982 THE NERVOUS SYSTEM 



of its three cords divides into two terminal branches, and it hes 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 behind with the scalenus medius (figs. 751, 755). In the posterior triangle it is 

 covered superficially by the skin and superficial fascia, the platysma, 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 sub- 

 clavius 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 trunks 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 swpra-davicular , the infra-clavicular and the subscapular por- 

 tions of the plexus. 



The branches of the supra-clavicular portion. — After the roots of the plexus 

 have received communications from the sj^mpathetic, 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 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 the fourth nerve (fig. 751). 



It passes downward and dorsalward, across the middle scalene, parallel with and laelow the 

 spinal accessory nerve to the anterior border of the levator scapula}, 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 fibres of the levator and the smaller and larger rhomboid muscles. 



(6) 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 (figs. 751 and 754). The upper two roots traverse the substance of 

 the scalenus medius; the root from the seventh passes in front of that muscle. Twigs are fur- 

 nished 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 ])asscs downward behind the brachial plexus and the first stage of the axillary artery, 

 and runs along the axillary surface of the serratus anterior (magnus), supplying twigs to each of 

 the digitations of that muscle (fig. 755). 



The suprascapular nerve (fig. 751) supplies the supraspinatus and infraspi- 

 natus muscles. 



It receives fibres 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 (fig. 755), whore it comes into relation with 

 the transver.se scapular (suprascapular) artery. It is separated from the art(n-y at the notch 

 by the superior transverse ligament, the nerve passing through the notch and the artery above 

 the ligaiiKsnt. After entering the sui)ras|)inous fossa the nerve supplies branches to the supra- 

 spinatus 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 ter- 

 minates in the infraspinatus muscle. 



The twig to the phrenic (fig. 751) arises from the fifth cervical nerve close to the point where 

 the latter nerve receives its twig from the cervical plexus. 



