Chapter XII: N. HYPOGLOSSUS (Fig. 156, b).--The twelfth or hypoglossal nerve

arises from the ventral side of the medulla (Fig. 138, _XII_), as previously described. It emerges from the skull through the hypoglossal canal. At first it passes ventrad, then gradually turns craniad, following thus a curved course and passing successively laterad of the vagus and sympathetic nerves (Fig. 156, _d_ and _e_), the common carotid artery, and the cranial cornu of the hyoid bone. A short distance from its origin the nerve gives off a =ramus descendens= (Fig. 156, _f_), which receives a communicating branch from the first cervical nerve (_j_), and divides into two branches, one (_g_) to the thyrohyoid muscle (9), the other to the sternohyoid (8) and sternothyroid muscles (7). The main nerve passes beneath the mylohyoid muscle into the tongue, at first following the lingual artery, then laterad of it, then crossing it again. It sends branches to all the muscles of the tongue, and may be traced to its tip.

The hypoglossal nerve is involved in the plexus just outside of the jugular foramen, with which the vagus, sympathetic, and accessory nerves are also connected.

2. =Spinal Nerves.=--The origin and general features of the spinal nerves have been described in connection with the account of the spinal cord (page 337). A description of their peripheral distribution will now be given.

A. CERVICAL NERVES.--There are eight pairs of cervical nerves. The first leaves the vertebral canal through the atlantal foramen. The second passes out between the arches of the atlas and axis, not through a special intervertebral foramen, so that its ganglion lies among the muscles of the back of the neck. The others emerge through the intervertebral foramina, the eighth one from between the last cervical and first thoracic vertebræ.

M. clavotrapezius has been partly removed. 1, N. accessorius; 2-4,
ventral rami of second to fourth cervical nerves; 5, N. auricularis
magnus; 6, N. cutaneus colli. _a_, M. clavotrapezius (cut); _b_, M.
splenius; _c_, M. levator scapulæ ventralis; _d_, M. cleidomastoideus;
_e_, M. sternomastoideus; _f_, parotid gland.]

=Dorsal Rami= (Rami posteriores).--The dorsal ramus of the first nerve (N. suboccipitalis) supplies the short dorsal muscles which move the head and connect the atlas and skull. In the second nerve the dorsal ramus is much larger, forming =N. occipitalis major=. It sends small branches to the muscles about its origin, then turns craniad on the surface of the obliquus superior muscle, passes through the biventer cervicis and splenius, joins a small branch from the third nerve, and reaches the dorsal surface of the back of the head. It passes craniad, lying beneath the levator auris longus, emerges from between the two divisions of this muscle, and supplies the skin and cutaneous muscles between the two external ears. The dorsal rami of the other cervical nerves supply muscles and integument on the back of the neck.

=Ventral Rami= (=rami anteriores=).--These pass ventrad between the transverse processes of the vertebræ, except in the case of the =first= nerve (Fig. 156, _j_). This passes ventrad from the atlantal foramen along the groove for the vertebral artery, through the notch in the wing of the atlas, and across the lateral surface of the longus capitis muscle (5). Here it sends a branch caudad to join the second cervical (_k_), then crosses the vagus (_d′_) and sympathetic and the carotid artery, giving off communicating branches to the vagus and sympathetic, and uniting with a branch of the descending ramus of the hypoglossal (_f_). At the lateral surface of the larynx it turns caudad, following the lateral border of the sternothyroid muscle (7), and is distributed to the sternohyoid (8) and sternothyroid (7).

The ventral ramus of the =second= nerve (Fig. 158, 2) receives a branch from that of the first nerve, passes laterad between the levator scapulæ ventralis (_c_) and the cleidomastoid (_d_), receives a communicating branch from the third cervical (3), sends a branch to N. accessorius (1) and numerous small nerves into the sternomastoid (_e_) and cleidomastoid (_d_), then turns craniad and divides into =N. auricularis magnus= (5) and =N. cutaneus colli= (6).

=N. auricularis magnus= (5), the great auricular nerve, passes dorsocraniad across the lateral surface of the sternomastoid (_e_) to the lateral and caudal surface of the external ear and parotid gland (_f_), where it ramifies. The =cutaneus colli= (6) is the smaller, ventral, division of the second nerve; it may receive also an accession from the third. It passes to the integument over the ventral part of M. masseter and ventrad of that muscle.

The =third= nerve (Fig. 158, 3) communicates with the second and supplies the levator scapulæ ventralis (_c_), cleidomastoid (_d_), sternomastoid (_e_), longus capitis, and other muscles of this region and aids in forming the cutaneus colli (6). The =fourth= (4) and =fifth= are distributed to the muscles and integument of the sides of the neck. A branch of the fourth supplies the integument in the hollow of the shoulder, and one from the fifth follows the vena cephalica and supplies the integument over the shoulder. The fifth by sending a branch to aid in forming the phrenic nerve (Fig. 157, _f_) may be considered to enter partly into the brachial plexus.

Owing to the intercommunicating branches between the ventral roots of the first five cervical nerves, these are sometimes considered as forming a loose plexus which receives the name =cervical plexus=.

The sixth, seventh, and eighth cervical nerves (with a part of the fifth) become interconnected with each other and with the first thoracic to form the =brachial plexus=.

=The Brachial Plexus= (Fig. 159).--The brachial plexus is formed by the ventral rami of the fifth, sixth, seventh, and eighth cervical nerves and the first thoracic. Of the fifth cervical only a small part enters into the plexus, forming part of the phrenic nerve. The formation of the plexus is due to the union of the different nerves by means of strong connecting branches or =ansæ=. The plexus lies in the axilla, along with the axillary artery and vein; all its component nerves pass laterad in front of the first rib. Its branches supply the arm and shoulder.

The precise arrangement of the different strands is somewhat variable. The plexus is commonly made up in approximately the following manner (Fig. 159). From the =fifth= cervical nerve (_V_) a small branch joins a similar one from the sixth to form the phrenic nerve (_a_); the remainder of the fifth does not enter into the plexus. From the =sixth= cervical (_VI_) arise parts of the phrenic nerve (_a_), the suprascapular (_b_), the cranial one of the three subscapular nerves (_c_), the axillary (_d_), and the musculocutaneous (_f_). The sixth also gives off close to its origin a nerve (_b′_) which passes to the inner surface of the levator scapulæ and ramifies over the surface, supplying this muscle and extending to the rhomboideus, which it also innervates. The =seventh= cervical (_VII_) is the largest nerve entering into the plexus; it furnishes parts of one or both of the anterior thoracic nerves (_k_ and _n_), of the posterior thoracic (_m_), the three subscapular nerves (_c_, _e_, and _i_), the axillary (_d_), musculocutaneous (_f_), radial (_h_), and median (_g_) nerves. The =eighth= cervical (_VIII_) supplies parts of one of the anterior thoracic nerves (_k_), the caudal one of the subscapular nerves (_i_), the radial (_h_), median (_g_), and ulnar (_j_) nerves. The =first thoracic= (1) curves craniad, so as to leave the thorax on the cranial side of the first rib; it supplies the median cutaneous nerve (_l_) and parts of the radial (_h_), ulnar (_j_), and median (_g_) nerves; sometimes also a part of one (_k_) of the anterior thoracic nerves.

_V_, _VI_, _VII_, _VIII_, the fifth to eighth cervical nerves. 1, the
first thoracic nerve. _a_, phrenic nerve; _b_, suprascapular; _b′_,
nerve to serratus anterior and levator scapulæ muscles; _c_, first or
cranial subscapular nerve; _d_, axillary nerve; _e_, second
subscapular; _f_, musculocutaneous; _g_, median; _h_, radial; _i_,
third subscapular; _j_, ulnar; _k_, second anterior thoracic; _l_,
medial cutaneous; _m_, posterior thoracic; _n_, first anterior
thoracic.]

The following are the nerves which arise from the brachial plexus:

A. =Nerves of the Shoulder and Breast= (Fig. 160).

1. =Nn. thoracici anteriores.=--There are two anterior thoracic nerves. One (Fig. 160, _h_; Fig. 159, _n_) arises from the seventh cervical and passes to the pectoral muscles (9) along with the anterior thoracic artery. The other (Fig. 159, _k_; Fig. 160, _q_ and _r_) arises usually by two roots, one from the eighth cervical, one from the first thoracic; it passes along with the long thoracic artery to be distributed to the pectoralis muscles (Fig. 160, 8) and sometimes also to the latissimus dorsi (7).

2. =N. thoracicus posterior.=--The posterior thoracic (or long thoracic) nerve (external respiratory) (Fig. 159, _m_) arises from the seventh cervical nerve near its beginning. It passes within the scalenus muscle and extends caudad on the outer surface of the serratus anterior muscle, which it supplies.

3. =N. suprascapularis= (Figs. 159 and 160, _b_).--The suprascapular nerve arises from the sixth (and sometimes the seventh) cervical. It passes laterad and gives off a branch which passes over the shoulder-joint and penetrates the clavodeltoid muscle to be distributed to the integument on the ventral surface of the upper arm. It then follows the transversa scapulæ artery into the supraspinatus fossa and accompanies the artery to be distributed to the supraspinatus and infraspinatus muscles.

4. =Nn. subscapulares.=--The subscapular nerves are three. The cranial one (Figs. 159 and 160, _c_) arises from the sixth and seventh cervical and supplies the subscapular muscle. The middle one (Fig. 159, _e_) arises from the seventh; it supplies principally the teres major. The caudal one (Fig. 159, _i_) arises from the seventh and eighth cervical and supplies the latissimus dorsi. (The points of origin from the plexus vary.)

5. =N. axillaris= (or =circumflexus=) (Fig. 159, _d_).--The axillary nerve arises from the sixth and seventh cervical nerves. It passes toward the shoulder-joint, then follows the posterior circumflex artery ventrad of the long head of the triceps to be distributed to the spinodeltoid and acromiodeltoid. A branch of it continues to the clavobrachial, which it supplies.

B. =The Phrenic Nerve= (Fig. 157, _f_, page 381).

6. =N. phrenicus.=--The phrenic nerve (internal respiratory) is formed by the junction of two slender branches, one from the fifth and one from the sixth cervical nerves (Fig. 159, _a_). (It is said to receive sometimes a branch from the fourth.) It passes caudad into the thorax on the ventral surface of the subclavian artery and then extends to the diaphragm lying at the side of the inferior and superior venæ cavæ. It is the motor nerve of the diaphragm.

The pectoral and clavobrachial muscles have been cut and laid aside,
their ends being shown; only a part of the nerves are exhibited. 1, M.
clavobrachialis; 2, cut end of M. pectoralis major; 3, cut end of M.
pectoralis minor; 4, M. biceps; 5, M. teres major; 6, M.
epitrochlearis, partly cut and turned back; 7, M. latissimus dorsi,
partly cut; 8, M. pectoralis minor; 9, M. pectoralis major; 10, short
portion of caput mediale of M. triceps brachii. _a_, branches of the
thyrocervical axis to clavobrachial and clavotrapezius muscles; _b_,
suprascapular nerve; _c_, first subscapular nerve; _d_,
musculocutaneous nerve; _e_, median nerve; _f_, V. axillaris; _g_, A.
axillaris; _h_, first anterior thoracic nerve, accompanied by the
anterior thoracic artery and vein; _i_, V. subscapularis; _j_, large
muscular branch (to subscapular muscle) of the subscapular vein and
brachial artery; _l_, A. subscapularis; _l′_, A. circumflexa humeri
anterior; _m_, radial nerve; _n_, ulnar nerve; _o_, medial cutaneous
nerve; _p_, A. thoracica longa; _q_, _r_, second anterior thoracic
nerve; _s_, V. thoracica longa; _t_, A. and V. thoracicodorsalis; _u_,
A. profunda brachii; _v_, branch of brachial artery accompanying
medial cutaneous nerve; _w_, branch of A. collateralis radialis
superior; _x_, A. collateralis radialis superior; _y_, V. mediana
cubiti; _z_, A. collateralis ulnaris.]

C. =Nerves of the Arm.=

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.

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.

B. THORACIC NERVES.

=Dorsal Rami.=--The dorsal rami of the thoracic nerves are small and supply the muscles and integument of the back. Each gives off a branch directly dorsad to the spinal muscles, and a lateral branch which reaches the integument some distance from the middle line.

=Ventral Rami.=--The ventral ramus of the first thoracic nerve enters into the brachial plexus, as already described. The ventral rami of the other thoracic nerves form the =intercostal nerves=. Each of these passes ventrolaterad, lying close to the caudal border of a rib, in company with the intercostal artery. Branches are given to the intercostal muscles, and at about the middle of the length of the rib a large lateral branch is given off, which passes to the more superficial muscles of the thoracic wall (serrati posteriores, obliquus externus, etc.), its main branches running dorsad and ventrad. The main intercostal nerve extends ventrad to the transversus costarum and rectus abdominis, supplying these muscles.