1 III. HKAKT 85 



columns on the walls of the ventricle. The cords arc called 

 chordae tendineae, and the columns papillary muscles. 

 The chorda- tendincoj arc to prevent the flaps from being forced 

 back so far as to give way. At the top of the right ventricle, 

 towards the front of the heart, is a vessel leaving the right ven- 

 tricle. This is a large, thick-walled vessel, not collapsed, called 

 the pulmonary artery. It carries the blood from the right 

 ventricle to the lungs, and lies the farthest in front or most 

 anterior of all the vessels at the base of the heart. Cut it open 

 for a short distance. Just where it leaves the ventricle three 

 half-moon-shaped flaps of transparent membrane are arranged 

 round the inside of the vessel. Each flap forms a pocket open 

 on the side away from the ventricle. From their half-moon 

 shape these are called the semilunar valves. The valves 

 will allow blood to pass from the ventricle into the artery, but 

 not back again. Hold the heart under a tap so that water 

 runs down the artery towards the ventricle ; the pockets will 

 fill and swell out, and if you close up the artery again you will 

 see that they meet one another and block the way from the 

 artery to the ventricle. 



Open the left ventricle by cutting with a knife just on the 

 left of the oblique groove in front, and carry the knife round 

 the tip and up the back. The wall of the left ventricle is more 

 than half an inch thick. The partition or septum between the 

 two ventricles is also about half an inch in thickness and mus- 

 cular like the rest. The cavity of the left ventricle seems to 

 be smaller than that of the right, but there is really very little 

 difference, if any, when both are filled, because the septum 

 bulges into the right ventricle and so makes its cavity smaller 

 than it appears to be when empty. The two flaps of the mitral 

 valve hang down on each side of the opening from the left 

 auricle into the left ventricle. Each flap is a transparent 

 but tough membrane, from the edges of which the white 

 cords, the chorda? tendincx, pass to thick muscular pro- 

 jections, the papillary muscles, situated on the wall of 

 the ventricle. At the top of the ventricle, between the 

 mitral valve and the septum, is the opening of the aorta. 

 Put your finger into this and see which of the vessels at the 

 IMSC of the heart it is. Notice that it is large, thick-walled, 

 and not collapsed. It carries the blood to all parts of the 



