The Thorax. 



THE Thorax is a conical framework, formed partly of bones, and partly of 

 the soft tissues by which they are connected together. It is supported and its 

 back part is formed by the middle, or dorsal, region of the spine. It is narrow 

 above, broad below, flattened before and behind, and somewhat cordiform on a 

 transverse section. 



Boundaries. The thorax is bounded in front by the sternum, the six upper 

 costal cartilages, the ribs, and intercostal muscles ; at the sides, by the ribs and 

 intercostal 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 

 Diaphragm filling in the intervening space. Its direction is obliquely down- 

 wards and backwards ; so that the cavity of the thorax is much deeper on the 

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

 backwards. Its outer surface is convex ; but it is more flattened at the centre 

 than at the sides. Its floor is higher on the right than on the left side, corre- 

 sponding 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 Longus Colli 

 muscles on each side ; 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 innominatae, and the inferior thyroid veins, the pneumo- 

 gastric, sympathetic, phrenic, and cardiac 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 viscera contained in the thorax are, the heart, inclosed in its membranous 

 bag, the pericardium ; and the lungs, invested by the pleura. 



THE PEKICARDIUM. 



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 third, 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 g]and 

 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 fs covered by the pleurae ; the phrenic 

 vessels and nerve descending between the two membranes on either side. 



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