BRANCHES OF CERVICAL PLEXUS 979 



3, The nerves to the levator scapulae (fig. 752) are derived from the third and fourth cervical 

 nerves, and occasionally from the second or fifth. They pierce the superficial surface of*the 

 levator scapulae, and supply the upper three divisions of that muscle. 



4. The branches to the trapezius (fig. 752) are usually in the form of two stout twigs which 

 are given off by the third and fourth cervical nerves. They emerge from under cover of the 

 stemo-mastoid at its posterior border and cross the posterior superior triangle of the neck at a 

 lower level than the spinal accessory nerve (fig. 753). They pass under cover of the trapezius 

 in company with the last-named nerve, and communicate with it to form the subtrapezial 

 plexus, from which the trapezius is supplied. 



The medial branches of the deep series also comprise communicating and mus- 

 cular branches. 



The communicating branches (figs. 751, 752) include (1) branches which connect each 

 of the first four cervical nerves with the superior cervical ganghon of the sympathetic; (2) a 

 branch to the vagus; (3) a branch to the hypoglossal; and (4) branches which pass from the 

 second and third cervical nerves to the descendens cervicalis (hypoglossi) . The ultimate dis- 

 tribution of the twigs connected with the sympathetic and the vagus nerves is not known, but 

 the fibres which pass to the hypoglossal nerve pass from it to the thyreo-hyoideus muscle, and 

 to the descendens cervicalis and the latter joins with the branches from the second and third 

 cervical nerves, forming with them the cervical or hypoglossal loop [ansa hypoglossi] which 

 lies on the carotid sheath. From this loop the two bellies of the omo-hyoid muscle and the 

 stemo-hyoid and sterno -thyreoid muscles are supplied as described above. 



The muscular branches supply the rectus capitis lateralis, the longus capHis 

 (rectus capitis anterior major), the rectus capitis anterior (minor), the scalenus 

 anterior, and the diaphragm. The nerve to the latter muscle is the phrenic. 



1. The branch to the rectus capitis lateralis is furnished to that muscle by the first cervical 

 nerve as it crosses the deep surface of the muscle. 



2. The nerve to the rectus capitis anterior (minor) is given off by the first nerve at the 

 upper part of the loop in front of the transverse process of the atlas. 



3. The longus capitis (rectus capitis anterior major) receives twigs from the upper four 

 cervical nerves. 



4. The longus colli receives branches from the second, third, and fourth cervical nerves, 

 and additional branches also from the fifth and sixth nerves. 



5. The phrenic nerve (fig. 752) springs chiefly from the fourth cervical nerve, 

 but it usually receives a twig from the third and another from the fifth cervical 

 nerve, a small communicating branch from the sympathetic, and, rarely, a branch 

 from the vagus. The twig from the fifth cervical nerve is frequently connected 

 with the nerve to the subclavius. After the union of its roots the phrenic nerve 

 passes downward and medialward on the scalenus anterior (fig. 755). In this 

 part of its course it is crossed by the tendon of the omo-hyoid and by the trans- 

 verse cervical and transverse scapular (suprascapular) arteries. It is overlapped 

 by the internal jugular vein, and it is covered by the sterno-mastoid muscle. At 

 the root of the neck the left phrenic nerve lies behind the terminal portion of the 

 thoracic duct, and each nerve passes off the anterior border of the scalenus anterior 

 and descends in front of the first part of the subclavian artery and the pleura imme- 

 diately below that artery; each nerve passes dorsal to the terminus of the sub- 

 clavian vein, crosses either in front of or dorsal to the internal mammary artery and 

 gains the medial surface of the pleural sac. From the root of the neck the rela- 

 tions of the phrenic nerves differ. The right phrenic nerve descends along the medial 

 surface of the right pleural sac and crosses in front of the root of the lung. It is 

 accompanied by the pericardiaco-phrenic artery (comes nervi phrenici), and it is 

 in relation medially, and from above downward, with the right innominate vein, 

 the superior vena cava, and the pericardium, the latter membrane separating it 

 from the wall of the right atrium (auricle) . The left phrenic nerve descends along 

 the medial surface of the left pleural sac accompanied by the pericardiaco-phrenic 

 (comes nervi phrenici) artery. In the superior mediastinum it lies between the 

 left common carotid and the left subclavian arteries, and it crosses in front of the 

 left vagus, the left superior intercostal vein, and the arch of the aorta. Below 

 the arch of the aorta it crosses in front of the root of the left lung, and then Hes 

 along the left lateral surface of the pericardium, which separates it from the wall of 

 the left ventricle. 



Branches. — Both phrenic nerves distribute branches to the pericardium and to the pleura. 

 The right nerve gives off a branch, pericardiac, which accompanies the superior vena cava and 

 supplies the pericardium. Each phrenic nerve divides into numerous terminal phrenico- 

 abdominal branches. As a rule, the right phrenic nerve divides into two main terminal branches, 

 an anterior and a posterior. The anterior branch runs forward and one of its terminal filaments 



