THE VASCULAR SYSTEM 61 



lated blood. Observe the shape of the right ventricular 

 cavity. It communicates with the pulmonary artery 

 through the pulmonary orifice, which is surrounded by the 

 three pocket-like segments of the pulmonary valve. Locate 

 these valves and determine how they function. The right 

 atrioventricular, or tricuspid, orifice is the aperture 

 through which blood enters the right ventricle from the 

 right atrium. It is guarded by the tricuspid valve. The 

 inner surface of the ventricle bears muscular protuber- 

 ances {trahecidae carneae), some of which (the papillary 

 muscles) are attached to the tricuspid valves by slender 

 strands, the chordae tendineae. When the ventricle con- 

 tracts the blood starts to rush back into the atrium, but in 

 so doing the tricuspid valve is thrust across the aperture 

 by the blood stream. The edges of the valve would be 

 forced up into the atrium were it not for the chordae 

 tendineae and the papillary muscles, which hold the flaps 

 athwart the opening. Thus blood having once entered the 

 right ventricle is unable to retreat into the atrium when 

 the ventricle contracts, but must find its exit through the 

 pulmonary artery. Insert the scissors' point into the 

 severed inferior vena cava and open the right atrium by 

 a cut extending forward to the right superior vena cava 

 along the anterior surface of the atrium. Locate the open- 

 ings of the superior and inferior venae cavae, and the 

 atrioventricular aperture. Find the membranous valves 

 near the entrance of the superior vena cava. 



Slit the ventral wall of the left ventricle lengthwise by 

 inserting one point of the scissors into the cut end of the 

 aorta. Extend the incision to the .apex of the heart. Try 

 not to injure the aortic valve whose semilunar segments 

 surround the opening of the ventricle into the aorta. The 

 muscular wall of the left ventricle is considerably thicker 

 than the wall of the right, for more pressure is required 



