CIRCULATION. 



401 



opposed parts of the surfaces of the curtains, those parts undergo no strain, 

 and hence are enabled to be exquisitely delicate and flexible and therefore 

 easily fitted to one another. On the other hand, the parts of the valve which 

 intervene between the surfaces of contact and the auriculo-ventricular ring are 



O 



tough and much thicker, as they have to bear the brunt of the pressure within 

 the contracting ventricle. As the systole of the ventricle increases, the auric- 

 ulo-ventricular ring probably becomes smaller, and the curtains of the valve 

 probably become somewhat fluted from base to apex, so that their line of con- 

 tact is a zig-zag. At the same time their surfaces of contact may increase in 

 extent. 



Tendinous Cords and their Uses. The structure so far described is 

 wonderfully effective because it is combined with an arrangement to prevent a 

 reversal of the valve into the auricle, which otherwise would occur at once. 

 This arrangement consists in the disposition of the tendinous cords, which act 



FIG. 103. The left ventricle and aorta laid open, to show the mitral and aortic semilunar valves (Henle). 



as guy-ropes stretched between the muscular wall of the ventricle and the 

 valve, whether mitral or tricuspid. These cords are tough and inelastic, and, 

 like the valve, are coated with the slippery lining of the heart. They are 

 stout where they spring from the muscle, but divide and subdivide into 

 branches, strong but sometimes very fine, which proceed fan-wise from their 



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