THE HEART. 



335 



level with the second dorsal vertebra. In the intervals 

 between the ribs are the intercostal muscles. 



Posteriorly the thoracic cavity is concave along the 

 spine, and along either side of it are the broad, concave, 

 posterior walls formed by the ribs. The sternum is 

 directed obliquely downward and forward. 



THE HEART. 



The heart lies near the centre of the chest and is in- 

 vested by a strong, loose, fibro-serous sac, the pericardium. 



f 



I. 



FIG. 143. DIAGRAM OF THE HEART. 



a, left ventricle ; b, right ventricle : c, left auricle : d, right auricle : f, aorta ; g. g, pul- 

 monary arteries: h, inferior vena cava; i, superior vena cara : k, orifice of the superior 

 vena cava; 1, orifice of inferior vena cava; m, orifice of coronary vein ; o, left pulmonary 

 veins; p, right pulmonary veins; r, orifices of right pulmonary veins; B, orifice of left 

 pulmonary veins. 



That portion of the chest-cavity posterior to the pericar- 

 dium is called the posterior mediastinum, that portion in 

 front the anterior mediastinum ; the heart lies in the 

 middle mediastinum. 



The pericardium is a loose, fibro-serous sac, which 

 contains the heart and origin of the great vessels, to 



