388 NERVOUS SYSTEM. 
2. N. thoracicus posterior.—The posterior thoracic (or 
long thoracic) nerve (external respiratory) (Fig. 159, 72) 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, 4).—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 scapule 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, ¢) arises from the seventh; it supplies 
principally the teres major. The caudal one (Fig. 159, 2) 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, @).—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, 7, 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 venz cave. It is the 
motor nerve of the diaphragm. 
C. Nerves of the Arm. 
7. N. musculocutaneus (Fig. 160, @).—The musculo- 
cutaneous herve or external cutaneous arises from the ventral 
