CIBCULA TION. 



109 



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

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

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

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

 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, aud the curtains of the valve 

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

 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 au arrangement to prevent a 

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

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



Fig. is.— 'rile left ventricle and aorta Laid open, t>> show the mitral ami aortic semilunar valves i Efenle). 



as guy-ropes stretched between the muscular wall of the ventricle aud 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 Prom their 



