THORACIC CAVITY 43 



lower border of the transverse process of the vertebra above. 

 Laterally, it is continuous with the fascial layer between the 

 internal and external intercostal muscles, and above and 

 below, it is attached to the adjacent ribs. On the pleural 

 surface of the posterior intercostal membrane, in each space, 

 lie the corresponding intercostal nerve and vessels (see Figs. 

 13 and 14); they pass laterally, on the internal surface 

 of the membrane, and disappear posterior to the border of 

 the internal intercostal muscle. When the membranes are 

 removed the posterior fibres of the external intercostal 

 muscles will be exposed. They extend, medially, as far as the 

 tubercles of the ribs. 



Nervi Phrenici. Each phrenic nerve arises in the neck from 

 the cervical plexus, receiving fibres from the third, fourth, and 

 fifth cervical nerves. It descends on the scalenus anterior 

 muscle and, on the right side, at the root of the neck, crosses 

 the front of the subclavian artery and runs posterior to the 

 innominate vein ; but on the left side, as it leaves the scalenus 

 anterior, it descends parallel with and in front of the sub- 

 clavian artery and it passes behind the commencement of 

 the innominate vein. As it enters the upper aperture of the 

 thorax it crosses the internal mammary artery, passing from 

 its lateral to its medial side; then it descends along the 

 lateral border of the mediastinum, anterior to the root of 

 the lung, to reach the diaphragm, where it breaks up into 

 branches. The majority of the branches pass between the 

 muscular fibres of the diaphragm and, after communicating 

 with the abdominal sympathetic nerve fibres which form 

 the diaphragmatic plexus, they are distributed to the muscle 

 from its lower surface. The relations of the phrenic nerves 

 in the thorax are different on the two sides. 



The right phrenic nerve descends along the lateral borders 

 of the right innominate vein and the superior vena cava to 

 the point where the latter enters the pericardium ; then along 

 the side of the pericardium, which separates it from the 

 sinus venarum of the right atrium (see Figs. 13, 20, 21, 22, 



*3). 



In the upper part of the thorax the left phrenic nerve runs 

 downwards between the left common carotid and the left 

 subclavian arteries and, whilst lying between them,^it crosses 

 in front of the left vagus and behind to the left innominate 

 vein. In the lower part of the superior mediastinum it 



