VALVES OF HEART 289 



from gathering, under the influence of gravity, in the lower parts 

 of the body. Harvey saw that this explanation did not account 

 for the setting of the valves in the veins of the neck, and we saw 

 how, by noting the direction in which the valves would allow 

 fluid to pass, he discovered the circulation of the blood. 



It is clear that the pockets offer practically no resistance to the 

 passage of blood towards the heart. If, however, the pressure 

 on the heart (or central) side of a valve becomes greater than the 

 pressure in the preceding segment, the pockets will fill with blood, 

 become distended and effectively prevent a back-flow. That 

 this is so can be proved by repeating one of Harvey's experiments. 

 He tied a ligature round the upper part of his arm and so dammed 

 up the blood in the lower part of the arm. When he milked these 

 swollen veins towards the hand he noticed that the blood could 

 not pass certain points where lie knew valves were placed. No 

 valves are necessary in the arteries as there is always a positive 

 driving pressure. The type of two of the valves of the heart is 

 indicated in Figs. 58 and 61. 



(1) The auriculo- ventricular valves are triangular sheets of 

 fibrous tissue tough but flexible fixed by one side to the 

 auriclo-ventricular ring and hanging apex downwards into the 

 ventricular cavity. The pointed part of each flap or cusp is tied 

 to the ventricular wall by a number of cords, chordae tendineae. 

 The main cords are, however, not inserted directly into the 

 ventricular wall but are attached to the finger-like papillary 

 muscles. These muscles regulate the tension of the valve-flaps. 

 When the ventricles contract so do the papillary muscles- 

 pulling on the chordae and thus bringing the cusps closer together. 

 The increasing pressure of the blood in the ventricle causes the 

 flaps to bellow out and block the passage-way so that the blood 

 cannot pass back into the auricles. The greater the pressure 

 developed in the ventricle, the more tightly is the valve shut. 

 The cusps may even bulge up into the auricles. Valves con- 

 structed on this principle are obviously fitted to occlude openings 

 which vary in size and shape during the various phases of the 

 cardiac cycle. 



The right and left sides of the heart differ in the number of 

 cusps in their valves and in the details of their movements. 

 The mitral valve on the left side of the heart has only two triangular 

 flaps like a bishop's mitre, while on the other hand the passage 

 way from right auricle to right ventricle is guarded by the three 

 cusps of the tricuspid valve. 



B B. 19 



