The Spinal Nerves脊神经
The spinal nerves are formed by the union of ventral and dorsal spinal nerve roots which are attached in series to the sides of the spinal cord.There are 31 pairs of these nerves grouped as follows:cervical，8；thoracic，12；lumber，5；sacral，5；coccygeal，1.The abbreviations C，T，L，S and Co.，followed by the appropriate numeral are commonly used to identify the individual nerves.They emerge through the intervertebral foramina.The first cervical nerve escapes from the vertebral canal between the occipital bone and the atlas，and is therefore called the suboccipital nerve；the eighth issues between the seventh cervical and first thoracic vertebrae.
Each nerve is connected with the spinal cord by ventral and dorsal roots，the latter being characterized by the presence of a spinal ganglion.
The ventral(anterior)roots contain the axons of cells in the anterior and lateral grey columns of the spinal cord.Each root emerges as a series of rootlets arranged in two or three irregular rows over a distance of about 3 mm across the anterolateral aspect of the spinal cord.
The dorsal(posterior)roots contain the processes of cells in the spinal ganglia which are swellings on the roots.Each root consists of two fascicles，medial and lateral，each of which divides of rootlets entering along the posterolateral sulcus.Dissections in a number of mammals，including man，have shown that the dorsal nerve roots of adjacent segments are often interconnected by fine，oblique bundles of nerve fibres，particularly in the lower cervical and lumbosacral regions of the cord.
The spinal ganglia are collections of nerve cells on the dorsal roots of spinal nerves.Each ganglion is oval，reddish，and in its sizes related to that of the nerve root on which it is situated；it is bifid medially where the two bundles of the dorsal nerve root emerge from it to approach and enter the cord.The ganglia are usually in the intervertebral foramina,immediately lateral to the sites where the nerve roots perforate the dura mater；the ganglia of the first and second cervical nerves，however，lie on the vertebral arches of the atlas and axis，and those of the sacral nerves are inside the vertebral canal，while that of the coccygeal nerve is usually within the dura mater.
Applied Anatomy.The nerve roots may be compressed or otherwise irritated in their course from the spinal cord to their exit through the vertebral foramina.In the cervical region，disease of a vertebral body，degeneration of an intervertebral disc or osteoarthritis of the intervertebral joints may affect nerve roots as they traverse the intervertebral foramen,causing pain,diminished cutaneous sensibility and some muscular weakness in the field of supply.In the lumbar region,posterior protrusion of an intervertebral disc or rupture of its annular fibres with herniation of the nucleus pulposus is very common，affecting particularly the disc between the fifth lumbar and the first sacral vertebrae，or that between the fourth and fifth lumbar vertebrae.These posterior disc lesions may compress one or more nerve roots as they pass towards their intervertebral foramina，resulting in low back pain(‘lumbago’)，with or without radiation of the pain to one or both lower limbs(‘sciatica’)，diminished cutaneous sensibility in the area of supply and weakness of the muscles innervated.Less commonly，tumors in the vertebral canal，or spinal bifida with meningomyelocele，may cause lesions of one or more nerve roots of the cauda equina.
The Brachial Plexus
The brachial plexus is formed by the union of the ventral rami of the lower four cervical nerves and the greater part of the ventral ramus of the first thoracic nerve; the fourth cervical nerve usually gives a branch to the fifth cervical,and the first thoracic nerve frequently receives one from the second thoracic.The contributions made to the plexus by C4 and T2 are subject to frequent variation.When the branch from C4 is large,the branch from T2 is frequently absent and the branch from T1 is reduced in size.This constitutes the prefixed type of plexus.On the other hand the branch from C4 may be very small or entirely absent.In that event the contribution of C5 is reduced in size but that of T2 is larger and branch from T2 is always present.This arrangement constitutes the postfixed type of plexus.These nerves constitute the roots of the plexus.The roots are nearly equal in size，but the way in which they form the plexus is subject to some variation.The following is，however，the most constant arrangement.The fifth and sixth cervical nerves unite at the lateral border of the scalenus medius to form the upper trunk of the plexus.The eighth cervical and first thoracic nerves unite behind the scalenus anterior to form the lower trunk of the plexus，while the seventh cervical nerve itself constitutes the middle trunk.These three trunks run downwards and laterally and just above or behind the clavicle，each splits into an anterior and a posterior division.The anterior divisions of the upper and middle trunks unite to form a cord，which is situated on the lateral side of the axillary artery,and is called the lateral cord of the plexus.The anterior division of the lower trunk passes down at first behind and then on the medial side of the axillary artery,and forms the medial cord of the brachial plexus; this cord frequently receives fibres from the seventh cervical nerve.The posterior divisions of all three trunks unite to form the posterior cord of the plexus，which is situated at first above and then behind the axillary artery.The posterior division of the lower trunk is very much smaller than the others，and contains few fibres form the first thoracic nerve.It is frequently derived from the eighth cervical nerve before the trunk is formed.
Morphologically the brachial plexus still shows，despite much adaptation to the evolutionary changes in the upper limb musculature,a clear reflexion of the original flexor-extensor organization of a primitive fin.The posterior cord represents the extensor nerve supply,the medial and lateral cords,the flexor supply.The migration of muscle masses has in some instance modified this basic pattern; for example,the brachialis and the anterior part of deltoid are both supplied(the former only in part)from ‘extensor’ nerves.For details of the comparative morphology of the plexus consult footnote reference.
Relations of the Brachial Plexus
In the neck，the brachial plexus lies in the posterior triangle in the angle between the clavicle and the lower part of the posterior border of the sternocleidomastoid，being covered by the skin，platysma and deep fascia.When the arm is by the side,it can be felt in this situation as a bunch of tense cords.The plexus is crossed by the supraclavicular nerves，the nerve to the subclavius,the inferior belly of the omohyoid,external jugular vein and the transverse cervical artery.It emerges between the scalenus anterior and scalenus medius；its proximal part is above the third part of the subclavian artery，while the lower trunk is posterior to the artery；the plexus next passes behind the anterior convexity of the medial two-thirds of the clavicle，the subclavius and the suprascapular vessels,and lies upon the first digitation of the serratus anterior and the subscapularis.In the axilla the lateral and posterior cords of the plexus are on the medial cord behind it.The cords surround the second part of the axillary artery on three sides,the medial cord lying on the medial side,the posterior cord behind，and the lateral cord on the lateral side of the artery.In the lower part of the axilla the axilla the cords split into the nerves for the upper limb.With the exception of the medial root of the medial nerve，the branches of the three cords bear the same relationships to the third part of the axillary artery as the cords from which they spring bear to the second part，i.e.branches from the lateral cord are lateral,branches of the posterior cord are behind,and branches of the medial cord are medial to the artery.
Close to their exit from the intervertebral foramina the fifth and sixth cervical nerves receive grey rami communicates from the middle cervical ganglion,and the seventh and eighth cervical similar rami from the cervicothoracic ganglion of the sympathetic trunk.The first thoracic nerve also receives a grey ramus from，and contributes a white ramus to，the cervicothoracic ganglion.
The branches of the brachial plexus are usually considered,for convenience,as to groups—those arising above the clavicle(supraclavicular)and those below it(infraclavicular).
SUPRACLAVICULAR BRANCHES
The supraclavicular branches may be grouped as follows:(a)those arising form the roots，and(b)those arising from the trunk of the plexus.
The branches for the scaleni and longus colli muscles arise from the lower cervical nerves close to their points of exit from the intervertebral foramina.
On the scalenus anterior the phrenic nerve is joined by a branch from the fifth cervical nerve.
The dorsal scapular nerve arises from the fifth cervical nerve,pierces the scalenus medius,passes on to the deep surface of the levator scapulae,to which it occasionally gives a twig，and runs in company with the deep branch of the dorsal scapular artery on the anterior surfaces of the rhomboids:it ends by supplying these muscles.
The long thoracic nerve is usually formed by three roots from the fifth,sixth and seventh cervical nerves,but the root from the seventh nerve may by absent.(A study of seventy dissections of the nerve demonstrated all three roots in only 42 percent of cases.)The upper two roots pierce the scalenus medius obliquely,uniting cither in the substance of the muscle or on its lateral surface,and the nerve so formed descends dorsal to the brachial plexus and the first part or the axillary artery.Having crossed the upper border of the serratus anterior to gain its outer surface，it is soon joined by the root fromC.7，which emerges from the interval between the scalenus anterior and the scalenus medius at a lower level and descends on the lateral surface of the latter muscle.The nerve is continued downwards to the lower border of the serratus anterior，supplying，in its course，filaments to each of its digitations.
The nerve to the subclavius is small and is derived near the point of junction of the fifth and sixth cervical nerves; it descends in front of the plexus and the third part of the subclavian artery,and is usually connected by a filament with the phrenic nerve.It then passes above the subclavian vein and reaches the subclavius，which it supplies.
The suprascapular nerve is a large branch from the superior trunk of the brachial plexus.It runs laterally deep to the trapezius and the omohyoid，and enters the supraspinous fossa through the suprascapular notch,inferior to the superior transverse scapular ligament:it then runs deep to the supraspinatus，and curves round the lateral border of the spine of the scapula in company with the suprascapular artery to gain the infraspinous fossa.In the supraspinous fossa it gives two branches to the supraspinatus and articular filaments to the shoulder joint and acromioclavicular joint；and in the infraspinous fossa it gives two branches to the infraspinatus,besides some filaments to the shoulder joint and scapula.
INFRACLAVICULAR BRANCHES
The infraclavicular branches are derived from the three cords of the brachial plexus,but their fibres may be traced through the plexus to the spinal nerves from which they originate.
The pectoral nerves supply the pectoralis major and pectoralis minor.
The lateral pectoral nerve，the larger of the two，may arise by two roots from the anterior divisions of the upper and middle trunks，or by a single root from the point where these divisions unite to form the lateral cord of the plexus；it receives its fibres from the fifth，sixth and seventh cervical nerves.It crosses the axillary artery and vein anteriorly,pierces the claripectoral fascia,and is distributed to the deep surface of the pectoralis major.It sends a filament to join the medial pectoral nerve and forms with it a loop in front of the first part of the axillary artery; through this loop the lateral pectoral nerve distributes some fibres to the pectoralis minor.
The medial pectoral nerve receives its fibres from the eighth cervical and first thoracic nerves，and arises from the medial cord of the plexus while that cord is still posterior to the axillary artery.It curves forwards between the axillary artery and vein，and unites in front of the artery with a filament from the lateral pectoral nerve.It then enters the deep surface of the pectoralis minor and supplies that muscle.Two or three branches pierce the pectoralis minor，and others may pass round its inferior border，to end in the pectoralis major.
The subscapular nerves，two in number，spring from the posterior cord of the plexus，and through it from the fifth and sixth cervical nerves.
The superior subscapular nerve,the smaller,enters the subscapularis at a cranial level，and is frequently represented by two branches.
The inferior subscapular nerve supplies the caudal part of subscapularis,and ends in teres major; the latter muscle is sometimes supplied by a separate branch.
The thoracodorsal nerve,a branch of the posterior cord of the plexus,derives its fibres from the sixth,seventh and eighth cervical nerves; it arises between the upper and lower subscapular nerves and then accompanies the subscapular artery along the posterior wall of the axilla and supplies the latissimus dorsi，in which it may be traced as far as the lower border of the muscle.
The axillary(circumflex humeral)nerve arises from the posterior cord of the brachial plexus,its fibres being derived from the fifth and sixth cervical nerves.It lies at first on the lateral side of the radial nerve and is placed behind the axillary artery，and in front of the subscapularis.At the lower border of that muscle it winds backwards in close relation to the lowest part of the articular capsule of the shoulder joint，and，in company with the posterior circumflex humeral vessels,passes through a quadrangular space bounded above by the subscapularis，in front，and the teres minor，behind，below by the teres major，medially by the long head of the triceps，and laterally by the surgical neck of the humerus.The nerve ends by dividing into an anterior and a posterior branch.
The anterior branch,accompanied by the posterior circumflex humeral vessels，winds round the surgical neck of the humerus，deep to the deltoid，as far as the anterior border of the muscle，supplying it，and giving a few small cutaneous branches which pierce the muscle and ramify in the skin covering its lower part.
The posterior branch supplies the teres minor and the posterior part of the deltoid; upon the branch to the teres minor an oval enlargement(pseudoganglion)usually exists.The posterior branch pierces the deep fascia at the lower part of the posterior border of the deltoid and is continued as the upper lateral cutaneous nerve of the arm，which supplies the skin over the lower part of the deltoid and the skin covering the upper part of the long head of the triceps.
The trunk of the axillary nerve gives an articular filament which enters the shoulder joint below the subscapularis.
The musculocutaneous nerve arises from the lateral cord of the brachial plexus，opposite the lower border of the pectoralis minor，its fibres being derived from the fifth,sixth and seventh cervical nerves,It pierces the coracobrachialis and runs downwards and laterally between the biceps and the brachialis to reach the lateral side of the arm；a little beyond the elbow it pierces the deep fascia on the lateral side of the tendon of the biceps and is continued into the forearm as the lateral cutaneous nerve of the forearm.A line drawn distally from the lateral side of the third part of the axillary artery,and laterally across the elevations produced by the coracobrachialis and the biceps to the lateral side of the biceps tendon of insertion,indicates the position of the musculocutaneous nerve relative to the surface; but variation in the point of entry of the nerve into the coracobrachialis may modify considerably its surface marking.In its course through the arm it supplies the coracobrachialis,both heads of the biceps and the greater part of the brachialis.The branch to the coracobrachialis leaves the musculocutaneous before that nerve enters the muscle; it receives its fibres from the seventh cervical nerve,and in some instances arises directly from the lateral cord of the brachial plexus.The branches to the biceps and brachialis leave the musculocutaneous nerve after it has pierced the coracobrachialis; that supplying the brachialis gives a filament to the elbow joint.The nerve also sends a small branch to the humerus；this branch enters the bone with the nutrient artery.
The lateral cutaneous nerve of the forearm passes deep to the cephalic vein，and descends along the radial border of forearm to the wrist.It supplies the skin over the lateral half of the anterior surface of the forearm and distributes branches which turn round the radial border of the forearm to communicate with the posterior cutaneous nerve of the forearm and the terminal branch of the radial nerve.At the wrist joint it is placed in front of the radial artery，and some filaments,piercing the deep fascia,accompany that vessel to the dorsal surface of the carpus.The nerve then passes downwards to the base of the thenar eminence，where it ends in cutaneous filaments.It communicates with the terminal branch of the radial nerve，and with the palmar cutaneous branch of the median nerve.
The musculocutaneous nerve presents frequent variations.It may run behind the coracobrachialis or it may adhere for some distance to the median nerve and then pass behind the biceps instead of through the coracobrachialis.Some of the musculocutaneous nerve and then leave it to join their proper trunk；less frequently the reverse is the case,and the median nerve sends a branch to join the musculocutaneous nerve.Occasionally it gives a filament to the pronator teres and,sometimes,it may replace the branches of the radial nerve to the dorsal surface of the thumb.
The medial cutaneous nerve of the forearm arises from the medial cord of the brachial plexus.It is derived from the medial cord of the brachial plexus.It is derived from the eighth cervical and first thoracic nerves，and at its commencement is placed between the axillary artery and vein.Near the axilla it supplies a filament which pierces the fascia and is distributed to the skin covering the biceps，almost as far as the elbow.The nerve then runs down the arm on the medial the basilic vein about the middle of the arm,and divides into an anterior and a posterior branch.
The anterior branch,the larger,usually passes in front of,but occasionally behind，the median cubital vein.It then descends on the front of the medial side of the forearm，distributing filaments to the skin as far as the wrist，and communicating with the palmar cutaneous branch of the ulnar nerve.
The posterior branch passes obliquely downwards on the medial side of the basilica vein，in front of the medial epicondyle of the humerus，winds round to the back of the forearm，and descends on its medial side as far as the wrist，distributing filaments on the skin.It communicates with the medial cutaneous nerve of the arm,the posterior cutaneous nerve of the forearm,and the dorsal branch of the ulnar.
The medial cutaneous nerve of the arm is distributed to the skin on the medial side of the arm.It is the smallest branch of the branchial plexus，and arising from the medial cord,receives its fibres from the eighth and first thoracic nerves.It passes through the axilla and crosses in front of，or behind，the axillary vein.It then runs on the medial side of this vein and communicate with the intercostobrachial nerve.It descends along the medial side of the brachial artery and basilic vein to the middle of the upper arm，where it pierces the deep fascia，and is distributed to the skin of the medial side of the distal third of the arm，extending on to its anterior and posterior aspects；some filaments reach the skin in front of the medial epicondyle and others over the olecranon.It communicates with the posterior branch of the medial cutaneous nerve of the forearm.
In some subjects the medial cutaneous nerve of the arm and the intercostobrachial nerve are connected by two or three filaments，which form a plexus in the axilla.In others the intercostobrachial nerve is large and may be reinforced by a part of the lateral cutaneous branch of the third intercostal nerve；it then takes the place of the media brachial cutaneous nerve,receiving from the brachial plexus a communicating filament which represents the latter nerve；occasionally this filament is absent.
THE MEDIAN NERVE
The median nerve arises by two roots，one from the lateral cord，C(5)，6，7，and the other from the medial …cord C.8，T.of … the brachial plexus；the roots embrace the third part of the axillary artery,uniting either in front,or on the lateral side of the artery.Often some of the fibres derived from C.7 leave the lateral root in the lower part of the axilla and pass distally and medially behind the medial root and usually in front of the axillary artery to join the ulnar nerve.These fibres from C.7 may arise from the lateral cord or even directly from the seventh cervical nerve root of the brachial plexus.On clinical grounds they are believed to be mainly motor fibres to the flexor carpi ulnaris.If the lateral root is small the musculocutaneous nerve(C.5，6，7)sends a communicating branch to the median nerve in the arm.The median nerve descends into the arm，lying at first lateral to the brachial artery; about the level of insertion of the coracobrachialis it crosses in front of(rarely behind)the artery and descends on its medial side to the cubital fossa，where it lies behind the bicipital aponeurosis and in front of the brachialis，which separates it from the elbow joint.The nerve enters the forearm between the two heads of the pronator teres，crossing from the medial to the lateral side of the ulnar artery,but separated from it by the deep head of the muscle.(The nerve has been observed to pass instead through the humeral fibres of this muscle)It then passes deep to the tendinous bridge that connects the humero-ulnar to the radial head of the flexor digitorum superficialis and descends through the forearm deep to,and adherent to,the flexor digitorum superficialis，lying on the flexor digitorum profundus.About 5cm above the flexor retinaculum it emerges from behind the lateral edge of the flexor digitorum superficialis and，becoming more superficial just above the wrist，it lies between the tendons of the flexor digitorum superficialis and the flexor carpi radialis,projecting laterally from under cover of the tendon of the palmaris longus(if present ).The nerve then passes deep to the flexor retinaculum to gain the palm of the hand.In the forearm it is accompanied by the median branch of the anterior interosseous artery and its course can be represented on the surface by a line running approximately along the mid-line of the front of the forearm from the medial side of the end of the brachial artery in the cubital fossa.
Branches of the median nerve in the Arm
These comprise vascular branches to the brachial artery and usually a branch to the pronator teres which is given off at a variable distance proximal to the elbow joint。
Branches of the Median Nerve in the Forearm
These are muscular，articular，anterior interosseous palmar cutaneous and communicating.The muscular branches are，with one exception，given off in the proximal part of the forearm(near the elbow)to the superficial flexor forearm muscles(except the flexor carpi ulnaris)，namely,the pronator teres,flexor carpi radialis，palmaris longus and flexor digitorum superficialis.The branch of the median nerve supplying the part of the flexor digitorum superficialis that passes to the index finger arises near the middle of the forearm and it may be a branch of the anterior interosseous nerve.Articular branches arise from the median nerve at，or just distal to，the level of the elbow joint as well as the proximal radio-ulnar joint.
The anterior interosseous nerve is given off from the posterior surface of the median nerve as it passes between the two heads of the pronator teres and just distal to the origin of the branches of the median nerve to the superficial flexor forearm muscles described above.Accompanied by the anterior interosseous artery，it passes distally on the front of the interosseous membrane，between and deep to the flexor pollicis longus and the flexor digitorum.It supplies a number of branches to these two muscles；those supplying the flexor digitorum profundus are limited to the lateral part of the muscle which sends tendons to the index and middle fingers.The nerve finally passes beneath the pronator quadratus，supplying a branch that enters its deep surface and ends by supplying the distal radio-ulnar，radio-carpal and carpal joints.
The palmar cutaneous branch of the median nerve commences a short distance above the flexor retinaculum; it pierces the deep fascia or proximal edge of the retinaculum and divides into lateral branches which supply the skin over the thenar eminence and communicate with the lateral cutaneous nerve of the forearm，and medial branches which supply the skin of the central part of the palm and communicate with the palmar cutaneous branch of the ulnar nerve(https://www.daowen.com)
The communicating branch of the median nerve，which may take the form of a number of slender nerves，is frequently present.It arises from the median nerve(or sometimes its anterior interosseous branch)in the upper part of the forearm and passes distally and medially between the flexor digitorum superficialis and flexor digitorum profundus and behind the ulnar artery to join the ulnar nerve.This communication is of importance in relation to the anomalous nerve supply of certain hand muscles.
The Median Nerve in the Hand
Just proximal to the flexor reticulum,the nerve is lateral to the tendons of the flexor digitorum superficialis,but dorsal to the retinaculum it lies immediately deep to the retinaculum and on the front of the tendons,in the limited space of the ‘carpal tunnel’，i.e.the space bounded in front bl the retinaculum and behind by the anterior surfaces of the carpal bones; in certain circumstances the never may be compressed in this situation(carpal tunnel syndrome).Immediately distal to the flexor retinaculum the never becomes enlarged and flattened and usually divides into five or six branches,though the exact mode and level of division of the nerve variable.
The muscular branch is a short stout nerve that arises from the lateral side of the median nerve; it may be the first branch of the median nerve in the palm or it may arise as a terminal branch at the same level as the digital branches.It runs laterally，just distal to the flexor retinaculum，with a slight recurrent curve proximally,lying beneath the lateral portion of the palmar aponeurosis covering the thenar muscles.It winds round the distal border of the flexor retinaculum and comes to lie on the superficial surface of the flexor pollicis brevis,usually giving a branch to this surface of the muscle.It then continues on the superficial surface of the muscle，or it may pass through the muscle itself.It gives a branch to the abductor pollicis brevis，which enters the muscle at its medial edge，then passes deep to this muscle to supply the opponens pollicis,entering the muscle through its medial edge.Rarely，the terminal part of the nerve gives a branch to the first dorsal interosseus，which may be the sole or part nerve supply of this muscle.
The muscular branch may arise in the carpal tunnel and pierce the flexor retinaculum，a point of surgical importance.
The palmar digital branches—most frequently the median nerve divides into four or five digital branches，though it often at first divides into two divisions，a lateral，which provides the digital branches to the thumb and radial side of the index finger，and a medial，which supplies the digital branches to the adjacent sides of the index，middle and ring fingers.Other variations in the mode of terminal branching of the nerve occur，as well as in the level in the hand at which the branching takes place.The arrangement of the digital nerve is commonly as follows.They pass distally，deep to the superficial palmar arch and its digital branches，lying at first in front of the long flexor tendons.Two proper palmar digital nerves，which may arise by a common stem，pass to the sides of the the thumb,the one supplying its lateral side crossing in front of the tendon of the flexor pollicis longus.The proper palmar digital nerve to the lateral side of the index finger supplies a branch to the first lumbrical.Two common palmar digital nerves pass distally between the long flexor tendons，the lateral one dividing in the distal palm into two proper palmar digital nerves traversing the adjacent sides of the index and middle fingers,and the medial one dividing into two proper palmar digital nerves which supply the adjacent sides of the middle and ring fingers.The lateral common digital nerve gives a branch to the second lumbrical，and the medial one receives a communicating twig from the common palmar digital branch of the ulnar nerve and may supply a branch to the third lumbrical.In the distal part of the palm，the digital arteries pass deeply between the divisions of the digital nerves,so that on the sides of the digits the nerves lie immediately in front of the arteries.In most cases the median nerve supplies palmar cutaneous digital branches to the lateral three and one-half digits(thumb,index,middle and lateral side of the ring finger); in some cases the lateral side of the ring finger is supplied by the ulnar nerve.The proper palmar digital nerves that run along the medial side of the index finger,both sides of the middle finger and the lateral side of the ring finger,enter the bases of these digits in the fat between the slips into which the central portion of the palmar aponeurosis divides.
They pass，with the lumbricals and palmar digital arteries，dorsal to the superficial transverse metacarpal ligament and ventral to the deep transverse metacarpal ligament.In the digits，the nerve run distally on the side of the long flexor tendons,outside their fibrous sheaths,on the plane of the anterior surfaces of the phalanges.And immediately in front of the accompanying digital arteries.Each nerve gives off several branches to the skin on the front and sides of the digit，many of which end in lamellated corpuscles，and branches to the metacarpophalangeal and interphalangeal joins.Branches are also supplied to the fibrous sheaths of the long flexor tendons,to the digital arteries(vasomotor),and to the sweat glands(secretomotor)，A little beyond the base of the distal phalanx，the digital nerve gives off a branch passing dorsally to supply the nail bed，while the main nerve divides into branches supplying the skin of the terminal part of the digit and the pulp.Just beyond the base of the proximal phalanx，each palmar digital nerve gives off a dorsal branch that runs obliquely,distally and dorsally,to supply branches to the skin over the back of the middle and distal phalanges.The proper palmar digital nerves to the thumb and lateral side of the index finger run from beneath the and lateral edge of the central part of the palmar aponeurosis，in company with the long flexor tendons to these digits，and in the digits themselves have the same arrangements as described above,but in the case of the thumb small branches from the distal part of the palmar digital nerves are given off to supply the skin on the back of the distal phalanx only.
In addition to the branches of the median nerve described above,variable vasomotor branches pass to supply the radial and ulnar arteries and their branches.Some of the intercarpal，carpometacarpal and intermetacarpal joints are said to receive branches from the median nerve or its anterior interosseous branch,though the precise details of the origin and distribution of these branches are uncertain.
THE ULNAR NERVE
The ulnar nerve(Figure-5)arises from the medial cord of the brachial plexus,C,8,T 1,though,as described above,it often receives fibres from the seventh cervical nerve.It runs distally through the axilla on the medial side of the axillary artery，intervening between it and the axillary vein，and continues distally on the medial side of the brachial artery as the middle of the arm.Here it pierces the medial intermuscular septum,and inclines medially,as it descends in front of the medial head of the triceps to the interval between the medial epicondyle and the olecranon，accompanied by the superior ulnar collateral artery.At the elbow it lies in a groove on the dorsum of the medial epicondyle,and as it enters the forearm between the two heads of the flexor carpi ulnaris，it lies on the posterior and oblique parts of the ulnar collateral ligament of the elbow joint.It descends along the medial side of the forearm,lying upon the flexor digitorum profundus; its upper half is covered by the flexor carpi ulnaris；its lower half lies on the lateral side of this muscle，and is covered by the skin and fasciae.In the upper one-third of the forearm,the ulnar nerve is separated from the ulnar artery by a considerable interval，but in the rest of its extent it lies close to the medial side of the vessel.About 5 cm above the wrist it gives off a dorsal branch，and it is then continued downwards into the hand，passing in front of the flexor retinaculum on the lateral side of the pisiform bone and lying medial to and somewhat behind the ulnar artery.In company with the artery the nerve passes behind the superficial part of the retinaculum and ends by dividing into a superficial and a deep terminal branch.Its relationship to the branchial artery in the arm and to the medial epicondyle at the elbow renders the nerve easy to map out in the upper part of its course；a line drawn from the medial epicondyle to the lateral edge of the pisiform bone represents its course through the forearm.
The branches of the ulnar nerve are:articular，muscular，palmar cutaneous，dorsal，superficial and deep terminal.
The articular branches to the elbow joint are several small filaments which issue from the nerve as it lies between the medial epicondyle and olecranon.Other articular branches are described below.
The muscular branches，two in number，begin near the elbow；one supplies the flexor carpi ulnaris,the other,the medial half of the flexor digitorum profundus.
The palmar cutaneous branch arises about the middle of the forearm,and descends on the ulnar artery，giving some filaments to the vessel.It perforates the deep fascia and ends in the skin of the palm，after communicating with the palmar branch of the median nerve.It sometimes supplies the palmaris brevis.
The dorsal branch arises about 5 cm above the wrist；it passes distally and backwards deep to the flexor carpi ulnaris，perforates the deep fascia，and，running along the medial side of the back of the wrist and hand，divides into two，frequently three，dorsal digital nerves；one supplies the medial side of the little finger；the second the adjacent sides of the little and ring finger.The third，when present，supplies the adjoining sides of the ring and middle fingers，but it may be replaced，wholly or partially，by a branch of the radial nerve，with which it always communicates on the dorsum of the hand.In the little finger the dorsal digital nerves extend only as far as the base of the distal phalanx,and in the ring finger as far as the base of the middle phalanx; the more distal parts of these digits are supplied by dorsal branches derived from the digital branches of the ulnar and -on the lateral side of the ring finger—medial nerves.
The superficial terminal branch supplies the palmaris brevis and the skin on the medial side of the hand，and divides into two palmar digital nerves，which can be compressed against the hook of the hamate bone.
One of these palmar digital nerves supplies the medial side of the little finger；the other(a common palmar digital nerve)sends a twig to join the median nerve and then divides into two proper digital nerves for the adjoining sides of the little and ring fingers.The proper digital branches are distributed to the fingers in the same manner as those of the median nerve.
The deep terminal branch,accompanied by the deep branch of the ulnar artery，passes between the abductor digiti minimi and flexor digiti minimi；it then perforates the opponens digiti minimi and follows the course of the deep palmar arch behind the flexor tendons.At its origin it supplies the three short muscles of the little finger.As it crosses the hand，it gives branches to the interossei and to the third and fourth lumbricals; it ends by supplying the adductor pollicis,the first palmar pollicis brevis.It also sends articular filaments to the wrist joint.
It has been pointed out that the medial part of the flexor digitorum profundus is supplied by the ulnar nerve；the third and fourth lumbricals which are connected with the tendons of this part of the muscle，are supplied by the same nerve.In like manner the lateral part of the flexor digitorum profundus and the first and second lumbricals are supplied by the median nerve.The third lumbrical frequently receives an additional twig from the median nerve.
The deep terminal branch of the ulnar nerve is said to give branches to some of the intercarpal,carpometacarpal and intermetacarpal joints,though,as in the case of the median nerve,the precise details of the origin and distribution of these branches are uncertain:Vascular(vasomotor)branches are given off from the ulnar nerve in the forearm and hand to supply the ulnar artery and the palmar arteries.
Anomalous Nerve Supply of Hand Muscles
The nerve supply to the short thenar muscles(flexor pollicis brevis,abductor pollicis brevis and opponens pollicis)is subject to considerable variation.From a study of the results of lesions of the median and ulnar nerves in the forearm，the following variations have been deduced，expressed here in percentages of 226 hands examined,relating to the innervation of these thenar muscles by the median nerve，the ulnar nerve or both nerves:flexor pollicis brevis-median 36,ulnar-48,both nerves 17; abductor pollicis brevis —median 95,ulnar 25,both nerves 2; opponens pollicis—median 83,ulnar 9,both nerves 75.The usual description of the nerve supply to these muscles is that they are all supplied by the median nerve; the results of the investigations cited are probably to be explained by the variable connexions between the median and ulnar nerves in the axilla，arm or forearm，whereby truly median nerve fibres are aberrantly conveyed distally in the ulnar nerve to these muscles.
An arcuate connecting loop has long been known to occur between the median and ulnar nerves in the substance of the flexor pollicis brevis,either part of which，superficial or deep，may be supplied by fibres from both nerves.In a recent survey of the considerable literature on this topic,results of dissections of 35 hands showed a loop in 77 percent of individuals，indicating that it should be a feature of ‘normal’ description.
Clinically these variations are important in that even with a complete lesion of the median nerve some of these muscles may not be paralysed,and this may lead to the erroneous conclusion that the median nerve has not suffered a complete lesion.Clinical evidence reveals that the short muscles of the thumb，whether apparently supplied by the median or ulnar nerves,receive their segmental supply from the eighth cervical and first thoracic segments of the spinal cord.
THE RADIAL NERVE
The radial nerve arises from the posterior cord of the brachial plexus,C 5，6，7，8；T 1.It is the largest branch of the brachial plexus，and it descends behind the third part of the axillary artery and the upper part of the brachial artery,and in front of the subscapularis and the tendons of the latissimus dorsi and teres major.Accompanied by the arteria profunda brachii and,later,its radial collateral branch,it inclines dorsally between the long and medial heads of the triceps.From here it passes obliquely across the back of the humerus，first between the lateral and medial heads of the triceps and then in a shallow groove deep to the lateral head of the triceps muscle.On reaching the lateral side of the humerus it pierces the lateral intermuscular septum and enters the anterior compartment of the arm.It then descends，lying deeply in the intermuscular furrow，which is bounded of the medial side by the brachialis and on the lateral side by the brachioradialis,above,and the extensor carpi radialis longus，below.On reaching the front of the lateral epicondyle，it divides into terminal rami，superficial and deep.
In the arm the radial nerve is indicated by a line drawn from the commencement of the brachial artery and carried distally and laterally across the elevations produced by the long and the lateral heads of the triceps to the junction of the upper and middle thirds of a line joining the lateral epicondyle to the deltoid tuberosity.The line of the nerve is then continued on the anterior aspect of the arm to the level of the lateral epicondyle，where it lies 1cm or less to the lateral side of the biceps tendon.
The branches of the radial nerve are:muscular，cutaneous，articular and superficial and deep terminal branches.
The muscular branches of the radial nerve supply the triceps,anconeus,brachioradialis,extensor carpi radialis longus and brachialis,and are grouped as medial，posterior and lateral.
The medial muscular branches arise from the radial nerve on the medial side of the arm and supply the medial and long heads of the triceps; the branch to the medial head is a long，slender filament，which lies close to the ulnar nerve as far as the distal third of the arm，and is therefore frequently named the ulnar collateral nerve.
The posterior muscular branch,of large size，arises form the nerve as it lies in the groove.It divides into filaments which supply the medial and lateral heads of the triceps and the anconeus.The branch for the latter muscle is a long nerve which descends in the substance of the medial head of the triceps，and gives numerous branches to it.It is accompanied by the middle collateral branch of the arteria profunda brachii，and passes behind the elbow joint to end in the anconeus.
The lateral muscular branches arise from the nerve as it lies in front of the lateral intermuscular septum; they supply the lateral part of the brachialis,the brachioradialis，and the extensor carpi radialis longus.
The cutaneous branches of the radial nerve are the posterior cutaneous and the lower lateral cutaneous nerves of the arm and the posterior cutaneous nerve of the forearm.
The posterior cutaneous nerve of the arm,of small size,arises in the axilla and passes to the medial side of the arm to supply the skin on its dorsal surface nearly as far as the olecranon.It crosses posterior to，and communicates with，the intercostobrachial nerve.
The lower lateral cutaneous nerve of the arm perforates the lateral head of the triceps just below the insertion of the deltoid.It then passes to the front of the elbow，lying close to the cephalic vein，and supplies the skin of the lateral part of the lower half of the arm.
The posterior cutaneous nerve of the forearm arises in common with the preceding branch.Perforating the lateral head of the triceps，it descends along the lateral side of the arm，and then along the dorsum of the forearm to the wrist，supplying the skin in its course，and joining，near its termination，with dorsal branches of the lateral cutaneous nerve of the forearm.
The articular branches of the radial nerve are distributed to the elbow joint.
The superficial terminal branch descends from the front of the lateral epicondyle along the front of the lateral side of the upper two-thirds of the forearm.Lying at first upon the supinator，lateral to the redial artery and behind the brachioradialis.In the middle third of the forearm it lies behind the brachioradialis，but is now close to the lateral side of the artery.Here it lies first on the pronator teres,next on the radial head of the flexor digitorum superficialis and then on the flexor pollicis longus.It quits the artery about 7cm above the wrist，passes deep to the tendon of the brachioradialis and，winding round the lateral side of the radius as it descends,pierces the deep fascia and divides into five，sometimes four，dorsal digital nerves.
The dorsal digital nerves are small and four or five in number.The first supplies the skin of the radial side of the thumb and the adjoining part of the thenar eminence，communicating with branches of the lateral cutaneous nerve of the forearm; the second supplies the medial side of the thumb; the third,the lateral side of the index finger; the fourth,the adjoining sides of the index and middle fingers；the fifth communicates with a filament from the dorsal branch of the ulnar nerve and supplies the adjoining sides of the middle and ring fingers,but it is frequently replaced by the dorsal branch of the ulnar nerve.On the dorsum of the hand the superficial branch of the radial nerve usually communicates with the posterior and lateral cutaneous nerves of the forearm.The digital nerves to the thumb reach only as far as the root of the nail; those to the index finger as far as the middle of the middle phalanx and those to the middle and ring fingers not farther than the proximal interphalangeal joints.The remaining distal areas of skin of the dorsal surface of these digits are supplied by the palmar digital branches of the median and ulnar nerves.The superficial radial nerve may supply the whole of the dorsum of the hand；for this and other variations consult footnote reference.
The deep terminal branch(posterior interosseous nerve)winds to the back of the forearm round the lateral side of the radius between the two planes of fibres of the supinator.It gives a branch to the extensor carpi radialis brevis，and another to the supinator before it enters the latter muscle,and as it traverses its substance it supplies additional branches to it.The branch to the extensor carpi radialis brevis may spring from the commencement of the superficial branch of the radial nerve.As soon as it emerges from the supinator on the back of the forearm the deep branch of the radial nerve gives off three short branches—to the extensor digitorum,extensor digiti minimi and extensor carpi ulnaris—and two long branches—a medial to the extensor pollicis longus and the extensor indicis，and a lateral，which supplies the abductor pollicis longus and ends in the extensor pollicis brevis.The nerve lies at first between the superficial and the deep muscles of the back of the forearm，but，at the distal border of the extensor pollicis brevis,it passes deep to the extensor pollicis longus and,diminished to a fine thread，runs down on the dorsal aspect of the interosseous membrane of the forearm.Finally it reaches the dorsum of the carpus，where it presents a flattened and somewhat expanded termination(‘pseudoganglion’)from which filaments are distributed to the ligaments and articulations of the carpus.
Articular branches from the deep branch of the radial nerve are distributed to the carpal and distal radio-ulnar joints and to some of the intercarpal and intermetacarpal joints，while the digital branches of the radial nerve supply branches to the metacarpophalangeal and proximal interphalangeal joints of the appropriate digits.
Applied Anatomy.The brachial plexus may be injured by falls from a height on to the side of the head and shoulder，whereby the nerves of the plexus are violently stretched；the upper trunk of the plexus sustains the greatest injury，and the subsequent paralysis may be confined to the muscles supplied by the fifth nerve—the deltoid，biceps，brachialis and brachioradialis，with sometimes the supraspinatus，infraspinatus and supinator.The position of the limb under such conditions，is characteristic:the arm hangs by the side and is rotated medially；the forearm is extended and pronated.The arm cannot be raised from the side；all power of flexion of the elbow is lost，as is also supination of the forearm.This is known as Erb’s paralysis，and a very similar condition is occasionally met with in newborn children，either from injury to the upper trunk from the pressure of the forceps used in effecting delivery,or from traction of the head in breech presentations.A second variety of partial palsy of the brachial plexus is known as Klumpke’s paralysis.In this it is the eighth cervical and first thoracic nerves that are injured，eighth before or after they have joined to form the lower trunk.The subsequent paralysis affects，principally,the intrinsic muscles of the hand and the flexors of the wrist and fingers.
The brachial plexus may also be injured by direct violence or gunshot wounds，by violent traction on the arm,or by efforts at reducing a dislocation of the shoulder joint；the amount of paralysis will depend upon the amount of injury to the constituent nerves.When the entire plexus is involved，the whole of the upper extremity will be paralysed and anaesthetic.In some cases the injury appears to be rather a tearing away of the roots of the nerves from the spinal cord than a rupture of the nerves themselves，and where this involves the first thoracic nerve the pupil on the same side may be constricted,on account of damage to the preganglionic fibres emerging in it to supply the dilatator pupillae.The brachial plexus in the axilla is often damaged from the pressure of a crutch，producing the condition known as ‘crutch paralysis’.In these cases the radial is the nerve most frequently implicated；the ulnar nerve suffers next in frequency.The median and radial nerves often suffer from ‘sleep palsies’，paralysis from pressure coming on while the patient is profoundly asleep under the influence of alcohol or some narcotic.
Paralysis of the long thoracic nerve throws the serratus anterior out of action，and may occur in porters who have to carry heavy weights on the shoulder,for the nerve is exposed to injury as it lies in the posterior triangle of the neck.The inferior angle of the scapula is drawn towards the median plane，by the unopposed action of the rhomboids and levator scapulae,and tends to project backwards(‘winging’ of the scapula)when the arm is held horizontally forwards or when forward pushing movements are attempted against resistance.The arm cannot be raised above the horizontal unless the inferior angle of the scapula is pushed anterolaterally for the patient.
The axillary(circumflex humeral)nerve,on account of its course round the surgical neck of the humerus，is liable to be injured in fractures of this part of the bone,and in dislocations of the shoulder joint; paralysis of the deltoid,and anaesthesia of the skin over the lower part of that muscle，result.Paralysis of the deltoid renders effective abduction fo the arm impossible.The associated paralysis of the teres minor is not easily demonstrated.
The median nerve is liable to injury in wounds of the forearm.When it is completely divided proximal to its muscular and anterior interosseous branches,there is loss of flexion of the second phalanges of all the fingers,and of the terminal phalanges of the index and middle fingers.Flexion of the terminal phalanges of the ring and little fingers is affected by the section of the flexor digitorum profundus which is supplied by the ulnar nerve.There is power to flex the proximal phalanges through the interossei.The thumb cannot be opposed or abducted,nor can it be flexed at the interphalangeal joint,and it is maintained in a position of extension and adduction.There is loss in the power of pronating the forearm；the brachioradialis has the power of bringing the forearm into a position of mid-pronation，but beyond this no further pronation can be effected.The wrist can be flexed by the flexor carpi ulnaris，but flexion is combined with adduction of the hand.There is loss or impairment of sensation on the palmar surfaces of the thumb,index,middle,and radial half of the ring fingers,and on the dorsal surfaces of the same fingers over the last two phalanges，except in the thumb，where the loss of sensation is limited to the back of the distal phalanx.Owing to the paralysis of the short muscles of the thumb and the unopposed action of the extensor pollicis longus，an ‘ape-like’ hand is produced.Injury of the nerve just above the middle of the forearm may result only in weakness of flexion of the index finger(‘pointing’ index finger)，since the branch to the part of the flexor digitorum superficialis distributed to that finger arises at about the middle of the forearm.More commonly，however，the nerve is injured proximal to the flexor retinaculum，when the power of flexion of the fingers and pronation of the forearm remains intact，unless the flexor tendons are also divided.The chief effect of lesions here is usually inability to oppose the thumb,though the intact abductor pollicis longus and adductor pollicis may combine to imitate this action.As it lies deep to the flexor retinaculum，the median nerve is situated in the restricted space(carpal tunnel or carpal canal)between the retinaculum and the carpal bones.Any pathological condition which diminishes the size of this tunnel(e.g.carpal dislocation or arthritis，tenosynovitis of the long flexor tendons，etc.)will cause pressure on the nerve with resultant pain and slight sensory impairment in the digits supplied by the nerve and sometimes slight wasting of the thenar muscles，these clinical features constituting the ‘carpal tunnel syndrome’.Most commonly there is no apparent cause for this syndrome and，in any case，complete division of the retinaculum is curative.
The ulnar nerve is also liable to be injured in wounds of the forearm.The commonest cause of complete or partial ulnar nerve lesion is injury to the nerve as it lies behind the medial humeral epicondyle.Such injury leads to impaired power of adduction，and，upon an attempt being made to flex the wrist，the hand is drawn to the radial side by the flexor carpi radialis；there is inability to spread out the fingers owing to paralysis of the dorsal interossei，and for the same reason the fingers,especially the ring and little fingers,cannot be flexed at the metacarpophalangeal joints or extended at the interphalangeal joints,and the hand assumes a claw shape from the action of the opposing muscles；there is loss of power of flexion in the fourth and fifth digits；and there is inability to adduct the thumb.The muscles of the hypothenar eminence become wasted.Sensation is lost，or impaired，in the skin supplied by the nerve.
The radial nerve also is frequently injured.In consequence of its close relationship to the humerus,it may be injured,but is seldom torn,in fractures of this bone.Callus formation in repair of the fracture does not interfere with the function of the nerve.In fractures of the middle part of the humerus，the triceps is not paralysed,since its supplying nerves arise from the radial nerve more proximally.It is also liable to be contused against the bone by kicks or blows，or to be divided in wounds of the arm.When paralysed，the hand is flexed at the wrist and lies flaccid.This is known as wrist drop.The fingers are also flexed，and when an attempt is made to extend them，the last two phalanges only will be extended,through the action of the lumbrical and interosseous muscles; the first phalanges remain flexed.Extension of the wrist is impossible.Supination is completely lost when the forearm is extended on the arm，but is possible to a certain extent if the forearm be flexed to allow effective action of the biceps.The power of extension of the forearm is lost on account of paralysis of the triceps，if the injury to the nerve has taken place near its origin.As the radial nerve has only a very small area of exclusive supply,the extent of the anaesthesia associated with even severe injuries to it is surprisingly small and is confined to a limited region on the lateral part of the dorsum of the hand.
Dislocations and epiphyseal separations of the elbow and supracondylar fractures of the humerus in children frequently lead to ulnar,median or posterior interosseous injury.