THE THORAX 973 



artery, which lies in its posterior part ; (3) a portion of the great 

 cardiac or coronary vein, which lies in its anterior part, and also 

 slightly in its posterior part ; and (4) the coronary sinus, which lies 

 in its posterior part. 



The auricular portion of the heart has the form of a crescent, the 

 horns of which represent the auricular appendices, and are directed 

 forwards. The concavity of the crescent is also directed forwards, 

 and it lodges the roots of the aorta and pulmonary artery, the latter 

 being the more anterior of the two. The walls of the auricular 

 portion are thin, and it is divided into two auricles, right and 

 left. The external indication of this division is the inferauricular 

 groove, which is situated vertically on the posterior surface, to the 

 left of the openings of the superior and inferior venae cavae. This 

 grooA-e corresponds to the posterior attachment of the interauricular 

 septimi in the interior. The greater part of the posterior surface 

 of the auricular portion is formed by the left auricle. The large 

 posterior part of each auricle is called the atrium (' reception- 

 chamber '), and projecting forwards from the anterior and upper 

 part of each atrium there is the auricular appendix. The two 

 auricular appendices embrace between them the roots of the aorta 

 and pulmonary artery'. 



The right auricle forms the anterior and right part of the base of 

 the heart, and is quadrangular. The superior vena cava enters its 

 atrium at the postero-superior angle, and the inferior vena cava 

 at the postero- inferior angle. Near the latter vein the coronary 

 sinus also opens into the atrium. The right auricular appendix 

 is prolonged forwards from the antero-superior ang^s of the atrium, 

 and inclines to the left in front of the root of the ascending aorta. 

 It is shorter, broader, and less curved than the left auricular 

 appendix, and its margin is notched, but not so much so as in the 

 left appendix. The right auricle is traversed by a groove, called 

 the sulcus terminalis, which extends from the front of the ter- 

 mination of the superior vena cava to the right side of the termina- 

 tion of the inferior vena cava. This groove indicates where the 

 saccus reuniens of embryonic life meets the primitive auricle. 



The left auricle forms the posterior and left part of the base of 

 the heart. It is quadrilateral, and the greater part of it lies flattened 

 behind the ascending aorta and trunk of the pulmonary artery. 

 The pulmonary veins, two right and two left, open into the posterior 

 part of its atrium. The left auricular appendix is prolonged for- 

 wards from its left aspect, and inclines to the right over the left 

 side of the root of the pulmonary artery. It is longer, narrower, 

 and more curved than the right auricular appendix, and its margin 

 is more deeply notched. The back of the left auricle is related to 

 the oesophagus, with the intervention of the pericardimn, and the 

 small oblique vein of Marshall passes downwards and inwards upon 

 it, to open into the coronary sinus. 



The ventricular portion of the heart is conical, its walls are thick, 

 and it is divisible into two ventricles, right and left. The ex- 



