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 protuberances (trahecidae carneae), some of which (the papillary muscles) are attached to the tricuspid valves by slender strands, the chordae tendineae. When the ventricle contracts 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 openings 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
