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anterior muscle and the axillary artery. It continues its downward course over the axillary surface of the serratus, to the slips of which it is distributed.
There is a more or less definite relation between the roots of this nerve and the parts of the serratus muscle. The first part of the muscle is innervated by the fifth nerve alone; the second part by the fifth and sixth, or the sixth alone; the third part by the sixth and seventh, or the seventh nerve alone.
N. Suprascapularis.-The supra-scapular nerve arises from the back of the cord formed by the fifth and sixth cervical nerves in the posterior triangle of the neck. It occupies a position above the main cords of the brachial plexus, and courses downwards and laterally parallel to them towards the superior margin of the scapula. It passes through the scapular notch to reach the dorsum of the scapula. After supplying the supra-spinatus muscle it winds round the great scapular notch in company with the transverse scapular artery and terminates in the infra-spinatus muscle. It also supplies articular branches to the back of the shoulder-joint.
Pars Infraclavicularis.-The so-called infra-clavicular branches of the brachial plexus are distributed to the chest, shoulder, arm and forearm. According to their origin they are divisible into two sets-an anterior set, derived from the lateral and medial cords, and a posterior set, derived from the posterior cord. In their distribution the same arrangement is maintained. The anterior nerves of distribution, springing from the lateral and medial cords, supply the chest and the front of the limb; the posterior nerves, springing from the posterior cord, supply the shoulder and the back of the limb.
The anterior thoracic nerves are two in number, lateral and medial. The lateral anterior thoracic nerve arises from the lateral cord of the brachial plexus by three roots-from the fifth, sixth, and seventh cervical nerves. The medial anterior thoracic nerve arises from the medial cord of the plexus, from the eighth cervical and first thoracic nerves. They course downwards and forwards, one on each side of the axillary artery, and a loop of communication is formed between them in front of the artery. They are finally distributed to the pectoralis major and minor muscles (Fig. 615).
The nerves are distributed to the pectoral muscles in the following way. Two sets of branches from the lateral anterior thoracic nerve pierce the costo-coracoid membrane. The superior branches supply the clavicular part of the pectoralis major; the inferior branches are distributed to the superior fibres of the sternal portion of the muscle. The superior branches come from the fifth and sixth cervical nerves; the inferior branches, from the fifth, sixth, and seventh nerves. The pectoralis minor is pierced by two sets of nerves-the superior set is derived from the loop of communication between the two anterior thoracic nerves over the axillary artery; the inferior set is derived from the medial anterior
thoracic nerve alone. These nerves supply the pectoralis minor muscle, and, after piercing it, supply the sternal part of the pectoralis major. The inferior nerve, in many cases, sends its branches to the pectoralis major round the inferior border of the pectoralis minor, and on its way it may supply the axillary arches, if present. These two branches are derived-the superior from the seventh and eighth cervical, and first thoracic nerves; the inferior from the eighth cervical and first thoracic nerves. The pectoral muscles are thus both supplied by the two anterior thoracic nerves. The clavicular fibres of the pectoralis major are innervated by the fifth and sixth nerves; the sternal fibres, from above downwards, by the fifth, sixth, seventh, and eighth cervical, and first thoracic nerves; and the pectoralis minor is supplied by the seventh and eighth cervical, and first thoracic
The musculo - cutaneous nerve takes origin from the lateral cord of the plexus, from the fifth and sixth cervical nerves (Fig. 614). The nerve to the coracobrachialis muscle, arising from the seventh or sixth and seventh nerves, is usually associated with it. Separating from the lateral head of the median nerve, the musculo-cutaneous nerve lies at first between the coracobrachialis muscle and the axillary artery. It is then directed distally between the two parts of the coracobrachialis, and passes between the biceps and brachialis muscles, to the bend of the elbow. It pierces the deep fascia over the front of the elbow, between the biceps and brachioradialis, and terminates as the lateral cutaneous nerve for the supply of the lateral aspect of the forearm. In its course it
may send a branch under the biceps to join the median nerve.
The branches of the nerve are muscular and cutaneous. The muscular branches are supplied to the two heads of the biceps and the brachialis, as the nerve lies between the muscles. The nerve to the coracobrachialis (usually incorporated with the trunk of the musculo-cutaneous nerve) has an independent origin from
the seventh or sixth and seventh nerves. It is usually double, one branch entering each portion of the muscle. The lateral cutaneous nerve of the forearm divides into volar and dorsal branches (Fig. 616, p. 704). The volar branch runs distally along the front of the lateral aspect of the forearm to the wrist, and supplies an area extending medially to the middle line of the forearm anteriorly, and distally so as to include the ball of the thumb. It communicates, proximal to the wrist, with the superficial ramus of the radial nerve, and supplies branches to the radial artery. The dorsal branch passes backwards and distally over the extensor muscles and supplies the skin on the lateral aspect of the forearm posteriorly in its proximal three-fourths, communicating with the cutaneous branches of the radial nerve.
In addition to the above branches, the musculo-cutaneous nerve supplies in many cases the following small twigs in the arm: (1) a medullary branch to the humerus; (2) a periosteal branch to the distal end of the humerus on its anterior surface; and (3) a branch to the brachial artery.
The median nerve arises by two heads-one from the lateral cord, the other from the medial cord of the brachial plexus. The lateral head, from the (fifth), sixth, and seventh nerves, descends along the lateral side of the axillary artery; the medial head, from the eighth cervical and first thoracic nerves, crosses the end of the axillary artery or the beginning of the brachial artery, to join the other head in the proximal part of the arm. Descending along the lateral aspect of the brachial artery, the nerve crosses over it obliquely in the distal half of the arm. the hollow of the elbow, it lies on the medial side of the brachial artery, behind the lacertus fibrosus and the median basilic vein. It passes into the forearm between the two heads of the pronator teres muscle, separated from the ulnar artery by the deep origin of that muscle. Extending distally along the middle of the forearm, between the superficial and deep muscles, to the wrist, it enters the palm of the hand on the lateral side of the flexor tendons of the fingers, and deep to the transverse carpal ligament. In the hand, it spreads out at the distal border of the transverse carpal ligament, under cover of the palmar aponeurosis and superficial volar arch, and separates into its six terminal branches. In the forearm a small artery accompanies it, the median branch of the volar interosseous artery. Immediately proximal to the wrist it is comparatively superficial, lying on the lateral side of the superficial flexor tendons and directly behind the tendon of the palmaris longus. Branches. The median nerve usually gives off no branches in the (upper)
Branches in the Forearm.-(1) Articular Branches.-Minute articular filaments are distributed to the front of the elbow-joint.
(2) Muscular Branches. Just below the elbow a bundle of nerves arises to be distributed to the following muscles: pronator teres, flexor carpi radialis, palmaris longus, flexor digitorum sublimis. Nerves are also generally traceable from this bundle to the upper fibres of the flexor pollicis longus and flexor digitorum profundus. The nerve to the pronator teres often arises independently in the hollow of the elbow.
(3) The volar interosseous nerve of the forearm (O.T. anterior interosseous) arises from the posterior surface of the median nerve in the forearm. It passes distally on the volar aspect of the interosseous membrane along with the volar interosseous artery, lies dorsal to the pronator quadratus muscle, and terminates by supplying articular filaments to the radio-carpal articulation. In its course the nerve supplies muscular branches to the flexor pollicis longus, the lateral half of the flexor digitorum profundus, and the pronator quadratus, minute medullary branches to the radius and ulna, and twigs to the periosteum and interosseous membrane.
(4) Palmar Ramus. In the distal third of the forearm a small cutaneous branch arises, which pierces the deep fascia and crosses the transverse carpal ligament to reach the palm of the hand. It supplies the skin of the palm and com
municates with a similar branch of the ulnar nerve. This branch is not always present.
Branches in the Hand.-In the hand the median nerve gives off its terminal branches. These are muscular and cutaneous.
The main muscular branch arises just distal to the transverse carpal ligament and passes to the base of the thenar eminence; entering the ball of the thumb superficially on the medial side, it supplies branches to the abductor pollicis brevis, opponens pollicis, and the flexor pollicis brevis.
FIG. 617. THE DISTRIBUTION OF CUTANEOUS NERVES ON THE FRONT OF THE ARM AND HAND.
(B) is a schematic representation of the areas supplied by the above nerves, the lettering indicating the spinal origin of the branches of distribution to each area. V.A.L., Ventral axial line.
The cutaneous branches are five in number. Three separate branches supply each side of the thumb and the lateral side of the index finger. The two remaining branches (nn. digitales volares communes) subdivide at the cleft between the second and third, and the third and fourth fingers respectively, into branches (nn. digitales volares proprii) which supply the adjacent sides of the second and third, and the third and fourth fingers. From the nerves which supply respectively the lateral side of the index finger, and the contiguous sides of the index and third fingers, fine muscular branches arise for the first two lumbrical muscles. The cutaneous branches of the median nerve are placed in the palm between the superficial palmar arch and the flexor tendons. They become super
ficial at the roots of the fingers between the slips of the palmar aponeurosis, or, in the case of the nerves to the thumb and lateral side of the index finger, at the lateral edge of the central portion of the palmar aponeurosis. In the fingers they are placed superficial to the digital arteries, and are distributed to the sides and volar aspects of the fingers. Each nerve supplies one or more dorsal branches, distributed to the skin on the dorsal aspect of the terminal phalanx of the thumb and the distal two
FIG. 618. THE DISTRIBUTION OF CUTANEOUS NERVES ON THE BACK OF THE ARM AND HAND. (B) is a schematic representation of the areas supplied by the above nerves, the lettering indicating the spinal origin of the branches of distribution to each area. D.A.L., Dorsal axial line.
phalanges of the first two and a half fingers, thus making up for the deficiency of the superficial branch of the radial nerve in those situations.
Communications. (1) The median nerve, in some cases, receives a communicating branch from the musculo-cutaneous nerve in the arm. (2) It communicates in some cases, in the proximal part of the forearm, with the ulnar nerve beneath the flexor muscles. (3) It communicates by means of its cutaneous branches with the ulnar nerve in the palm of the hand (ramus anastomoticus cum nervo ulnari).