376 CIRCULATION 



vessels at the root of the neck. These sluice-gates are very 

 simple contrivances — just little pockets set in pairs opposite each 

 other in the vein. Fabricius noted that the openings of the pockets 

 were always directed toward the central part of the body. He 

 interpreted this as indicating a mechanism to prevent the blood 

 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, by 

 noting the direction in Mdiich 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 he 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. 85 and 88. 



(1) The atrio- ventricular valves are triangular sheets of fibrous 

 tissue — tough but flexible — fixed by one side to the atrio- 

 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 tendirieae. 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. The bases of the 

 valve-flaps are approximated by the ventricular contraction which 

 begins at the base. When the ventricles contract so do the papillary 

 muscles- — pulling on the chordae and thus preventing the cusps 

 from being pushed through into the atria. The increasing pressure 

 of the blood in the ventricle causes the flaps to belly out and block 

 the passage-way so that the blood cannot pass back into the 

 atria. The greater the pressure developed in the ventricle, the 

 more tightly is the valve shut. The cusps may even bulge up 

 into the atria. Valves constructed 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 



