The Thorax. 175 



dorsal surface of the aortic arch. It ends in the cardiac 

 plexus (plexus cardiacus), a network of sympathetic nerves 

 lying between the aortic arch and the pulmonary artery. 



(e) The phrenic nerve (n. phrenicus) is a stout cord arising 

 chiefly from the fourth cervical spinal nerve. That of the 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. 



(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 arterv. 



The nerves proceeding from the inferior cervical gangHon enter 

 the cardiac plexus and the sympathetic plexuses of the sub- 

 clavian and its branches. 



3. Dissection of the heart.- 



The character and relations of the enclosing serous sac, the peri- 

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

 that of the peritoneum and pleura investing other visceral organs (p. 49). 

 It comprises a parietal layer, that portion commonly known as the peri- 

 cardium, and a visceral layer, the epicardium, 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 broadlv 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 : 



(a) The posterior, somewhat conical, ventricular portion of the 

 heart. The left ventricle (ventriculus sinister) may be dis- 

 tinguished both by its position and by the more solid charac- 

 ter of its wall. The right ventricle (ventriculus dexter) is less 

 muscular, and the wall is readily pressed inward. The line 

 of division is indicated on the ventral surface by a faint de- 

 pression, the anterior longitudinal sulcus. 



