236 HUMAN PHYSIOLOGY 



10. Nature of Cardiac Contraction. The contraction of the 

 ventricle lasts for a considerable period 0*3 seconds. Is it 

 of the nature of a single contraction, or of a tetanus ? 



It is impossible to tetanise heart muscle, even by rapidly 

 repeated induction shocks. A single stimulus applied to heart 

 muscle produces a single prolonged contraction. Again, the 

 mode of development of the currents of action does not indi- 

 cate anything of the nature of a tetanus. With each beat of 

 the ventricles the variation in the electric potential begins 

 at the base and travels rapidly to the apex. This passage 

 of the contraction wave along the fibres explains the great 

 length of the ventricular systole as a whole. There can be 

 no doubt that each contraction of heart muscle is of the 

 nature of a muscle twitch. In this respect heart muscle 

 resembles non-striped muscle. 



It further resembles it in that the minimum stimulus is 

 also a maximum stimulus i.e. the smallest stimulus which 

 will make the muscle contract makes it contract to the 

 utmost. But while this is the case the strength of stimulus 

 necessary to call forth a contraction varies at different 

 periods. To produce another contraction while the muscle is 

 alreadyjin the period of contraction is difficult, but as it relaxes 

 it reacts more and more readily to stimuli In cardiac muscle, 

 perhaps more than in any other, the staircase increase in the 

 extent of contraction with a series of stimuli is manifested. 



11. How is the Rhythmic Contraction of the Heart 

 maintained? The mechanism is in the heart itself, for 

 the excised heart continues to beat. 



In considering what this mechanism is, it must be borne 

 in mind^that two distinct questions have to be investigated. 



1st. How does the contraction, once started, pass in regular 

 sequence from one part of the heart to the other ? 

 2nd. What starts each rhythmic contraction ? 



1st. Propagation of the Wave of Contraction. In the 



heart of many of the lower animals, and in the embryo of 

 mammals, no nervous structures are to be found, and the 

 rhythmic contraction is manifestly simply a function of the 

 muscular fibres. 



