THE CHAMBERS OF THE HEAKT. 



873 



coronary sulcus, which runs obliquely from above downwards, and from left to 



right, from . the level of the third left to that of the sixth right costal cartilage. 



The upper section of the surface, which is concave anteriorly, is formed by the 



atria ; it is separated from the sternum by the ascending aorta and the pulmonary 



artery, and is continuous laterally with the auricles of the atria which, projecting 



forwards, embrace the great vessels. 



The lower section of the sterno-costal 



surface is convex ; it is formed by the 



ventricular part of the heart, and is 



divided, by an anterior interventricn- 



lar sulcus, into a smaller left and a 



larger right part. At the junction of 



the atrial and ventricular parts of 



this surface are the orifices of the 



pulmonary artery and the aorta, the 



former lying anterior to the latter. 



The right margin of the heart is 

 formed by the right atrium. It lies 

 posterior to the cartilages of the third, 

 fourth, fifth, and sixth ribs on the 

 right side, about half an inch from the 

 margin of the sternum ; it is in re- 

 lation with the right pleura and lung, 

 the phrenic nerve with its accompany- 

 ing vessels intervening, and it is 

 marked by a shallow groove the 

 sulcus terminalis which passes from 

 the front of the superior vena cava to 

 the front of the inferior vena cava. 

 The inferior margin of the sterno-costal surface is sharp, thin, and usually concave 

 corresponding with the curvature of the anterior part of the diaphragm; it is 

 formed mainly by the right ventricle and only near the apex by the left ventricle 

 It lies, almost horizontally, in the angle between the diaphragm and the anterior 

 wall of the thorax, passing from the sixth right costal cartilage, posterior to the 

 lower part of the body of the sternum, or the xiphoid process, and the cartilages 

 rf the sixth and seventh ribs on the left side, to the apex of the heart. 



The left margin is formed mainly by the left ventricle, and only to a small 

 3xtent by the left atrium ; it is thick and rounded. It lies in relation with 

 :he left pleura and lung, the phrenic nerve and its accompanying vessels inter- 

 vening, and it passes from just above the third left costal cartilage, about an inch 

 Tom the sternum, to the apex of the heart, descending obliquely and with, a con- 

 vexity to the left. 



Fio/752. THE RELATION OF THE HEART TO THE 

 ANTERIOR WALL OF THE THORAX. 



i, n, in, iv, v, vi, the upper six costal cartilages. 



THE CHAMBEKS OF THE HEAKT. 



Atria. The atrial or basal portion of the heart is cuboidal in form. Its long 



, which lies transversely, is curved, with the concavity of the curve forwards. 



s cavity is divided into two chambers the right and left atria by a septum 



ich runs from the anterior wall backwards and to the right, so obliquely that 



)he right atrium lies anterior and to the right, and the left atrium posterior and 



;o the left. 



Each atrium is also somewhat cuboidal in form, the long axes of both being 



rtical, and each possesses a well-marked ear-shaped, forward prolongation, known 



the auricle, which projects from its anterior and upper angle. 



Atrium Dextrum. The right atrium receives, posteriorly, the superior vena 



i above and the inferior vena cava below. Between them, and a little above 



niddle, it is crossed posteriorly by the lower right pulmonary vein. It is coii- 



luous below and anteriorly with the right ventricle, at the atrio-ventricular 



