166 



CIRCULATION OF THE BLOOD 



the arteries. Without the valves not a drop of blood would reach the arteries, 

 for the resistance in the arteries is considerably greater than that in the auri- 

 cles and in the great veins emptying into them. 



During the ventricular diastole the atrio-ventricular flaps are more or 

 less closely approximated by simply floating into position on the blood as it 

 fills the ventricle. When the ventricular systole sets in and the pressure in 

 the ventricles rises suddenly, the valvular flaps naturally strike together and 

 so cut off connection of the ventricle with the auricle. 



Because the pressure in the auricle at the systole of the ventricle is infini- 

 tesimally small as compared with that in the ventricle itself, the valves must 

 close so quickly that at most only a very insignificant quantity of blood can 

 get back into the auricles before the closure is complete. It appears even 

 that the valves work so promptly that absolutely no regurgitation of the blood 



into the auricle takes place. When the auricle 

 contracts, the ventricle is somewhat distended 

 owing to the flexibility of its resting wall and 

 its contents are subjected to a certain tension. 

 At the moment the contraction of the auricle 

 abates somewhat or ceases altogether the pres- 

 sure in the ventricle becomes greater than that 

 in the auricle, and by this means the valvular 

 flaps are brought together even before the ven- 

 tricular systole begins (Baumgarten). 



The great pressure which is brought to bear 

 on the valves during the ventricular systole 

 would cause them to turn up into the auricles, 

 and thereby cause serious disturbance to the cir- 

 culation, were it not for the chordae tendinse. 

 Since the chordae are attached not only at the 

 free edges but also on the flat surface of the 



valves, the latter are prevented not only from turning up into the auricle, but 

 even from bulging toward it. 



Because of the chordae tendinse the closed valve takes a perfectly definite 

 position, the central parts of the flaps being pushed up to the level of their 

 attachment, and their turned-down edges being applied to each other as in Fig. 

 54. By this means closure is established over a greater surface and is secured 

 also by the fact that the bent portions of the edges dovetail into each other by 

 toothlike folds, so that the valves are able to sustain the great pressure. The 

 circumference of the base of the heart, and at the same time that of the atrio- 

 ventricular opening, becomes very much smaller in ventricular systole : the mus- 

 cles surrounding the opening therefore must be credited with a share in the 

 closure of the passage. 



The role of the papillary muscles in the closing of the atrio-ventricular valves 

 has been conceived in very different ways. The most probable view is, that they 

 prevent swinging of the valves into the auricles, the approach of the heart base 

 toward the apex being compensated by their contraction and consequent pull on 

 the chordae tendinae. 



When the auricles contract, regurgitation of blood into the great veins is 

 prevented by contraction of the circular musculature passing around the latter, 



FIG. 54. Position of the atrio- 

 ventricular valves when closed, 

 schematic drawing, after Krehl. 



