78 PEACTICAL PHYSIOLOGY 



the inferior vena cava. It is directed from the posterior wall towards 

 the internal wall. 



Note also the size and form of the auriculo-ventricular orifice. Cut 

 away most of the auricle, and put the auriculo-ventricular orifice for a 

 moment under the tap. The valve will float up. The flaps are brought 

 into opposition by eddies the moment "the ventricular pressure becomes 

 greater than the auricular pressure. Note the shape of each flap, and 

 the upward convexity of the valve flaps when closed, and the star- 

 shaped figure formed by their opposition. Note also the papillary 

 muscles and chordae tendineae. A band of muscle the moderator 

 band crosses the right ventricle of the sheep's heart. 



Next cut through the chordae tendineae, and then place the auriculo- 

 ventricular orifice for a moment under the tap. The valve-flaps are now 

 driven towards the auricle, and the flap is no longer competent. In- 

 troduce a pair of scissors between two of the valve-flaps, and cut down 

 to the bottom of the ventricle. Then turn round the scissors and cut up 

 close to the septum, towards, but not as far as, the pulmonary artery. 

 Observe the columnae carneae and papillary muscles in the lower part of 

 the ventricle. These pack together and obliterate the lower part of 

 the ventricle during systole. Acting as elastic cushions, they rebound 

 in diastole and produce a momentary negative pressure in the ventricle. 

 Note the funnel-shaped, smooth-walled upper part of the ventricle the 

 conus arteriosus which leads into the pulmonary artery. This part is 

 not emptied during systole, and blood thus remains in contact with the 

 auriculo-ventricular valve, and ensures its closure. Note the form of the 

 flaps of this valve, and their attachment to the auriculo-ventricular ring. 

 Some of the chordae tendineae are attached to the edges, and others to 

 the under surface of the valves. Owing to the papillary muscles and 

 chordae tendineae, the auriculo-ventricular valve presses on the blood 

 during systole, equally with the rest of the ventricular wall. 



Now lay open the pulmonary orifice and note the shape and attach- 

 ment of the semi-lunar valves and the small nodule of tissue in the 

 free edge of each flap. Observe also the sinuses of Valsalva. These 

 favour the formation of eddies, which bring the valves in opposition the 

 moment the intraventricular pressure becomes less than the pulmonary 

 arterial pressure. 



Cut open the left auricle in the same manner as the right, and 

 observe the two flaps of the left auriculo-ventricular valve, the papillary 

 muscles, etc., and thickness of the left ventricular wall. Cut across the 

 aorta just above its origin and observe the three aortic semi-lunar 

 valves. Insert the nozzle of the tap through this valve into the left 

 ventricle and turn on the water. The auriculo-ventricular valve closes 



