THE BEAT OF THE HEART 51 



over a delicate bridge of tissue, which is the functional 

 connection between the auricle and the ventricle, and 

 finally the whole ventricle becomes involved. 



If the heart of a living animal be removed from the body, 

 it will, if its substance is fed with a suitable nutrient fluid, 

 go on beating much as it did while in the body. ^ And care- 

 ful attention to these beats will show that they consist of : — 

 (1) A simultaneous contraction of the walls of both 

 auricles. (2) Immediately following this, a simultaneous 

 contraction of the walls of both ventricles. (3) Then 

 comes a pause, or state of rest ; after which the auricles 

 and ventricles contract again in the same order as before, 

 and their contractions are followed by the same pause as 

 before. 



The state of contraction of the ventricle or auricle is 

 called its syatole ; the state of relaxation, during which it 

 undergoes dilation, its diastole. 



If the auricular contraction be represented by A", the 

 ventricular by V", and the pauses by—, the series of 

 actions will be as follows: A~'V — A'V"" — A"V 

 — &c. Thus the contraction of the heart is rhyth- 

 mical, two short contractions of its upper and lower 

 halves respectively being followed by a pause of the 

 whole, which occupies nearly as much time as the two 

 contractions. 



The movements taking place in the heart during one 

 complete beat and the pause are usually spoken of 

 as a "cardiac cycle." This cycle is repeated, or as 

 we more ordinarily say, "the heart beats" in an 

 average healthy adult person about 72 times in a 

 minute. From this it follows that the ordinary duration 

 of each beat is /„ of a second. Of this period the 

 contraction of the auricles occupies ^V ^"^^ ^^^^^ °^ ^^^ 

 ventricles fg, the remaining /„ bemg taken up by the 

 pause of the heart as a whole. During each cycle or beat 

 the heart undergoes certain changes of shape and position, 

 as to the details of whicli there is some uncertainty, but 

 which are, on the whole, as follows. During each systole 



E 2 



