934 A MANUAL OF ANATOMY 



Thoracic Cavity. 



Contents and their General Position. — ^The thoracic cavity is 

 chiefly occupied by the lungs and heart . The lungs are situated one 

 in each half of the cavity, and each lung is provided with a serous 

 membrane, called the pleura. The heart lies obliquely between the 

 lungs, projecting more to the left of the sternum than to the right, 

 and it is enclosed within a fibro-serous sac, called the pericardium. 

 Each lung is free to expand except at the hilum pulmonis, which is 

 situated on its inner surface. Through this hilum the bronchus, 

 pulmonary artery, and pulmonary veins pass, along with other 

 structures, and the pedicle so formed is called the root of the lung. 

 The upper part, or apex, of the lung rises into the root of the neck, 

 where it is covered by the dome of the pleura, that in turn being 

 covered by Sibson's fascia. The lower part, or base, of the lung 

 rests upon the corresponding half of the diaphragm, the heart lying 

 upon the central tendon of that muscle. 



The pulmonary artery springs from the base of the right ventricle 

 of the heart, and the aorta from the base of the left ventricle, being 

 at first covered by the pulmonary artery. The left innominate 

 vein courses along the upper aspect of the arch of the aorta in front 

 of the origins of the innominate, left common carotid, and left sub- 

 clavian arteries, and it unites with the right innominate vein behind 

 the sternal end of the first right costal cartilage to form the superior 

 vena cava. This latter vessel opens into the postero-superior angle 

 of the right auricle of the heart, and, just before piercing the 

 pericardium, it receives the right azygos vein, which arches forwards 

 over the right bronchus. The inferior vena cava, having entered 

 the thorax through the foramen quadratum in the central tendon 

 of the diaphragm, almost immediately opens into the postero- 

 inferior angle of the right auricle of the heart. 



The phrenic nerve, on each side, descends in front of the root of 

 the lung, and is intimately related to the pericardium, especially 

 on the left side. The small anterior pulmonary plexus of nerves 

 lies in front of the root of each lung. The pneumogastric nerve, on 

 each side, descends behind the root of the lung, and forms in that 

 part of its course the much larger posterior pulmonary plexus. 

 The following important nerves descend in front of the arch of the 

 aorta : the left phrenic, the left pneumogastric, the upper cervical 

 cardiac branch of the left sympathetic, and the lower cervical 

 eardiac branch of the left pneumogastric. The left superior inter- 

 costal vein lies in front of the back part of the aortic arch. The 

 superficial cardiac plexus of nerves lies within the concavity of the 

 arch of the aorta, and the deep cardiac plexus lies behind the arch 

 and in front of the trachea, close to its bifurcation into the two 

 bronchi. The left recurrent laryngeal nerve passes backwards under 



