768 THE NERVOUS SYSTEM. 



tion of the Serratus magnus, and the Subscapularis muscles. In the axilla, it is 

 placed on the outer side of the first portion of the axillary artery ; it surrounds 

 the artery in the second part of its course, one cord lying upon the outer side of 

 that vessel, one on the inner side, and one behind it, and at the lower part of the 

 axillary space gives off its terminal branches to the upper extremity. 



Branches. The branches of the brachial plexus are arranged in two groups 

 viz., those given off above the clavicle, and those below that bone. 



Branches above the Clavicle. 



Communicating. Posterior thoracic. 



Muscular. Suprascapular. 



The communicating branch with the phrenic is derived from the fifth cervical 

 nerve or from the loop between the fifth and sixth ; it joins the phrenic on the 

 Anterior scalenus muscle. The communications with the sympathetic have already 

 been referred to. 



The muscular branches supply the Longus colli, Scaleni, Rhomboidei, and Sub- 

 clavius muscles. Those for the Lougus colli and Scaleni arise from the four lower 

 cervical nerves at their exit from the intervertebral foramina. The Rhomboid 

 branch arises from the fifth cervical, pierces the Scalenus medius, and passes 

 beneath the Levator anguli scapulae, which it occasionally supplies, to the Rhom- 

 boid muscles. The nerve to the Subclavius is a small filament which arises from 

 the fifth cervical at its point of junction with the sixth nerve; it descends in 

 front of the third part of the subclavian artery to the Subclavius muscle, and is 

 usually connected by a filament with the phrenic nerve. 



The posterior thoracic nerve (long thoracic, external respiratory of Sell) (Fig. 

 413) supplies the Serratus magnus, and is remarkable for the length of its course. 

 It sometimes arises by two roots from the fifth and sixth cervical nerves imme- 

 diately after their exit from the intervertebral foramina, but generally by three 

 roots from the fifth, sixth, and seventh nerves. These unite in the substance of 

 the Middle scalenus muscle, and, after emerging from it, the nerve passes down 

 behind the brachial plexus and the axillary vessels, resting on the outer surface 

 of the Serratus magnus. It extends along the side of the chest to the lower 

 border of that muscle, supplying filaments to each of its digitations. 



The suprascapular nerve (Fig. 414) arises from the cord formed by the fifth 

 and sixth cervical nerves ; passing obliquely outward beneath the Trapezius and 

 the Orno-hyoid, it enters ; the supraspinous fossa below the transverse or supra- 

 scapular ligament, and, passing beneath the Supraspinatus muscle, curves round 

 the external border of the spine of the scapula to the infraspinous fossa. In the 

 supraspinous fossa it gives off two branches to the Supraspinatus muscle, and an 

 articular filament to the shoulder-joint; and in the infraspinous fossa it gives off 

 two branches to the Infraspinatus muscle, besides some filaments to the shoulder- 

 joint and scapula. 



Branches below the Clavicle. 



The branches given off below the clavicle are derived from the three cords of 

 the brachial plexus, in the following manner : 



From the outer cord arise the external anterior thoracic nerve, the musculo- 

 cutaneous, and the outer head of the median. 



From the inner cord arise the internal anterior thoracic nerve, the internal 

 cutaneous, the lesser internal cutaneous (nerve of Wrisberg), the ulnar, and inner 

 head of the median. 



From the posterior cord arise two of the three subscapular nerves, the third 

 taking origin from the posterior division of the trunk formed by the fifth and sixth 

 cervical nerves ; the cord then divides into the musculo-spiral and circumflex 

 nerves. 



