154 THE CIRCULATION OF THE BLOOD 



descent of the auriculoventricular groove, thus tending to open out some- 

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

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

 of the sudden diminution in intrathoracic pressure which is produced 

 at each heartbeat by the discharge of blood from the heart and intra- 

 thoracic blood vessels into those of the rest of the body. Under these 

 conditions all thin-walled structures in the thoracic cavity, the auricles 

 included, will expand to take up the extra room thus created in the 

 thoracic cavity. Similar negative heart pulses, or cardiopneumatic move- 

 ments, as they are called, can be observed accompanying each cardiac 

 systole in the air passages and in the esophagus. This is done by con- 

 necting sensitive tambours through tubing, either with one nostril (while 

 the other nostril and the mouth are closed and the breath held) or with 

 a thin rubber bag lying in the esophagus. Cardiopneumatic movements 

 can sometimes be seen in the intercostal spaces of emaciated persons, 

 and they often become pronounced in cardiac hypertrophy. 



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 presystole 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 



