The Thorax. 259 



left side crosses the ventral surface of the subclavian artery 

 and the aortic arch, passing then along the pericardium to 

 the diaphragm. That of the right side accompanies the 

 thoracic portion of the inferior caval vein. The nerve 

 \/^c ontrols the respiratory movements of the d iaphragm. 

 (f) The sympathetic trunk. At the base of the neck the 

 cervical portion of the sympathetic trunk enters the inferior 

 cervical ganglion (g. cervicale inferius). The latter lies 

 in front of and somewhat dorsal to the subclavian artery. 

 The first thoracic ganglion lies behind, the artery and is 

 connected with the inferior cervical by the ansa subclavia, 

 a loop formed by two cords, one of which passes to the dorsal, 

 the other to the ventral side of the subclavian artery. 

 The-nerves proceeding from the inferior cervical ganglion 

 enter the cardiac plexus and the sympathetic plexuses of the 

 subclavian and its branches. 

 3. Dissection of the heart. 



The character and relations of the enclosing serous sac, the 

 pericardium, should first be noted. Its relation to the heart is 

 similar to that of the peritoneum and pleura investing other visceral 

 organs (p. 99). It comprises a paiie_fc al layer , that portion com- 

 monly kn^iVTT-^ the- peri^nrdjiirri, and a visceral layer, the epi- 

 cardium, which forms an immediate investment for the heart 

 substance. The parietal layer forms a loose, capacious sac, the 

 serous membrane being greatly strengthened by the presence of a 

 thin layer of connective tissue which forms an external investment 

 and is usually considered as part of the pericardium. 



The paired pleural cavities containing the lungs are broadly separated by a 

 longitudinal vertical partition, the mediastinum or mediastinal septum, the 

 space enclosed by the latter being largely occupied by the heart and by the cavity 

 of the pericardium. For a considerable area ventrally the pericardium is loosely 

 applied to the thoracic wall, the intervening space, which is bounded laterally 

 by the membrane lining the pleural cavities, being known as the anterior 

 mediastinum. A corresponding dorsal space lying between the heart and the 

 bodies of the thoracic vertebrae, and also bounded laterally by the pleura, is the 

 posterior mediastinum. It is occupied by several structures, namely, the 

 oesophagus, the thoracic aorta, the bronchi, and the pulmonary bloodvessels. 



The pericardium should be removed, and the external features 

 of the heart and its great vessels examined as follows : 



