44 THORAX 



passes lateral to the arch of the aorta and the left superior 

 intercostal vein ; then, descending into the middle medi- 

 astinum, it lies at first anterior to the root of the left lung, 

 and afterwards it runs downwards along the side of the 

 pericardium. The pericardium separates it from the auricle 

 of the left atrium and from the left ventricle of the heart. 



The left phrenic nerve is longer than its fellow of the right side, partly 

 on account of the lower position of the diaphragm, and partly on account 

 of the greater projection of the heart on the left side. 



Branches of the Phrenic Nerves. The main distribution 

 of the phrenic nerves is to the diaphragm, but some minute 

 sensory twigs are given off by each nerve to the pericardium 

 and to the pleura. The student should note the great import- 

 ance of the phrenic nerves. They are the nerves of supply 

 to the diaphragm, which is the chief muscle of respiration. 



Arterise Pericardiacophrenicae. The pericardiaco-phrenic 

 artery, one on each side, is given off from the upper part 

 of the internal mammary artery at the root of the neck. 

 Each accompanies the corresponding phrenic nerve, through 

 the superior and middle mediastina, to the diaphragm. It 

 gives branches to the pleura and the pericardium, and it 

 terminates in branches which anastomose, in and on the 

 diaphragm, with the ramifications of the inferior phrenic and 

 musculo-phrenic arteries. Each pericardiaco-phrenic artery is 

 accompanied by venae comites, which end in the internal 

 mammary vein of the same side. 



Dissection. Before the lungs are examined the pericardium 

 should be opened on each side in order that the dissectors may 

 make themselves familiar with the relations of the heart to the 

 mediastinal pleura, and to the mediastinal surface of the lungs. 



Two longitudinal incisions must be made on each side, one 

 anterior and one posterior to the phrenic nerve (see Figs. 13 and 

 14). On the right side the incisions should commence at the level 

 of the upper pulmonary vein. On the left side the anterior incision 

 should begin at the lower border of the aortic arch, and the 

 posterior at the level of the left pulmonary artery (see Fig. 14). 

 On both sides the longitudinal incisions must descend to the 

 lower border of the pericardium. On both sides incisions should 

 be carried forwards from the upper and lower ends of the 

 anterior longitudinal incision to the line along which the medi- 

 astinal pleura was left attached to the anterior surface of the 

 pericardium (see Figs. 13 and 14). From the upper end of the 

 posterior longitudinal incision on the right side a cut should be 

 made downwards and backwards, along the anterior aspect of 

 the root of the lung, to the upper end of the inferior vena cava 

 (see Fig. 13). 



