168 



STUDIES IN ADVANCED PHYSIOLOGY. 



represented by the syllable " dupp." This sound is due to 

 the sudden closing of the two semilunar valves. Ordinarily 

 the semilunar valves of the aorta and pulmonary artery close 

 so nearly at the same time that the sound is heard as one. 

 After violent exercise, however, when the pressure in the 

 aorta is materially increased, the semilunar valves at its 

 origin close perceptibly sooner than those of the pulmonary 

 artery, and there are two distinct second sounds. The 

 earlier closing of the valves in the aorta is due to the greater 

 pressure or blood forcing them together, just as two doors 

 entirely alike would close at different moments if winds of 

 different strengths should strike them. 



Cardiograms. 



All these observations on the events of a heart beat may 

 be best studied by means of cardiograms. These cardio- 



Fig. 81. CARDIOGRAMS OF THE RIGHT AURICLK (R V) AND THE RIGHT VENTRICLE 

 (E K) OF A HORSE. THE DOTTED LINES CONNECT SAME INSTANTS OF TIME. 



grams are tracings made by placing systems of levers on the 

 ventricle and auricle, so arranged that the systole of these 

 will raise the levers and the diastole lower them. If, now, 

 the ends of such moving levers be made to trace a 

 line on a plate moving past them at a known speed, there 

 will result curves such as those pictured in figures 81 and 82. 

 The cardiogram in figure 82 is one made on a pathologic- 

 ally exposed human heart, and so is peculiarly valuable as 

 giving the actual course of events in man himself. In this 



