The brachial plexus is formed by the anterior primary divisions of the four lower cervical nerves and the greater part of that of the first thoracic nerve. It is usually joined by small twigs from the fourth cervical and second thoracic nerves.

The anterior primary divisions of the lower four cervical nerves, after passing dorsal to the vertebral artery and between the anterior and posterior parts of the intertransverse muscles, pass into the posterior triangle in the interval between the adjacent borders of the anterior and middle scalene muscles, where the fifth and sixth nerves receive a grey ramus communicans each from the middle cervical sympathetic ganglion, and the seventh and eighth nerves each receive a grey ramus from the inferior cervical sympathetic ganglion. The first thoracic is connected by two rami communicantes with the first thoracic sympathetic ganglion, and it divides into a smaller and a larger branch. The smaller branch passes along the intercostal space as the first intercostal nerve, and the larger branch, after being joined by a twig from the second thoracic nerve, passes upward and lateralward, in front of the neck of the first rib and behind the apex of the pleural sac, into the lower part of the posterior triangle of the neck, where it takes part in the formation of the plexus.

Diagbam of a Common Form of Brachial Plexus. The posterior cord of the plexus is darkly shaded.

The anterior primary divisions of those cervical nerves that form the brachial plexus may be considered as typically giving off anterior and posterior branches, except that the fifth and sixth nerves often unite before branching and give off their posterior branches as a common trunk, and the eighth nerve often receives its branch from the first thoracic nerve before giving off its posterior branch.

It is on account of this variation in the point of union of the fifth and sixth cervical nerves and of the eighth cervical and first thoracic nerves that so many different forms of the plexus have been pictured and described. But if the differences in primary branching be borne in mind, the formation of the plexus is always uniform and simple, notwithstanding its different appearances.

The Brachial Plexus and its Branches of the Region of the Neck and Shoulder. (After Toldt, "Atlas of Human Anatomy," Rebman, London and New York.)

Three cords are formed from these branches in the following manner:

  1. The lateral (outer) cord [fasciculus lateralis] is formed by the anterior branches of the fifth, sbcth, and seventh nerves;
  2. the medial (inner) cord [fasciculus medialis], by the anterior branches of the eighth cervical and first thoracic nerves; and
  3. the posterior cord [fasciculus posterior], by the posterior branches of all of these cervical nerves.

Relations

The plexus extends from the lateral border of the scalenus anterior, where the roots of its constituent nerves appear, to the lower border of the peotoralis minor, where each of its three cords divides into two terminal branches, and it lies in the posterior triangle, in the root of the neck, and in the axillary fossa. In the posterior triangle and in the root of the neck it is in relation liehind with the scalenus medius. In the posterior triangle it is covered superficially by the skin and superficial fascia, the pJatysma, the supra-clavicular branches of the cervical plexus, and the deep fascia, and it is crossed by the lower part of the external jugular vein, by the nerve to the subclavius, the transverse cervical vein and the transverse scapular (supra-scapular) vein, the posterior belly of the omo-hyoid muscle, and by the transverse cervical artery. At the root of the neck it lies behind the clavicle and the subclavius muscle, and the transverse scapular (suprascapular) artery crosses in front of it. In the axillary fossa the cords are arranged around the axillary artery, the lateral (outer) cord lying lateral to the artery, the medial (inner) cord medial to it, and the posterior cord dorsal to the artery. In this region the posterior relations of the plexus are the fat in the upper part of the fossa and the subscapularis muscle, and it is covered in front by the pectoral muscles and the coraco-clavicular fascia. The lower border of the plexus is in relation in the posterior triangle and at the root of the neck with the pleura and the first rib, and it is overlapped in front by the third part of the subclavian artery. In the axillary fossa the medial cord which forms the lower border of the plexus is overlapped anteriorly by the axillary vein. The upper and lateral border of the plexus has no very important relations.

In gross, the brachial plexus may be formulated as beginning with five nerves and terminating in five nerves, with its intermediate portions displayed in sets of threes. It begins with the fifth, sixth, seventh and eighth cervical and first thoracic nerves; it terminates as a plexus with the formation of the musculo-cutaneous, radial, axillary, median, and ulnar nerves; in its intermediate portions, first main trunlcs are formed and these divide into two sets of threes which, by union, give rise to three cords. The branches from the cords are three main lateral branches from each and the terminal branches of the plexus. The lateral branches, according as they are given off above, below, and dorsal to the clavicle, are grouped as the supra-clavicular, the infra-clavicular and the subscapular portions of the plexus.

The branches of the supra-clavicular portion

After the roots of the plexus have received communications from the sympathetic, which have already been referred to, they give off a series of muscular branches, viz. - the posterior thoracic nerves (the dorsal scapular and the long thoracic nerve), the suprascapular nerve, a twig to the phrenic, the nerve to the subclavius, and small twigs to the scalene muscles and the longus colli muscle.

The posterior thoracic nerves

The posterior thoracic nerves are two in number:

(a) the dorsal scapular (nerve to the rhomboids) arises principally from the fifth cervical nerve, but it frequently receives a twig from tihe fourth nerve.

It passes downward and dorsalward, across the middle scalene, parallel with and below the spinal accessory nerve to the anterior border of the levator scapulae, under which it disappears. It continues its descent under cover of the levator scapulae and the rhomboids almost to the lower angle of the scapula, lying a little medial to the posterior border of the bone, and it supplies the lower fibers of the levator and the smaller and larger rhomboid muscles.

(b) The long thoracic nerve (external respiratory nerve of Bell) supplies the serratus anterior.

It usually arises, by three roots, from the fifth, sixth, and seventh cervical nerves. The last is sometimes absent. The upper two roots traverse the substance of the scalenus medius; the root from the seventh passes in front of that muscle. Twigs are furnished to the superior portion of the serratus anterior by the upper two roots; lower down they unite and are subsequently joined by the root from the seventh when present. The trunk of the nerve passes downward behind the brachial plexus and the first stage of the axillary artery, and runs along the axihary surface of the serratus anterior (magnus), supplying twigs to each of the digitations of that muscle.

The suprascapular nerve

The suprascapular nerve supplies the supraspinatus and infraspinatus muscles.

It receives fibers from the fifth and sixth cervical nerves, and occasionally also a twig from the fourth nerve. It is a nerve of considerable size, and it passes downward and dorsalward parallel with the dorsal scapular nerve, at first along the upper border of the posterior belly of the omo-hyoid muscle, then internal to the latter muscle and under cover of the anterior border of the trapezius to the suprascapular notch, where it comes into relation with the transverse scapular (suprascapular) artery. It is separated from the artery at the notch by the superior transverse ligament, the nerve passing through the notch and the artery above the ligament. After entering the supraspinous fossa the nerve supplies branches to the supraspinatus and a branch to the shoulder-joint; then it descends through the great scapular notch between the bone and the inferior transverse ligament to the infraspinous fossa, where it terminates in the infraspinatus muscle.

The phrenic nerve

The twig to the phrenic arises from the fifth cervical nerve close to the point where the latter nerve receives its twig from the cervical plexus.

The nerve to the subclavius is a small twig which arises from the fifth nerve or from the upper trunk of the plexus, but occasionally it receives additional fibers from the fourth and sixth nerves. It runs downward in front of the lower part of the plexus and the third stage of the subclavian artery and, after giving off sometimes a branch to the phrenic, pierces the posterior layer of the coraco-clavicular fascia, and enters the subclavius at its lower border.

Variety

In rare cases the entire phrenic nerve may pass vid the nerve to the subclavius in front of the third stage of the subclavian artery.

The scaleni and longus colli are supplied by twigs which arise from the lower three or four cervical nerves immediately after their exit from the intervertebral foramina.

The lateral branches of the infra-clavicular portion of the brachial plexus are the anterior thoracic nerves, from the lateral and medial cords respectively, the medial antibrachial (internal) cutaneous and the medial brachial (lesser internal) cutaneous nerves, from the medial cord, and the subscapular nerves and thoraco-dorsal from the posterior cord.

 

Distbibution of cutaneous nerves on the anterior and posterior aspects of the Superior Extremity.

The lateral anterior thoracic nerve

The lateral anterior thoracic nerve joins with the medial to form a loop which supplies the pectoralis major and minor. It arises from the lateral cord of the plexus and contains fibers from the fifth, sixth, and seventh cervical nerves. After joining the medial anterior thoracic it pierces the coraco-clavicular fascia and ends in branches that supply the pectoraUs major muscle. The medial anterior thoracic nerve arises from the medial cord, contains fibers from the eighth cervical and first thoracic nerves, and passes forward between the first stage of the axillary artery and the axillary vein. It unites with a branch from the lateral anterior thoracic, to form a loop which is placed in front of the first stage of the axillary artery; it gives branches to the pectoralis minor, and branches which pass through the latter muscle and end in the pectorahs major. From the loop additional branches are furnished to the pectoralis major.

The medial brachial (lesser internal) cutaneous nerve

The medial brachial (lesser internal) cutaneous nerve, or nerve of Wrisberg, arises from the medial cord of the brachial plexus and sometimes contains fibers from the eighth cervical and first thoracic nerves, but usually fibers from the first thoracic nerve alone. It runs downward on the medial side of the axillary vein, being separated by that vessel from the ulnar nerve, and it continues downward with a slight inclination dorsalward under cover of the deep fascia on the inner side of the arm. At the middle of the arm it pierces the deep fascia, and near the bend of the elbow it turns somewhat sharply dorsalward to supply the integument which covers the olecranon process.

As it traverses the axilla the nerve of Wrisberg communicates with the intercosto-braohial nerve, forming one, or sometimes two loops. In its course clown the arm it gives a few fine twigs to the integument. This nerve may be absent, its place being taken by the inter-costo-brachial or by part of the posterior brachial (internal) cutaneous branch of the radial (musculo-spu-al) or, rarely, by a branch from the fu'st intercostal nerve.

The medial antibrachial (internal) cutaneous nerve

The medial antibrachial (internal) cutaneous nerve arises from the medial cord in close relation with the ulnar nerve. It contains fibers from the eighth cervical and first thoracic nerves. At its origin it lies directly on the medial side of the axillary artery, but it soon becomes more superficial and then lies in the groove between the artery and the vein. In the upper two-thirds of the arm it lies in front and to the medial side of the brachial artery. It divides into two branches (volar and ulnar) which supply the medial aspect of the forearm.

At the junction of the middle and lower thirds of the arm this nerve pierces the deep fascia, in company with the basilic vein, and divides into an anterior and a posterior branch. Previous to its division it gives off twigs which pierce the deep fascia and supply the integument of the upper and medial part of the arm. The volar (anterior) branch is larger than the ulnar (posterior) ; it passes in front of or dorsal to the median basilic vein, and divides into several twigs which run down the forearm, supplying the integument covering its anterior and medial aspect as far as the wrist, and anastomosing with the branches of the ulnar nerve. The ulnar (posterior) branch passes downward and dorsalward in front of the medial condyle of the humerus, and divides into branches which supply the skin on the postero-medial aspect of the forearm. It anastomoses with the dorsal antibrachial (inferior external) cutaneous branch of the radial (musculo-spiral) nerve and the dorsal branch of the ulnar nerve).

The subscapular nerves

The subscapular nerves are branches of the posterior cord. They are three in number, are distinguished as upper, thoraco-dorsal or middle, and lower, and are distributed to the subscapularis, latissimus dorsi, and teres major muscles.

The upper or short subscapular nerve

The upper or short subscapular nerve is derived from the fifth and sixth cervical nerves. It hes in the upper and posterior part of the a.xillary fossa, and it is distributed exclusively to the subscapularis muscle. It is occasionally double.

The thoraco-dorsal, middle, or long subscapular nerve

The thoraco-dorsal, middle, or long subscapular nerve consists mainly of fibers from the seventh and eighth cervical nerves, but it may contain fibers from the fifth or the sixth nerve. It passes behind the axillary artery, accompanies the subscapular artery along the axillary margin of the subscapularis muscle, and ends in the latissimus dorsi.

The lower subscapular nerve

The lower subscapular nerve, carrying fibers from the fifth and sixth cervical nerves, passes behind the subscapular artery, below the circumflex branch (dorsahs scapulae), and is distributed to the teres major, and furnishes to the subscapularis one or two twigs which enter that muscle near its axillary margin.

Terminal branches of the plexus

The terminal branches of the plexus are two from each cord. The posterior cord divides into the axillary (circumflex) and the radial (musculo-spiral) nerves. The lateral cord divides into the musculo-cutaneous nerve, and the lateral root of the median nerve; the medial cord divides into the ulnar nerve, and the medial root of the median nerve, the median nerve as a whole being one of the five terminal branches of the plexus.

The axillary (circumflex) nerve

The axillary (circumflex) nerve is the smaller of the two terminal branches of the posterior cord, and contains fibers from the fifth and sixth cervical nerves. At the lower border of the subscapularis it passes dorsalward and accompanies the posterior circumflex artery through the quadrilateral space, which is bounded by the teres major, long head of triceps, and subscapularis muscles, and the surgical neck of the humerus, and it divides into a smaller superior and. a larger inferior division. Previous to its division it furnishes an articular twig to the shoulder-joint. This twig pierces the inferior part of the articular capsule.

The superior division accompanies the posterior circumflex artery around the neck of the humerus, and gives off a number of stout twigs which enter the deltoid muscle. A few fine filaments pierce the deltoid and end in the integument which covers the middle third of that muscle.

The inferior division divides into cutaneous and muscular branches. The cutaneous branch (the lateral brachial cutaneous nerve) turns around the posterior border of the deltoid, pierces the deep fascia, and supplies the skin covering the lower third of the deltoid and a small area of integument below the insertion of the muscle. One muscular branch is distributed to the teres minor; it swells out into an ovoid or fusiform, reddish, gangliform enlargement before entering the muscle. Other branches supply the lower and posterior part of the deltoid.

The radial (musculo-spiral) nerve

The radial (musculo-spiral) nerve is the largest branch of the brachial plexus. It contains fibers from the sixth, seventh, and eighth cervical and sometimes from the fifth cervical and first thoracic nerves. It commences at the lower border of the pectoralis minor, as the direct continuation of the posterior cord of the brachial plexus, and passes downward and lateralward in the axillary fossa behind the third part of the axillary artery and in front of the subscapulars, latissimus dorsi, and teres major muscles. From the lower border of the axillary fossa it descends into the arm, where it lies, at first, on the medial side of the upper third of the humerus, behind the brachial artery and in front of the long head of the triceps ; then it runs obliquely downward and lateralward behind the middle third of the humerus, in the groove for the radial nerve (musculo-spiral groove), and between the lateral and medial heads of the triceps. It is accompanied, in this part of its course, by the profunda artery. At the junction of the middle and lower thirds of the humerus it reaches the lateral side of the arm, pierces the external intermuscular septum, and runs downward and forward between the brachio-radialis and extensor carpi radialis longus externally, and the brachialis internally, and it terminates, a short distance above the capitulum, by dividing into deep and superficial terminal branches. In the last part of its course it is accompanied by the anterior terminal branch of the profunda artery.

Branches

The branches of the radial or musculo-spiral nerve are cutaneous, muscular, articular and terminal, but for practical purposes it is best to consider them in association with the situations of their origins. While it is in the axillary fossa the radial (musculo-spiral) nerve gives branches to the medial and long heads of the triceps, and a medial cutaneous branch. The branch to the long head of the triceps at once enters the substance of the muscle, that to the medial head breaks into branches which terminate in the muscle at different levels, and one of them, the ulnar collateral nerve, accompanies the ulnar nerve to the lower part of the arm. The posterior brachial (internal) cutaneous branch crosses the tendon of the latissimus dorsi, passes dorsal to the intercosto-brachial (intercosto-humeral) nerve, pierces the deep fascia, and is distributed to the skin of the middle of the back of the arm below the deltoid.

While it lies behind the middle third of the humerus, the radial nerve gives branches to the lateral and medial heads of the triceps and to the anconeus. The latter branch descends in the substance of the median head of the triceps, close to the bone, and it is accompanied by a small branch of the profunda artery. The dorsal antibrachial (external) cutaneous branch, passing down between the lateral and median heads of the triceps, divides near the elbow into its upper and lower branches, each of which perforates either the lateral head of the triceps muscle near its attachment to the humerus or the external intermuscular septum.

The upper branch, much the smaller, pierces the deep fascia in the line of the external intermuscular septum; it accompanies the lower part of the cephaUc vein, and supphes the skin over the lower half of the lateral and anterior aspect of the arm. The lower branch is of considerable size. It pierces the deep fascia a httle below the upper branch, runs behind the external condyle, and supplies the skin of the middle of the back of the forearm as far as the wrist, an astomosing with the medial antibrachial (internal) cutaneous and musculo-cutaneous nerves.

After the radial nerve has pierced the external intermuscular septum it gives branches to the brachio-radialis, extensor carpi radialis longus, and to the lateral portion of the brachialis. From one of these branches an articular filament is distributed to the elbow-joint.

The terminal branches of the radial nerve are: a motor branch, the deep radial, to the supinator and extensor muscles of the forearm, and a sensory branch, the superficial radial, which supplies the dorsal aspect of the radial half of the hand.

The deep radial [ramus profundus] (posterior interosseous) nerve runs downward in the interval between the brachialis and extensor carpi radialis longus. It passes in front of the lateral part of the elbow-joint, and after giving off branches to supply the extensor carpi radialis brevis and supinator, it is crossed in front by the radial recurrent artery. It then runs downward and dorsalward through the substance of the supinator, and enters the interval between the superficial and deep layers of muscles at the back of the forearm, where it comes into relation with the posterior interosseous artery, and accompanies it across

A Dissection or the cutaneous Nerves on the Dorsal Aspect of the Hand AND Fingers. (H. St. J. B.) The branches of the median nerve are shown in black.

the abductor pollicis longus. At the lower border of the latter muscle it gives off a branch to the extensor pollicis longus, and another which crosses this muscle to the extensor indicis proprius.

Continuing distalward as the dorsal antibrachial interosseous nerve the deep radial leaves the posterior interosseous artery, dips beneath the e.xtensor pollicis longus, and joins the volar inter-osseous artery. It accompanies this artery upon the interosseous membrane and upon the back of the radius, passes through the groove for the extensor digitorum communis and extensor indicis proprius to the dorsum of the wrist, and terminates in a gangliform enlargement which gives branches to the carpal articulations. The muscles supplied by the deep radial nerve are the extensor carpi radialis brevis, brachio-radialis (supinator longus), extensor digitorum communis, extensor digiti quinti proprius, extensor carpi ulnaris, extensor indicis proprius, and the extensor muscles of the thumb. The supinator (brevis) receives two twigs, one of which is given off before the nerve pierces the muscle and the other while it is passmg through it.

The superficial radial (radial) nerve [ramus superficialis n. radialis] is somewhat smaller than the deep radial (posterior interosseous), and is a purely cutaneous nerve. It runs downward under cover of the brachio-radialis, passing in front of the elbow-joint, the radial recurrent artery, and the supinator (brevis). At the lower border of the supinator it approaches the radial artery at an acute angle, and runs parallel to the lateral side of that vessel in the middle third of the forearm, across the pronator teres. At the lower border of the pronator teres it bends dorsalward on the deep surface of the tendon of the brachio-radialis, and appears on the back of the forearm. It pierces the deep fascia and is directed across the dorsal carpal (posterior annular) ligament toward the dorsum of the wrist, where it divides into its terminal branches.

The most lateral of these branches suppUes the skin on the radial part of the thenar eminence; the most medial, designated the ulnar anastomotic branch, communicates with the'dorsal branch of the ulnar nerve. The other terminal branches, the dorsal digital nerves, supply to a variable extent the skin on the dorsum of the first digit, both sides of the second and the radial side of the third digit. These branches usually extend to the base of the nail of the first digit, to the distal interphalangeal joint of the second, not quite to the proximal interphalangeal joint of the third, and to the metacarpo-phalangeal joint of the fourth digit.

Terminal branches of the lateral cord of the brachial plexus

The terminal branches of the lateral cord of the brachial plexus are the musculo-cutaneous and the lateral component of the median nerve. The latter nerve will be described with the medial cord.

The musculo-cutaneous nerve

The musculo-cutaneous nerve is composed of fibers derived chiefly from the anterior divisions of the fifth and sixth cervical nerves, together usually with some fibers from that of the seventh. The nerve to the coracobrachialis usually consists of two or three twigs given off from the nerve close to its origin before it enters the muscle. Sometimes, however, the fibers from the seventh cervical nerve pass directly to this muscle without joining the main trunk. The musculo-cutaneous nerve is placed at first close to the lateral sideof the axillary artery, but soon it leaves that vessel and, piercing the coraco-brachialis muscle, it passes obliquely downward and lateralward between the biceps and brachialis muscles. Soon after piercing the coraco-brachialis it gives off muscular branches to each head of the biceps and to the brachialis. It also gives twigs to the humerus, to the nutrient artery, and gives the chief supply to the elbow-joint. Below the branch to the brachialis the cutaneous portion of the nerve forms the lateral antibrachial cutaneous nerve. This portion continues downward between the biceps and brachialis, pierces the deep fascia at the lateral border of the former muscle a little above the bend of the elbow, receives a communication from the upper branch of the dorsal antibrachial (upper external) cutaneous branch of the radial (musculo-spiral) nerve, passes dorsal to the median cephalic vein, and divides into an anterior and a posterior branch.

The anterior branch runs downward on the lateral and anterior part of the forearm, supplying the integument of that region, and it terminates in the skin covering the middle part of the thenar eminence. A short distance above the wrist, after it has received a communicating twig from the superficial radial nerve, it gives off an articular branch to the carpal joints. This branch pierces the deep fascia and accompanies the radial artery to the dorsum of the wrist. The posterior terminal branch is small, and is directed downward and backward in front of the external condyle of the humerus, to be distributed to the skin on the lateral and posterior aspect of the forearm as low as the wrist. It anastomoses with the superficial radial and with the lower branch of the dorsal antibrachial (lower external) cutaneous branch of the radial nerve.

The terminal branches of the medial cord of the brachial plexus

The terminal branches of the medial cord of the brachial plexus are the ulnar nerve and the medial component of the median nerve. Neither of these gives any branches in the upper arm, and thus they differ from the other terminal branches of the plexus. They both supply the muscles and joints of the forearm, and the muscles, joints, and integument of the hand.

The ulnar nerve

The ulnar nerve, which is the largest branch of the medial cord of the brachial plexus, contains fibers from the anterior divisions of the eighth cervical and first thoracic nerves. It commences at the lower border of the pectoralis minor and runs downward in the axillary fossa in the posterior angle between the axillary artery and vein. In the upper half of the arm it lies on the medial side of the brachial artery, but at the level of the insertion of the coraco-brachialis it passes backward at an acute angle, and, accompanied by the superior ulnar collateral (inferior profunda) artery, it pierces the internal inter-muscular septum. After passing through the septum it runs downward, in a groove in the medial head of the triceps, to the interval between the olecranon process and the medial condyle of the humerus, and in this part of its course it is closely bound to the muscle by the deep fascia. Immediately below the medial condyle it passes between the two heads of the flexor carpi ulnaris, along the medial side of the medial collateral ligament of the elbow, and it comes into relation with the dorsal ulnar recurrent artery.

 

Nerves op the Right Upper Arm viewed from in Front. (Spalteholz.)

In the upper forearm the ulnar nerve has on the flexor digitorum profundus, covered by the flexor carpi uhiaris. Near the junction of the upper and middle thirds of the forearm it is joined by the ulnar artery, which accompanies it to its termination, lying throughout on its radial side. In the lower part of the forearm it still rests on the flexor digitorum profundus, but between the flexor carpi ulnaris and flexor digitorum sublimis, and is covered only by skin and fascia. At a variable point in this part of the forearm, usually about 5 to 8 cm. (2 to 3 in.) from the carpus, the nerve divides into its two terminal branches, a dorsal branch to the dorsal aspect of the hand, and a volar branch to the volar aspect.

 

Deep Nerves op the Volar Surface op the Forearm. (After Toldt, "Atlas of Human Anatomy," Rebman, London and New York.)

Branches

The ulnar resembles the median nerve in not furnishing any branches to the upper arm. As it passes between the olecranon process and the medial condyle it gives off two or three fine filaments to the elbow-joint. In the upper part of the forearm it supplies the flexor carpi ulnaris and the medial portion of the flexor digitorum profundus, and in the lower half it gives off the three cutaneous branches. In the palm of the hand it supplies the integument of the hypothenar eminence, the fifth digit, and half of the fourth digit, and part of the skin of the dorsum. It also supplies the short intrinsic muscles of the hand with the exception of the abductor poUicis, the opponens, the lateral head of the flexor poUicis brevis, and the two lateral lumbricales.

The nerves to the flexor carpi ulnaris and to the medial two divisions of the flexor digitorum profundus arise from the ulnar nerve in the upper third of the forearm.

Cutaneous branches

About the middle of the forearm the ulnar nerve gives off two cutaneous branches: one pierces the fascia and anastomoses with the volar branch of the medial antibrachial (internal) cutaneous nerve, and the other, the palmar cutaneous branch, runs downward in front of the ulnar artery and is conducted by this vessel into the palm

Diagrams Illustrating a Common Distribution of Cutaneous Nerves

It furnishes some filaments to the vessel, supplies a few twigs to the skin of the hypo-thenar eminence, and ends in the integument covering the central depressed surface of the palm.

The dorsal or posterior cutaneous branch, usually the smaller of the terminal branches, arises about 5 cm. (2 in.) above the wrist-joint, and passes backward under cover of the flexor carpi ulnaris to reach the dorsal aspect of the wrist, where it gives off dehoate branches to anastomose with branches of the medial antibrachial (internal) cutaneous, the dorsal anti-brachial (external) cutaneous branch of the radial (musculo-spiral), the lateral antibrachial cutaneous of the musculo-cutaneous nerve, and with branches of the superficial radial, and then divides into five branches, the dorsal digitals, which are distributed to the ulnar sides of the third, fourth, and fifth digits and the radial sides of the fourth and fifth digits. These branches usually extend on the fifth digit only as far as the base of the terminal phalanx, and on the fourth digit as far as the base of the second phalanx. The more distal parts of these digits are supplied by palmar digital branches of the ulnar nerve.

The volar branch, the other terminal branch of the ulnar nerve, continues its course between the flexor carpi ulnaris and flexor digitorum sublimis, on the medial side of the ulnar artery, to the wrist, where, on the lateral side of the pisiform bone, it divides into a superficial and a deep branch. The latter accompanies the deep branch of the ulnar artery into the interval between the abductor digiti quinti and flexor digiti quinti brevis, and then passes through the fibers of the opponens digiti quinti to reach the deep surface of the flexor tendons and the synovial sheaths. It supphes the abductor and opponens digiti quinti, the flexor digiti quinti brevis, the third and fourth lumbricales, all the interossei, the adductors of the thumb, and the medial head, and occasionally the lateral head, of the flexor poUicis brevis. The superficial branch gives off a branch to supply the palmaris brevis muscle, an anastomosing branch to the median nerve, and then divides into two branches, the proper volar digital branch, which is distributed to the medial side of the fifth digit on its volar aspect, and the common volar digital branch, which passes underneath the palmar aponeurosis and divides into two branches, which supply the contiguous margins of the fourth and fifth digits. These branches usually supply also the dorsal surface of the second and third phalanges of the same digits.

The median nerve

The median nerve contains fibers of the sixth, seventh, and eighth cervical nerves and of the first thoracic, and sometimes of the fifth cervical nerve. The trunk is formed a little below the lower margin of the pectorahs minor, by the

Nerves op the Palmar Surface op the Hand. (Testut.)

 

union of two components, one from the medial and one from the lateral cord "of the brachial plexus. The medial component passes obliquely across the third part of the axillary artery, and in the upper part of the trunk the fibers of the two components are felted together. From its commencement the median nerve runs almost vertically through the lower part of the axillary fossa and through the arm and forearm to the hand.

In the fossa it hes lateral to the axillary artery and it is overlapped, on its lateral side, by the cqraco-brachiahs muscle. In the upper half of the arm it lies along the lateral side of the brachial artery, and it is overlapped by the medial border of the biceps. At the middle of the arm it passes in front of the brachial artery, and then it descends, on the medial side of the artery, to the elbow. In the upper part of the antecubital fossa it is still at the medial side of the brachial artery, but separated from it by a small interval, and in the lower part of the fossa it lies along the medial side of the ulnar artery. In case of the high division of the brachial artery, when the radial and the ulnar arteries lie together in the upper arm, the median nerve may pass between them and then one or the other of the arteries will be superficial to the nerve. As it leaves the antecubital fossa it passes between the two heads of the pronator teres, and it crosses in front of the ulnar artery, from which it is separated by the deep head of the pronator. In the forearm it passes vertically downward, accompanied by the median (comes nervi mediani) artery. In the upper two-thirds of this region it lies deeply, between the flexor digitorum sublimis and the flexor digitorum profundus, but in the lower third it becomes more superficial, and is placed beneath the deep fascia, between the flexor carpi radialis on the radial side and the palmaris longus and flexor digitorum sublimis tendons on the ulnar side. It crosses beneath the transverse carpal (anterior annular) ligament, in front of the flexor tendons, and in the palm at the lower border of the ligament it enlarges and divides into three branches, the common volar digital nerves.

Branches

The median nerve does not supply any part of the upper arm. In front of the elbow-joint it furnishes one or two filaments to that articulation. In the forearm it supplies all the superficial anterior muscles (with the exception of the flexor carpi ulnaris) directly from its trunk, and it supplies the deep muscles (with the exception of the ulnar half of the flexor digitorum profundus) by its volar (anterior) interosseous branch. Thus in general it supplies the pronator and flexor muscles of the forearm (radial side). In the hand it supplies the group of short muscles of the thumb, which are placed on the radial side of the tendon of the flexor pollicis longus, the two lateral lumbricales, the integument covering the central, part of the palm and ulnar aspect of the thenar eminence, and the palmar aspect of the first, second, third, and radial half of the fourth digits. It also sends twigs to the dorsal aspect of these digits.

The nerve to the pronator teres usually arises a little above the bend of the elbow, and pierces the lateral border of the muscle. It may arise in a common trunk with the following nerves:

The nerves to the flexor carpi radialis, palmaris longus, and flexor digitorum sublimis arise a Uttle lower down, and pierce the pronator-flexor mass of muscles to end in the respective members of the group for which they are destined.

The volar (anterior) interosseous nerve arises from the median at the level of the bicipital tubercle of the radius, and runs downward, on the interosseous membrane, accompanied by the volar (anterior) interosseous artery. It passes under cover of the pronator quadratus, and pierces the deep surface of that muscle, which it supplies. The volar interosseous nerve also furnishes a twig to the front of the wrist-joint, and supphes the flexor digitorum profundus and the flexor poUicis longus. The nerve to the former muscle arises from the volar interosseous near its commencement; it supplies the outer two divisions of the muscle, and it communicates within the substance of the muscle with twigs derived from the ulnar nerve.

It also supplies a branch to the interosseous membrane which runs downward upon, or in, the membrane, supplying it and giving branches to the volar (anterior) interosseous and nutrient arteries and to the periosteum of the radius, the ulna, and the carpus.

The palmar cutaneous branch arises immediately above the transverse carpal (anterior annular) ligament and passes between the tendons of the flexor carpi radialis and the palmaris longus. It then crosses the superficial surface of the transverse carpal ligament, and is distributed to the integument and fascia on the central, depressed surface of the palm. It also supplies a few twigs to the medial border of the thenar eminence; these twigs communicate with the musculo-cutaneous and superficial radial nerves.

The three common volar digital nerves pass in the palm of the hand dorsal to the superficial palmar arch and its digital branches, while the proper volar digitals, branches of these nerves, lie on the volar side of the digital arteries.

The first of the common volar digital nerves gives off a branch to supply the abductor pollicis, the opponens, and the superficial head of the flexor pollicis brevis, and joins by a delicate branch with the deep branch of the ulnar nerve. It then divides into three proper volar digitals. The lateral of these passes obhquely across the long flexor tendon of the thumb and runs along the radial border of the thumb to its extremity. It gives numerous branches to the pulp of the thumb, and a strong twig which passes to the dorsum to supply the matrix of the nail. The second of these proper volar digitals supplies the medial side of the volar aspect of the thumb and gives off a twig to the matrix of the thumb nail. The third supplies the radial side of the second digit and gives a twig to the flrst lumbrical muscle.

The second common volar digital sends a twig to the second lumbrical muscle, and divides a httle above the metacarpo-phalangeal articulation into two proper volar digitals, which respectively supply the adjacent sides of the second and third digits.

The third common volar digital communicates with the ulnar nerve, often gives a branch to the third lumbrical muscle, and divides into two proper volar digitals which supply the adjacent sides of the third and fourth digits.

As the proper volar digitals pass along the margins of the fingers they give off twigs for the innervation of the skin on the dorsum of the second and third phalanges and the matrix of their nails. Each of the nerves terminates in filaments to the pulp of the finger.

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