292 THE VASCULAR SYSTEM 



sac, as a result of pericarditis, give rise to no characteristic 

 signs beyond dilatation and hypertrophy of the heart. 



THE HEART 



The Heart is situated within the pericardium in the middle 

 mediastinum. Its posterior surface^ which consists of the two 

 atria (auricles), is placed opposite the fifth, sixth, seventh and 

 eighth thoracic vertebrae and is separated from them by the 

 pericardium, the oesophagus and the descending thoracic 

 aorta. The antero-superior surface is in relationship to the 

 lungs and pleural sacs and to the anterior chest wall, and it 

 is the outlines of this surface which are represented when the 

 heart is mapped out on the anterior surface of the body. It 

 comprises (i) The right atrium, which occupies the right 

 portion of the area ; (2) the right ventricle, which forms the 

 large central area ; (3) the left ventricle, which is only repre- 

 sented by a narrow strip along the left border ; (4) the left 

 auricle (auricular appendix), which forms the left upper 

 corner of the surface. 



The inferior surface of the heart rests on the diaphragm, 

 by which it is separated from the superior surface of the liver 

 and the antero-superior surface of the stomach. It consists 

 of a small portion of the right atrium, which is placed to the 

 right and posteriorly, but most of the surface is formed by 

 the ventricles. Hyperdistension of the stomach may influence 

 the heart's action (i) by mechanical pressure, and (2) reflexly, 

 through the vagus nerves. 



The Right Atrium (Auricle) receives the blood from the 

 great systemic veins and expels it through the right atrio- 

 ventricular orifice into the right ventricle. The opening of 

 the superior vena cava is placed at the right upper extremity 

 of the atrium and is not guarded by a valve. The opening 

 of the coronary sinus, which returns the blood from the heart 

 wall, and the opening of the inferior vena cava are placed at 

 the lower part of the chamber and both are guarded by slender 



