174 THE VASCULAR MECHANISM. 



sound, which, as we have seen, is certainly due to the closure of the semi- 

 lunar valves, has been taken to mark the close of the ventricular systole. 

 And on this supposition the interval between the beginning of the first and 

 the occurrence of the second sound has been regarded as indicating approxi- 

 mately the duration of the ventricular systole i. e., the period during which 

 the ventricular fibres are contracting. 



The determination of the separate duration of each of the three periods of 

 the ventricular systole viz., the getting up of the pressure, the discharge of 

 the contents, and the remaining emptied but contracted is subjected to so 

 much uncertainty that it need not be insisted on here ; it may, however, be 

 said that, roughly speaking, each phase occupies probably about 0.1 second. 

 In a heart beating 72 times a minute, which may be taken as the normal 

 rate, each entire cardiac cycle would last about 0.8 second, and taking 0.3 

 second as the duration of the ventricular systole, the deduction of this would 

 leave 0.5 second for the whole diastole of the ventricle, including its relaxa- 

 tion, the latter occupying about or somewhat less than 0.1 second. In the 

 latter part of this period there occurs the systole of the auricles, the exact 

 duration of which it is difficult to determine, it being hard to say when it 

 really begins, but which, if the contraction of the great veins be included, 

 may perhaps be taken as lasting, on an average, 0.1 second. The " passive 

 interval," therefore, during which neither auricle nor ventricle is undergoing 

 contractions, lasts about 0.4 second, and the absolute pause or rest during 

 which neither auricle nor ventricle is contracting or relaxing, about 0.3 

 second ; if, however, a longer period be allotted to the ventricular systole, 

 these periods must be proportionately shortened. The systole of the ventricle 

 follows so immediately upon that of the auricles, that practically no interval 

 exists between the two events. 



The duration of the several phases may, for convenience sake, be arranged 

 in a tabular form as follows : 



Second. Second. 



Systole of ventricle before the opening of the semilunar 

 valves, while pressure is still getting up (probably rather 



less than 0.1 



Escape of blood into aorta (about) 0.1 



Continued contraction of the emptied ventricle (possibly 



rather more than) 0.1 J 



Total systole of the ventricle (probably rather more than) 0.3 



Diastole of both auricle and ventricle, neither contracting, 



or "passive interval " (probably rather less than) 0.4 | 



Systole of auricle (about or less than) 0.1 J 



Diastole of ventricle, including relaxation and filling, up to 

 the beginning of the ventricular systole (probably rather 

 less than) 0.5 



Total cardiac cycle 0.8 



Summary. 



126. We may now briefly recapitulate the main facts connected with 

 the passage of blood through the heart. The right auricle during its dias- 

 tole, by the relaxation of its muscular fibres, and by the fact that all back- 

 ward pressure from the ventricle is removed by the closing of the tricuspid 

 valves, offers but little resistance to the ingress of blood from the veins. On 

 the other hand, the blood in the trunks of both the superior and inferior 

 vena cava is under a pressure, which, though diminishing toward the heart, 

 remains higher than the pressure obtaining in the interior of the auricle ; the 

 blood in consequence flows into the empty auricle, its progress in the case of 

 the superior vena cava beinsj assisted by gravity. At each inspiration this 

 flow (as we shall see in speaking of respiration) is favored by the diminution 



