THORACIC VISCERA. 



THE THORAX. 



THE thorax is the osseo-cartilaginous case which con- 

 tains the heart and lungs. In shape it is the frustum 

 of a cone compressed antero-posteriorly, and it is due 

 to this antero-posterior flattening that man in conditions 

 of extreme relaxation gravitates upon the back. In the 

 lower animals, as a rule, the chest is flattened from side 

 to side, and under similar conditions, or at death, they 

 rest upon the side. 



It is supported posteriorly by the spine, formed ante- 

 riorly by the sternum, and front and back and at the 

 sides by the ribs. The construction of the thorax is such 

 as to enable the ribs to rotate on their axes and enlarge 

 the transverse diameters of the chest. The upper open- 

 ing of the thorax is formed by the first ribs, sternum, and 

 first dorsal vertebra. It is nearly circular in form, and 

 transmits the trachea, oesophagus, great vessels and nerves, 

 and some muscles. 



The lower opening is very large and irregular. It 

 is formed by the last dorsal vertebra, the twelfth ribs, the 

 apices of the eleventh ribs, the costal cartilages of the 

 sixth, seventh, eighth, ninth, and tenth ribs, and the 

 ensiform cartilage. Stretched across it is the diaphragm, 

 which separates the thoracic from the abdominal cavities. 

 The antero-posterior diameter of the chest is two-thirds 

 the transverse. It is deeper posteriorly than anteriorly, 

 the ensiform cartilage being on a level with the ninth 

 dorsal vertebra ; the upper border of the sternum is on a 

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