916 THE NERVOUS SYSTEM 



sheath. From this loop the two bellies of the omo-hyoid muscle and the sterno- 

 hyoid and sterno-thyreoid muscles are supplied. 



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

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

 terior, 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 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 cervi- 

 cal nerves, and additional branches also from the fifth and sixth nerves. 



5. The Phrenic Nerve (fig. 676) 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 downwards 

 and inwards on the scalenus anterior (fig. 679) . In this part of its course it is crossed 

 by the tendon of the omo-hyoid and by the transverse 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 immediately below that artery; 

 it passes behind the terminal part of the subclavian vein, crosses either in front of or 

 behind the internal mammary artery and gains the inner surface of the pleural sac. 

 From the root of the neck the relations of the phrenic nerves differ. The right 

 phrenic nerve descends along the inner 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 internally, and from above downwards, 

 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 inner 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 lies 

 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 forwards and one of its terminal filaments anastomoses with the 

 phrenic of the opposite side in front of the pericardium; others descend between the 

 sternal and cos'tal origins of the diaphragm into the abdomen, where some of them 

 supply the diaphragm and others descend in the falciform ligament to the peritoneum 

 of the upper surface of the liver. The posterior branch passes through the vena 

 caval opening and ramifies upon the lower surface of the diaphragm, anastomosing 

 with the diaphragmatic plexus of the sympathetic, and its terminal branches supply 

 the muscular fibres of the right half of the diaphragm, the inferior vena cava, and the 

 right suprarenal gland. 



The left phrenic nerve divides into several branches. One of the most anterior 

 branches anastomoses with the right phrenic nerve; the others pierce the diaphragm 

 and ramify on its under surface, where they anastomose with filaments of the left 

 diaphragmatic plexus of the sympathetic and supply the left half of the diaphragm 

 and the left suprarenal gland. The left phrenic nerve is considerably longer than the 

 right nerve, partly on account of the lower level of the diaphragm on'the left side, and 

 partly on account of the greater convexity of the left side of the pericardium. 



