CIRCULATION AND THE BLOOD SYSTEM 77 



ing parts of the heart proper: right and left auricks, right and 

 left ventricles. By right and left. are meant the parts of the 

 heart that are right and left in its position in the body. 

 Which parts have the thickest walls? The waUs are made 

 cf muscle, and these thick-walled parts do the pumping. 



FIG. 36. Right Auricle and Ventricle (Sheep): RA, RV, right auricle and ven^ 

 tricle; IVC, SVC, inferior and superior venae cavse; a, b, bristle seekers showing 

 connections between auricle and ventricle, auricle and vena cava; PA, pulmonary 

 artery; tv, tricuspid valve; pp, papillary muscle; sv, semilunar valves. 



D. Cut off carefully the front walls of the right auricle 

 and ventricle. By means of the bristles locate the entrance 

 into the auricle of the inferior and superior vence cavce, and 

 the entrance into the ventricle of the pulmonary artery. 

 Find the connection between the auricle and the ventricle 

 and note the tricuspid valve that closes this entrance, Lo- 



