THORACIC CAVITY 35 



the side of the pericardium, which separates it from the 

 venous sinus of the right atrium (see Fig. 12). 



The Left Phrenic Nerve. 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 anterior to the left vagus and 

 posterior to the left innominate vein. In the lower part of 

 the superior mediastinum it passes lateral to the arch of the 

 aorta and the left superior intercostal vein, then, descending 

 into the middle mediastinum, it lies at first anterior to the 

 root of the left lung, and afterwards it runs downwards along 

 the side of the pericardium, which separates it from the 

 anterior part of the left atrium and from the lateral part of 

 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. 



Pulmones. Before proceeding to the further dissection 

 of the constituent parts of the mediastinum, the dissectors 

 should study the lungs which they previously removed. The 

 lungs are two soft, comparatively light, spongy organs placed 

 one on either side of the mediastinum. The weight of the 

 right lung, when it is filled with an average amount of blood, 

 is 22 oz. and that of the left 20 oz. When the thorax is 

 opened the lungs collapse to about one-third of their original 

 bulk (unless they have been hardened in situ\ and it is 

 difficult for the student to realise their proper size and 

 shape until they are distended to their original dimensions 

 with the aid of the bellows (see p. 1 6). 



When healthy and sound, the lungs lie free within the 

 cavity of the chest, and are attached only by their roots and 

 by their pulmonary ligaments. It is rare, however, that a 

 healthy lung is seen in the dissecting-room, for adhesions be- 

 tween the visceral and parietal portions of the pleura, due to 

 pleurisy, are generally present. Each lung is accurately 

 adapted to the space in which it lies, and, when hardened 



ii 3 a 



