COMPOSITION OF HEART. 



313 



The auricles are two (right and left) and receive their appellation from 

 the resemblance of the appendices to the dog's ears : they are placed so 

 deeply at the base of the heart, that only the tip of the right one comes 

 forwards to the sternum. The auricles are much thinner than the ven- 

 tricles. 



Fi 



DIAGRAM SHOWIXO THE POSITION OF THE HEART TO THE RIBS AND STKRNUM, the soft parts being 

 removed from the exterior of the thorax. The edge of each lun.,' is shown by a dotted Hue. 



The ventricles reach unequal distances on the two aspects of the heart: 

 thus the right one forms the thin right border and the greater part of the 

 anterior surface; but the left enters alone into the apex, and constructs 

 the left border, and most of the posterior surface of the heart. 



Dissection. Before opening the heart the coronary arteries are to be dis- 

 sected on the surface, with the small nerves and veins that accompany 

 them. The two arteries appear on the sides of the pulmonary artery, and 

 occupy the grooves on the surface of the heart, where they are surrounded 

 by fat : one branches over the right, and the other over the left side. With 

 the anterior artery is a plexus of nerves, which is to be followed upwards 

 to the superficial cardiac plexus ; and with the remaining artery another 

 plexus is to be sought. 



In the groove at the back of the heart between the auricles and ven- 

 tricles, the student will find the large coronary vein, and the dilated coro- 

 nary sinus in which it ends on the right : the last should be defined and 

 followed to its ending in the right auricle. 



The coronary arteries are two small vessels which are so named from 

 their course around the heart : they are the first brandies of the aorta. 

 One is distributed mostly on the right, and the other on the left side of the 

 heart. 



The riyht coronary branch appears on the right side of the pulmonary 



