154 THE CIRCULATION OF THE BLOOD 



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 i-ncrease 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), no doubt 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 semilunar 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 two opposing forces on the valve flaps: 

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

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

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

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

 tending the wall of the meanwhile relaxed ventricle and after a moment's 

 delay causing the reflected wave to become more pronounced. At the 

 same time the muscular fibers in the valve flaps (Kiirschner's fibers) 

 contract and make the flaps shorter, the total effect of the two factors 

 being that the valve takes up a position nearer that of closure. When 

 presystole suddenly stops, the reflected waves will persist for an instant 



