THE CIRCULATION OF THE BLOOD. 243 
posture; season, more frequent in summer; period of the day, 
more frequent in the afternoon and evening; elevation of tem- 
perature, the inspiratory act, emotions and mental activity, eating, 
muscular exercise, etc., render the heart-beats more frequent. 
2 The length of the systole, though variable, is more con- 
stant than that of the diastole. The estimated limits of the 
systole may be stated as 327 to ‘301 second. 
3. The force of the pulsation varies very greatly and exer- 
cises an important influence on the blood-pressure, and the 
velocity of the blood-stream. Asa rule, when the heart beats 
rapidly, especially for any considerable length of time, the force 
of the individual pulsations is diminished. 
4. The heart-beat may vary much and in ways it is quite 
possible to estimate, both directly by the hand placed over the 
organ on the chest, by the modifications of the cardiac sounds, 
and by the use of instruments. It is wonderful how much in- 
formation may be conveyed, without the employment of any 
instruments, through palpation and auscultation, to one who 
has long investigated the heart and the arteries with an intelli- 
gent, inquiring mind; and we strongly recommend the student 
to commence personal observations early and to maintain them 
persistently. 
Physicians recognize the pulse (and heart) as “slow” as dis- 
tinguished from “infrequent,” “slapping,” “ heaving,” “ thrill- 
ing,” “bounding,” etc. 
Now, if with these terms there arise in the mind correspond- 
ing mental pictures of the action of the heart under the cir- 
cumstances, well; if not, there is a very undesirable blank. 
How the student may be helped to a knowledge of the actual 
behavior of the heart under a variety of conditions we shall 
endeavor to explain later. 
Apart from all the above peculiarities, the heart may cease 
its action at regular intervals, or at intervals which seem to 
possess no definite relations to. each other—that is, the heart 
may be irregular in its action, which may be made evident 
either to the hand or the ear. 
There are certain deviations from the quicker rhythm which 
occur with such regularity and are so dependent on events that 
takes place in other parts of the body that they may be con- 
sidered normal. Reference will shortly be made to these and 
the causes of the variations enumerated in this section. 
Comparative.—The following table gives the mean number of 
cardiac pulsations per minute (after Gamgee) : 
i} 
