THE HEART AND ITS DISSECTION 



tion between the ventricle and the auricle above it. The three cusps thus 

 form a valve called the tricuspid valve ; and, from the arrangement we have 

 lust observed, it is clear that this valve will allow a free passage of blood 

 from auricle to ventricle, but not in the opposite direction. 



If we now prolong the cut towards the origin of the pulmonary arteries 

 until we lay open the trunk which gives rise to these vessels, we shall find 

 another set of valves called the semilunar valves (Lat. semi, half; and luna, 



Fig. 119. Interior of the Bight Side of the Human Heart. 



i, superior vena cava ; 2, inferior vena cava ; 3, interior of the right auricle ; 4, semilunar 

 valves of the pulmonary artery ; 4', papillary muscle ; 5, 5', and 5", cusps of the tricuspid 

 valve; 6, pulmonary artery; 7, 8, and 9, the aorta and its branches ; 10, left auricle; u, 

 left ventricle. 



the moon). These are three semi-circular membranous pouches, with their 

 convex surfaces turned toward the ventricle. They therefore offer no obstruc- 

 tion to the blood as it passes from the ventricle, but a backward tendency of 

 the blood would fill the pouches, causing their edges to meet, and thus closing 

 the passage into the ventricle. 



Now remove the left auricle completely, with its pulmonary veins, and 



