The Thorax. 



* 



THE THORAX is a conical, osseous framework, connected with the middle region 

 of the spine. It is narrow above, broad below, flattened before and behind, and 

 somewhat cordiform on a transverse section. 



Boundaries. It is bounded in front by the sternum, the six upper costal 

 cartilages, the ribs, and intercostal muscles ; at the sides, by the ribs and inter- 

 costal muscles ; and behind, by the same structures and the dorsal portion of the 

 vertebral column. 



The superior opening of the thorax is bounded on each side by the first rib ; 

 in front, by the upper border of the sternum ; and behind, by the first dorsal 

 vertebra. It is broader from side to side, than from before backwards ; and its 

 direction is backwards and upwards. 



The lower opening or base is bounded in front by the ensiform cartilage; 

 behind, by the last dorsal vertebra ; and on each side by the last rib, the Dia- 

 phragm filling in the intervening space. Its direction is obliquely downwards 

 and backwards, so that the cavity of the thorax is much deeper on the posterior 

 wall, than on the anterior. It is wider transversely than from before backwards, 

 and its general direction is convex towards the chest ; but it is more flattened at 

 the centre than at the sides, and rises higher on the right than on the left side, 

 corresponding in the dead body to the upper border of the fifth costal cartilage, 

 on the right side ; and to the corresponding part of the sixth cartilage on the left 

 side. 



The parts which pass through the upper opening of the thorax are, from before 

 backwards, the Sterno-hyoid and Sterno-thyroid muscles, the remains of the 

 thymus gland, the trachea, oesophagus, thoracic duct, and the Longi colli muscles ; 

 on the sides, the arteria innominata, the left carotid and left subclavian arteries, 

 the internal mammary and superior intercostal arteries, the right and left venae 

 innominate, and the inferior thyroid veins, the pneumogastric, sympathetic, 

 phrenic, and cardiac nerves, 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 viscera contained in the thorax are, the great central organ of circulation, 

 the heart, inclosed in its membranous bag, the pericardium ; and the organs of 

 respiration, the lungs, invested by the pleuras. 



THE PERICARDIUM. 



The Pericardium 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 fourth, fifth, sixth, and seventh ribs of the left side, in 

 the interval between the pleurae. 



Its apex is directed upwards, 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 of the Diaphragm, extending a little farther to the left than 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 vessels 

 and nerve descending between the two membranes on either side. 



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