus supplies the =flexor= and =pronator= muscles
(except the flexor carpi ulnaris). It then passes into the palm beneath
the transverse ligament and gives rise to three principal branches. The
first of these supplies the integument of the thumb, sending a branch on
either side of it; and onto the radial side of the second digit. The
second passes along the contiguous sides of the second and third digits
and supplies their integument. The third passes along the contiguous
sides of the third and fourth digits to their integument. Each sends
twigs into the trilobed pad in the palm, so that there are thus seven
terminal branches in the palm. From these, branches (probably) pass to
the three radial Mm. lumbricales.
[Illustration: FIG. 161.--NERVES AND ARTERIES OF THE FOREARM, SEEN
FROM THE FLEXOR SIDE.
Mm. palmaris longus, flexor carpi radialis, flexor carpi ulnaris, and
the third and fourth heads of M. flexor profundus digitorum have been
removed; also part of the pronator teres. 1, M. biceps; 2, conjoined
tendon of M. brachialis and M. clavobrachialis; 3, short portion of
caput mediale of M. triceps; 4, intermediate portion of caput mediale
of M. triceps brachii; 5, cut ends of M. pronator teres; 6, M.
brachioradialis; 7, M. extensor carpi radialis longus and brevis; 8,
fifth head of M. flexor profundus digitorum; 9, second head of M.
flexor profundus digitorum; 10, radial part of M. flexor sublimis
digitorum; 11, common tendon of M. flexor profundus digitorum; 12,
first head of M. flexor profundus digitorum; 13, cut origin of M.
flexor carpi ulnaris. _a_, ulnar nerve; _b_, median nerve; _c_, dorsal
cutaneous branch of the ulnar nerve; _d_, palmar branch of ulnar
nerve; _e_, deep palmar branch; _f_, superficial palmar branch; _g′_,
A. brachialis; _g_, A. radialis; _h_, A. collateralis ulnaris
superior; _i_, A. radialis recurrens; _j_, A. ulnaris recurrens; _k_,
A. ulnaris; _l_, A. interossea anterior; _m_, large branch of A.
radialis, joining A. ulnaris.]
10. =N. radialis.=--The radial (or musculospiral) nerve is formed by the
junction of branches from the seventh and eighth cervical and first
thoracic nerves. It immediately sends branches to the epitrochlearis,
long head of the triceps, and long and intermediate portions of the
medial head of the triceps. It then passes between the long and
intermediate portions of the medial head of the triceps, following the
profunda brachii artery, and curves about the humerus to its cranial
side. It supplies there the lateral head of the triceps and the
anconeus, and, lying on the brachialis muscle, divides into a
superficial and a deep branch. The superficial branch is the
=superficial radial nerve= (Fig. 130, _g_); the deep one forms the
=dorsal= (or posterior) =interosseous nerve=. The former is sensory and
the latter motor.
The =superficial radial= nerve (Fig. 130, _g_, page 319) becomes
cutaneous at the junction of the second and third thirds of the upper
arm, where it emerges from between the lateral head of the triceps and
the brachialis muscle. It then follows the course of the vena cephalica
(_c_) to the wrist and follows the dorsal tributary of the same vein
onto the dorsum of the hand. It supplies the integument of the distal
part of the ventral surface of the upper arm and that along the forearm.
On the hand it is distributed to the dorsal surface in the same manner
as the median nerve on the ventral surface, i.e. to the integument of
the thumb on both sides and to that of the medial side of the second
digit; to that of the contiguous sides of the second and third digits
and of the contiguous sides of the third and fourth digits. There are
thus seven terminal branches, one for each of these regions.
The =dorsal interosseous= nerve follows the brachialis muscle and passes
onto the forearm between it and the extensor carpi radialis longus. It
supplies the =supinator= and =extensor= muscles of the forearm
(brachioradialis, supinator, extensor carpi radialis longus, extensor
carpi radialis brevis, extensor communis digitorum, extensor digitorum
lateralis, extensor indicis, extensor brevis pollicis). The dorsal
interosseous and radial nerves thus supply all the extensor muscles of
the forearm and upper arm and the supinators (modified extensors).
11. =N. ulnaris= (Fig. 160, _n_).--The ulnar nerve arises from the
eighth cervical and first thoracic and accompanies the brachial artery
(_g_) and median nerve (_e_) through the upper arm. It does not pass
through the supracondyloid foramen, but curves over the dorsal end of
the medial epicondyle of the humerus within the short division of the
medial head of the triceps, and reaches the dorsal border of the
forearm. In the forearm (Fig. 161, _a_) it lies beneath the flexor carpi
ulnaris. It supplies muscular branches to the flexor carpi ulnaris and
the first or ulnar head of the flexor profundus digitorum. Near the
middle of the forearm it divides into two branches, =dorsal cutaneous=
(_c_) and =palmar= (_d_).
The =dorsal cutaneous= branch (_c_) curves about the ulnar side of the
wrist to the dorsum of the hand and divides into twigs which supply the
dorsal part of the ulnar side of the fifth digit and of the contiguous
sides of the fourth and fifth. The =palmar branch= (_d_) sends a twig to
the integument on the flexor surface of the arm near the wrist, and then
divides into deep palmar (_e_) and superficial palmar (_f_) branches.
The =superficial palmar branch= (_f_) passes into the palm and supplies
the ventral portion of the integument on the ulnar side of the fifth
digit, and on the contiguous sides of the fifth and fourth.
The =deep palmar branch= (_e_) passes into the palm, beneath the
ligament of the pisiform bone. Just distad of the pisiform bone it
curves toward the radial side and breaks up into a number of small
branches which are distributed to the short muscles in the palm.