940 ANIMAL PHYSIOLOGY. 
It thus appears that the heart is not only a force-pump but 
also to some extent a suction-pump; and, if so, the aspirating 
effect must express itself on the great veins, lacking valves as 
they do, at their entrance into the heart; hence, with each dias- 
tole the blood would be sucked on into the auricles, a result 
that is intensified by the respiratory movements of the 
thorax. 
Relative Time occupied by the Various Phases of the Cardiac Cycle. 
—The old and valuable diagram reproduced below is meant to 
convey through the eye the relations of the main events in a 
complete beat of the heart or cardiac cycle. The relative 
length of the sounds; the 
v2) 
oneavE EEE, 
long period occupied by the 
pause; the duration of the 
ventricular systole, which 
it is to be observed is in 
excess of that of the first 
sound, are among the chief 
facts to be noted. 
The tracings of Chau- 
veau and Marey, obtained 
from the heart of the horse, 
which has a very slow 
rhythm, show that of the 
whole period, the auricular 
Fic. 214. — Diagram regivesentiing the movements systole OCCUPIES, $ eas to of 
and sounds of the heart during a cardiac cycle a second | the ventricular 
(after Sharpey). 
systole, 2 or + of a sec- 
ond; and the diastole, # or 3 of a second. 
With the more rapid beat in man (70 to 80 per minute), the 
duration of the cardiac cycle may be estimated at about 58, of 
a second, and the probable proportions for each event are about 
these: The auricular systole, #4 of a second; the ventricular 
systole, 3, of a second; and the pause, + of a second. 
It will be noted that the pause of the heart is equal in dura- 
tion to the other events put together; and even assuming that 
there is some expenditure of energy in the return (relaxation) 
of the heart to its passive form, there still remains a consider- 
able interval for rest, so that this organ, the very type of cease- 
less activity, has its periods of complete repose. 
