THE HEART 331 



great veins as well as the muscular substance of the heart 

 itself. They arise from the aortic sinuses (p. 333), the right 

 artery from the ventral sinus, the left one from the left 

 dorsal sinus. 



The blood distributed to the walls of the heart is collected by four 

 groupsof freely anastomosing cardiac veins. Those draining the left side 

 unite in a left cardiac vein, which passes round in the left atrioventricular 

 groove and enters the caudal part of the left superior vena cava, or the 

 coronary sinus. The right cardiac vein, receiving the vessels from the 

 right side, lies in the right atrioventricular groove and opens into the 

 coronary sinus. The veins draining the dorsal surface of the heart 

 join the right cardiac vein as it enters the sinus. Minute veins from 

 the terminal portion of the right ventricle open directly into the right 

 atrium. 



(d) The left atrium (atrium sinistrum) is the thin -walled 

 chamber lying to the left at the base of the heart. The 

 pulmonary veins (venae pulmonales), passing from the 

 medial portions of the lungs, usually unite into two main 

 vessels on each side and these fourvesselsenter a short, wide, 

 funnel-shaped diverticulum on the left atrium. (This diver- 

 ticulum is a feature of more primitive mammals.) 



(e) The right atrium (atrium dextrum) resembles the left in 

 the character of its wall. It receives the right and left 

 superior caval veins and the unpaired inferior caval vein. 



The heart may be removed by dividing the great blood-vessels. 

 The arch of the aorta should be removed with the heart by dividing 

 the vessel at a point beyond the origin of the left subclavian, and 

 then severing the carotids and subclavians at their bases. This 

 exposes the surface for the subsequent examination of the posterior 

 end of the trachea and its connections with the lungs. 



Open the right ventricle by a longitudinal incision of the ventral 

 wall, extending the incision forward into the pulmonary artery. 

 Open both atria by transverse incisions. By washing out the 

 cavities, the internal features of the wall, including the arrange- 

 ment of the valvular structures, may be examined as follows: 



In the right ventricle: 



(a) The trabeculae carneae; muscular ridges of the internal 

 surface of the wall. 



