388 NERVOUS SYSTEM. 



2. N. thoracicus posterior. — The posterior thoracic (or 

 long thoracic) nerve (external respiratory) (Fig. 159, ;;/) arises 

 from the seventh cervical nerve near its beginning. It passes 

 within the scalenus muscle and extends caudad on the outer 

 surface of the serratus anterior muscle, which it supplies. 



3. N. suprascapularis (Figs. 159 and 160, d). — The supra- 

 scapular nerve arises from the sixth (and sometimes the seventh) 

 cervical. It passes laterad and gives off a branch which passes 

 over the shoulder-joint and penetrates the clavodeltoid muscle 

 to be distributed to the integument on the ventral surface of 

 the upper arm. It then follows the transversa scapulae artery 

 into the supraspinatus fossa and accompanies the artery to be 

 distributed to the supraspinatus and infraspinatus muscles. 



4. Nn. subscapulares. — The subscapular nerves are three. 

 The cranial one (Figs. 159 and 160, c) arises from the sixth and 

 seventh cervical and supplies the subscapular muscle. The 

 middle one (Fig. 159, e) arises from the seventh; it supplies 

 principally the teres, major. The caudal one (Fig. i 59, i) arises 

 from the seventh and eighth cervical and supplies the latissi- 

 mus dorsi. (The points of origin from the plexus vary.) 



5. N. axillaris (or circumflexus) (Fig. 159, d). — The axil- 

 lary nerve arises from the sixth and seventh cervical nerves. 

 It passes toward the shoulder-joint, then follows the posterior 

 circumflex artery ventrad of the long head of the triceps to be 

 distributed to the spinodeltoid and acromiodeltoid. A branch 

 of it continues to the clavobrachial, which it supplies. 



B. The Phrenic Nerve (Fig. 157,/, page 381). 



6. N. phrenicus. — The phrenic nerve (internal respiratory) 

 is formed by the junction of two slender branches, one from 

 the fifth and one from the sixth cervical nerves (Fig. 159, a). 

 (It is said to receive sometimes a branch from the fourth.) It 

 passes caudad into the thorax on the ventral surface of the 

 subclavian artery and then extends to the diaphragm lying 

 at the side of the inferior and superior vena^ cava;. It is the 

 motor nerve of the diaphragm. 



C. Nerves of the Arm. 



7. N. musculocutaneus (Fig. 160, d). — The musculo- 

 cutaneous nerve or external cutaneous arises from the ventral 



