CIRCULATION OF BLOOD 65 



y. During its contraction the cardiac muscle is not 

 irritable (refractive) from the beginning to the maximum of 

 the contraction. During this time a stimulation is inactive. 

 In its relaxed condition, the cardiac muscle is again irritable; 

 if a stimulus is introduced during this stage, a new contrac- 

 tion occurs which is the greater in proportion as the stimu- 

 lation occurs later. When, in an independent rhythmically 

 beating heart, such an " extra contraction " is called forth 

 by an artificial stimulus during the diastole, the pause fol- 

 lowing upon this contraction and lasting to the next inde- 

 pendent beat is longer than the ordinary pause between two 

 independent beats. This lengthened pause is called the 

 compensatory pause. 



The physiological contractions of the cardiac muscle con- 

 sist of single contractions which follow each other in a 

 definite rhythm. The contraction is called systole, and the 

 relaxation following it is called diastole. 



The contraction of the heart begins at the mouth of the 

 veins, from these it travels through the walls of the auricle 

 and then through the walls of the ventricle. The whole 

 cardiac cycle lasts about 0.86 second, which may be divided 

 as follows: 



1. Auricular systole (ventricles at rest), o. 1 6 second. 



2. Ventricular systole (auricles at rest), 0.3 second. 



3. Pause, during which both auricles and ventricles are 

 at rest, 0.4 second. 



The number of heart-beats in one minute in an adult 

 human being is on the average 70; in children it is higher 

 (first year 1 34) ; the number is increased by increase of tem- 

 perature (fever), muscular exertion, after the taking up of 

 food; it depends also upon mental conditions. 



The contraction of the ventricular or auricular wall does not 

 occur at all points simultaneously, but spreads itself along the 

 cardiac muscle, like the contraction waves in the fibres of skeletal 

 muscles. This is proven by the fact that the electrical phenomena 

 of the stimulated cardiac muscle dc not appear simultaneously at 

 all points, so that it is possible to demonstrate, as in the striated 

 skeletal muscle, a current of action. (See Chapter XIV.) From 

 the results of the electrical phenomena it can also be concluded 

 that the cardiac contraction corresponds to a twitch and not to a 

 short tetanus. 



