154 THE CIRCULATION OF THE BLOOD 



lettering is more or less arbitrary, we must accept it because of its gen- 

 eral usage in all Avork of this kind. 



As to the causes of the waves A is of course caused by auricular systole 

 or presystole; C, occurring as it does at the beginning of the period of 

 ventricular systole, is caused by the bulging into the auricle of the closed 

 auriculoventricular valve. The floor of the auricle, in other words, at 

 this moment becomes somewhat elevated and imparts to the blood which 

 is resting upon it a slight wave of pressure, which is transmitted along 

 the veins for a considerable distance. The succeeding depression is 

 marked x, and the negative pressure which it indicates is- probably due 

 to the co-operation of three forces, all tending to increase the auricular 

 capacity: (1) the diastole of the walls of the auricle; (2) the descent 

 of the auriculoventricular groove, thus tending to open out somewhat 

 the folds in the walls of the auricle; and (3), probably most important of 

 all, the tendency of the thin-walled auricles to become dilated as a result 

 of the sudden diminution in intrathoracic pressure produced at each heart- 

 beat by the discharge of blood from the heart and intrathoracic blood 

 vessels into those of the rest of the body. All thin-walled structures 

 in the thoracic cavity, the auricles included, will expand to take up the 

 extra room created in the thoracic cavity. Similar negative heart pulses, 

 as they are called, can be observed with each systole in the lungs and 

 in the esophagus. 



THE MECHANISM OF OPENING AND CLOSING OF THE VALVES 



When physical valves open and close as a result of the changes in pres- 

 sure on their two surfaces, a certain amount of fluid must succeed in 

 passing the valve flaps before these become perfectly closed. But there 

 is every reason to believe that such is not the case in the heart, the flaps 

 of both the auriculoventricular and the s'emilunar valves being already 

 completely closed before pressure conditions entailing a possible regur- 

 gitation of blood through them become established. 



Auriculoventricular Valves 



During diastole the flaps of the auriculoventricular valves are hanging 

 down into the ventricle and floating in a half-open position in the blood, 

 which is meanwhile accumulating in the chamber. This position is de- 

 pendent upon the operation of tAVo opposing forces on the valve flaps: 

 the pressure of the blood floAving from the auricle on their upper aspects, 

 and reflected waves of pressure from the Avails of the ventricle on their 

 under aspects (centripetal reflux). When presystole occurs, the pres- 

 sure of the auricular stream momentarily increases, thus slightly dis- 

 tending the Avail of the meanAvhile relaxed ventricle and after a moment 's 



