128 ELEMENTARY PHYSIOLOGY 



notice the thinness and irregularity of its walls. We are now enabled to look 

 down into the cavity of the left ventricle, which resembles the right in that a 

 membranous valve lies between it and the auricle. This valve is called the 

 mitral valve from its supposed resemblance to a bishop's mitre : it is also 

 called the bicuspid valve, being composed of two cusps or flaps. 



As the left ventricle is as yet uncut, we may easily study the action of the 

 valve as follows : Pour a little water into the ventricle, close the aorta 

 which leads from it, and then squeeze the ventricle between the ringers and 

 the thumb. Each time we do so, we notice that the cusps of the valve 

 float up on the surface of the water, till they partially or entirely close the 

 ventricle. 



If we remove the right auricle with its vessels (the venae cavae), we shall 

 find that its structure is much like that of the left auricle. Again, by cutting 

 through the walls of the left ventricle, we notice that this cavity closely 



Fig. 1 20. The Base of the Heart. 



The auricles have been cut away, and the valves are closed. The pericardium has also 



been removed to expose the muscular fibres. 



i and i , right ventricle ; 2, left ventricle ; 3, wall of right auricle ; 4, wall of left auricle ; 5, 

 5', and 5 , the tricuspid valve ; 6_and 6', the mitral valve ; 7, pulmonary artery ; 8, aorta ; 9 

 and 9', coronary arteries. 



resembles the ventricle already dissected, but that its walls are very much 

 thicker. 



Lastly, lay open the aorta as we did the pulmonary arteries, and here, at 

 the origin of this great artery, we shall notice another set of semilunar 

 valves, which are also arranged so as to prevent the blood from flowing back 

 to the heart. If instead of laying open the aorta, we cut it short immediately 

 above the semilunars, we may fill the three little pouches with water, thus 

 illustrating their action, i.e. their tendency to close the passage from the 

 aorta into the ventricle when acted on by a pressure from above. Just outside 

 the semilunars of the aorta may be seen two small openings. These are the 

 origins of the coronary arteries, by which the heart itself is supplied with 

 blood for its nourishment. This blood is afterwards collected up by the 

 coronary veins ; and branches of both the coronary arteries and the coronary 



