UPPER EXTREMITY AND THORAX, ANTERIOR. 315 



to the posterior thoracic nerve, (e) The posterior or long- 

 thoracic nerve is formed in the substance of the scalenus 

 medius by branches from the fifth, sixth, and seventh cer- 

 vical nerves. It passes downward behind the plexus and 

 the beginning of the axillary artery into the axillary space, 

 where it is found coursing along the middle of the serratus 

 magnus muscle, to which it is distributed. 



(2) From the second portion of the plexus, (a) Nerve 

 to the subclavius. This small nerve comes from the upper 

 trunk just after its formation, descends across the brachial 

 plexus and subclavian vessels to supply the subclavius 

 muscle. (b) Suprascapular nerve. This arises from 

 the upper trunk, passes outward and downward through 

 the suprascapular foramen in the scapula (notch con- 

 verted into a foramen by the transverse ligament), supplies 

 the supraspinatus muscle, the shoulder-joint, continues 

 around through the great scapular notch, and termi- 

 nates in the infraspinatus muscle. All the preceding nerves 

 arise from the plexus above the clavicle. 



(3) From the fourth portion of the plexus. Below the 

 clavicle. From the outer cord, (a) The external anterior 

 thoracic nerve receives filaments from the fifth, sixth, and 

 seventh cervical nerves, arises from the outer cord, turns 

 inward across the axillary artery and vein, perforates the 

 costocoracoid membrane, and supplies the pectoralis major. 

 (&) The musculocutaneous nerve. This nerve is the 

 direct continuation of the outer cord. It continues to 

 the outer side of the third portion of the axillary artery 

 and the beginning of the brachial, perforates the coraco- 

 brachialis muscle (supplying it), descends between the 

 biceps and brachialis anticus (supplying both), and as a 

 cutaneous nerve is distributed to the outer portion of the 



