942 A MANUAL OF ANATOMY 



Contents. — These are as follows: (i) the descending thoracic 

 aorta; (2) the oesophagus, and the two pneumogastric nerves; 

 (3) the thoracic duct ; (4) the right azygos vein ; (5) the upper and 

 lower left azygos veins, with the corresponding transverse azygos 

 veins; (6) the great splanchnic nerves, right and left; and (7) the 

 posterior mediastinal glands. 



Phrenic Nerve. — ^This nerve arises chiefly from the anterior 

 primary division of the fourth cervical nerve. It usually receives 

 a small root from the anterior primary division of the third cervical, 

 and, as a rule, an additional root from that of the fifth cervical. 

 Having descended on the superficial surface of the scalenus anticus 

 muscle to the root of the neck, it crosses the internal mammary 

 artery superficially from without inwards. Thereafter it enters the 

 thorax and descends in front of the root of the lung, in close contact 

 with the side of the pericardium, and under cover of the mediastinal 

 pleura. Having reached the diaphragm, the nerve divides into 

 several branches, which pierce that muscle to be distributed to its 

 abdominal surface. The terminal branches of each nerve are con- 

 nected, in the diaphragm, with filaments of the corresponding 

 inferior phrenic plexus of the sympathetic, which is an offshoot from 

 the solar plexus. At the place of junction of the two sets of fibres 

 on the right side there is a small ganglion, called the ganglion 

 diaphragmaticum. In the thorax the phrenic nerve occupies the 

 superior and middle mediastinal spaces. The right phrenic nerve 

 lies, in succession, on the outer side of the right innominate vein 

 and superior vena cava, and then descends in front of the root of 

 the right lung. The left phrenic nerve descends in the interval 

 between the left common carotid and left subclavian arteries, where 

 it crosses the left pneumogastric nerve from without inwards. 

 Thereafter it passes behind the left innominate vein, and crosses 

 over the arch of the aorta, after which it descends in front of the 

 root of the left lung. Each nerve is accompanied by the superior 

 phrenic artery, which is a branch of the internal mammary artery. 



Branches. — The nerve of each side furnishes twigs to the peri- 

 cardium and the mediastinal pleura, its principal branches being 

 distributed to the diaphragm. 



Differences between the two phrenic nerves — (i) The right nerve is shorter 

 than the left, because the right half of the diaphragm, having the bulk of the 

 liver below it, is higher than the left half. (2) The right nerve is straighter than 

 the left, because the heart, enclosed in the pericardium, projects less to the 

 right side than to the left. (3) The right nerve occupies a deeper position 

 in the upper part of the thorax than the left. 



Pericardium. — ^The pericardium is the fibro-serous sac which 

 loosely surrounds the heart in the middle mediastinum. It is some- 

 what conical, loeing wide below, where it is in contact with the dia- 

 phragm, and narrow above, where it surrounds the great vessels 

 connected with the base of the heart. On each side it is intimately 

 related to the mediastinal pleura, and is embraced by the inner 



