THE HEART. 



lar) that between the ventricles, and an indistinct groove 

 (interauricular) that between the auricles. [783] 



The base is quadrilateral, formed by the auricles (chiefly the 

 left) , and lies on the fifth to eighth vertebra?. The superior and 

 inferior venae cavae enter it at the right upper and lower 

 angles, the right pulmonary veins enter between them, and 

 the left pulmonary veins near the left border. The apex is 

 formed by the left ventricle and lies at the fifth intercostal 

 space, three and a quarter inches from the midline. The in- 

 ferior surface is entirely ventrictilar (chiefly the left) and lies 

 on the central tendon and (left side) a small part of the muscle 

 of the diaphragm. The antero-superior surface lies behind the 

 sternum and third to sixth right costal cartilages; the upper 

 part, entirely auricular, is concave; the lower part, entirely 

 ventricular (chiefly the left), is convex; the pulmonary artery 

 and, behind it, the aorta enter at the junction of the auricular 

 and ventricular parts. [784] 



The left margin is thick and rounded, and formed chiefly by 

 the left ventricle, slightly by the left auricle. Right margin: 

 The sharp lower part is nearly horizontal and formed by the 

 right ventricle. The upper part is nearly vertical, and formed 

 by the right auricle ; a shallow groove (sulcus terminalis) runs 

 on it from in front of the superior vena cava to the left of the 

 inferior vena cava. [786] 



Auricles. These are cuboidal and lined with endocar- 

 dium; the right one lies in front and to the right. A prolonga- 

 tion (auricular appendix) projects forward from the upper and 

 anterior part of each auricle. [786] 



Interior of Right Auricle. This is smooth, except in front 

 and in the appendix where there are vertical muscle columns 

 (musculi pectinati) which end above in a ridge (crista ter- 

 minalis). On the posterior surface the superior vena cava 

 enters above, and the inferior vena cava below; a small emi- 

 nence (tubercle of Lower) lies between these orifices. On the 

 septum is an oval depression (fossa ovalis) , the remains of the 



[125] 



