282 TEXT-BOOK OF PHYSIOLOGY. 



sure continues to rise, for the aortic pressure must not only be ex- 

 ceeded, but a certain velocity must be imparted to the blood. Between 

 the ordinates i and 4, the semilunar valves remain open and the 

 blood passes into the aorta. 



In accordance with the foregoing the ventricular systole may be 

 subdivided into two periods : 



1. The period of rising tension, from the beginning of the systole to 



the opening of the semilunar valves, occupying from 0.02 to 

 0.04 second. 



2. The period of ejection, from the opening of the semilunar valves 



to the end of the systole, occupying about 0.2 second. 

 The ventricular diastole may also be divided into two periods : 



1. The period of falling tension or relaxation, from the end of the 



systole to the time of lowest pressure in the ventricle, occupying 

 about 0.05 second. 



2. The period of filling, from the opening of the mitral valve to the 



beginning of the systole. 



Negative Pressure. As shown by the ventricular pressure curve 

 there is a moment when the pressure falls below atmospheric pres- 

 sure, becoming negative to it. The extent to which this takes place, 

 its duration and frequency, have never been satisfactorily determined. 

 The cause of the negative pressure, its influence on the opening of 

 the auriculo-ventricular valves, and on the entrance of blood into the 

 ventricles are equally unknown. The most probable cause is an 

 expansion of the base of the ventricles due to the enlargement of the 

 aorta and pulmonary artery. That it is not due to the expansion 

 of the thorax is evident from the fact that it is occurs when the thorax 

 is open and the heart exposed. 



Heart-sounds. Two sounds accompany each pulsation of the 

 heart, both of which may be heard by applying the ear or the stetho- 

 scope to the chest- walls, especially over the region of the heart. One 

 of these sounds is low in pitch, dull and prolonged; the other is high 

 in pitch, clear j,ncl_s_hort. These sounds can be approximately repro- 

 duced by pronouncing the syllables lubb-dirpp, lubb-dupp. The 

 rlong dull sound occurs with tfre systole. the~nrs~t phase of a new cardiac 

 cycle, and is therefore fermed the first sound; the short clear sound 

 occurs at the beginning of the diastole, with the second phase of the 

 cardiac cycle, and is therefore termed the second sound. The first 

 sound is the systolic, the second the diastolic, sound. With the ear 

 it can readily be determined that there is a brief pause between the 

 first and second sounds, and a longer pause between the second and the 

 first sounds. The duration of the first sound is almost equal to the 

 duration of the systole viz., 0.3 second; the duration of the second 

 sound is not more than o.i second. The systolic sound is heard 

 most distinctly over the body of the heart; the diastolic sound is 



