324 A MANUAL OF ANATOMY. 



tion of the pleura over the lung is the visceral layer. The 

 pleura also lines the interior of the thorax, diaphragm, and 

 sides of pericardium, and extends upward into the root of 

 the neck about an inch above the clavicle ; this portion is 

 the parietal pleura. These layers are continuous at the 

 root of the lung, and between them is the pleural cavity 

 (not a cavity unless the lungs have retracted into the back 

 of the chest). From the root of the lungs a fold of the 

 pleura descends to the diaphragm ; this is the broad liga- 

 ment of the lung. 



The lower level of the pleural reflection from the dia- 

 phragm on to the chest wall has been given on page 264. 



The Thymus Gland. 



The remains of this structure may be found lying below 

 the left innominate vein and over the transverse arch of 

 the aorta, reaching up into the root of the neck. 



The Phrenic Nerves. Figs. 18, 69, 70, 72. 



Each phrenic nerve is formed in the neck by branches 

 from the anterior divisions of the third, fourth, and fifth 

 cervical nerves. It descends obliquely downward and in- 

 ward across the front of the scalenus anticus muscle, be- 

 tween the subclavian artery and vein, to enter the thorax, 

 passing in front of the internal mammary artery. 



It then descends to the diaphragm under cover of the 

 pleura ; on the right side passing along the outer side of 

 the right innominate vein, superior vena cava, and the 

 pericardium ; on the left side, it is to the outside of the 

 transverse aorta and behind the left innominate vein ; on 

 both sides they pass in front of the roots of the lungs, 

 and when the diaphragm is reached pierce it and are dis- 

 tributed to the under side of the muscle. 



