THE THORAX. 





THE Thorax is a cone-shaped cavity containing and protecting the heart, 

 enclosed in its membranous bag, the pericardium, and the lungs, invested by 

 the pleura. Its shape and boundaries have already been described (see page 



The Cavity of the Thorax. The size of the cavity of the thorax does not 

 correspond with its apparent size externally, because (1) the space enclosed by the 

 lower ribs is occupied by some of the abdominal viscera, and (2) the cavity extends 

 above the first rib into the neck. The size of the cavity of the thorax is constantly 

 varying during life with the movements of the ribs and Diaphragm and with the 

 degree of distension of the abdominal viscera. From the collapsed state of the 

 lungs in the dead body it would appear as if the viscera only partly filled the 

 cavity of the thorax, but during life there is no vacant space, that which is seen 

 after death being filled up by the expanded lungs. 



The Upper Opening of the Thorax. The parts which pass through the upper 

 opening of the thorax are, from before backward in the middle line, the Sterno- 

 hyoid and Sterno-thyroid muscles, the remains of the thymus gland, the trachea, 

 oesophagus, thoracic duct, and the Longus colli muscle of each side ; at the sides, 

 the innominate artery, the left common carotid and left subclavian arteries, the 

 internal mammary and superior intercostal arteries, the right and left innom- 

 inate veins, and the inferior thyroid veins, the pneumogastric, cardiac, phrenic, 

 and sympathetic nerves, the anterior branch of the first dorsal nerve, and the 

 recurrent laryngeal nerve of the left side. The apex of each lung, covered by 

 the pleura, also projects through this aperture, a little above the margin of the 

 first rib. 



The Lower Opening of the Thorax is wider transversely than from before back- 

 ward. It slopes obliquely downward and backward, so that the cavity of the 

 thorax is much deeper behind than in front. The Diaphragm (see page 444) closes 

 in the opening, forming the floor of the thorax. The floor is flatter at the centre 

 than at the sides, and is higher on the right side than on the left, corresponding in 

 the dead body to the upper border of the fifth costal cartilage on the former, and 

 to the corresponding part of the sixth costal cartilage on the latter. From the 

 highest point on each side the floor slopes suddenly downward to the attachment 

 of the Diaphragm to the ribs ; this is more marked behind than in front, so that 

 only a narrow space is left between it and the wall of the thorax. 



For measurements of the thorax see page 1099. 



THE PERICARDIUM. 



The Pericardium (Figs. 692, 693) is a conical membranous sac in which the 

 heart and the commencement of the great vessels are contained. It is placed 

 behind the sternum and the cartilages of the third, fourth, fifth, sixth, and seventh 

 ribs of the left side, in the interval between the pleurae. 



Irs a/-x is directed upward, and surrounds the great vessels about two inches 

 above their origin from the base of the heart. Its base is attached to the central 

 tendon and part of the adjoining muscular structure of the Diaphragm, extending 

 a little farther to the left than to the right side. In front it is separated from the 

 sternum by the remains of the thymus gland above and a little loose areolar tissue 

 below, and is covered by the margins of the lungs, especially the left. Behind, it 

 rests upon the bronchi, the oesophagus, and the descending aorta. Laterally, it is 

 covered by the pleurae, the phrenic nerve with its accompanying vessels descending 

 between the two membranes on either side. 



1083 



