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 
controls the respiratory movements of the diaphragm. 
(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. 
G) Dissection of the heart. 
he 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 parietal layer, that portion com- 
monly known as the pericardium, 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 externa] investment 
and is usually considered as part of the pericardium, 
The paired pleural cavities containing the lungs are broadiy 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 aconsiderable 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: 
