THE CAUSATION OF THE HEART-BEAT 1077 



next succeeding excitatory wave from the sinus before it gives its next 

 beat. Hence the compensatory pause does not occur when we are 

 testing the effects of artificial stimuli on the sinus venosus. 



On account of the refractory period which ensues on the com- 

 mencement of the contractile process on heart muscle, it is impossible 

 to throw the muscle into a tetanus, since all the stimuli which fall 

 during systole are entirely ineffective. By using very strong stimuli 

 it is possible to intercalate extra contractions before the heart has 

 returned to the base line, i.e. before diastole is complete. So that in 

 this way one may obtain almost a continuous contraction, presenting, 

 however, waves on its summit, which differs from the tetanus of skeletal 

 muscle in the fact that its height is no greater than the height of a single 

 contraction. 



Only when the functional continuity of the heart-muscle is im- 

 paired by the ' block ' effect of vagal stimulation or the administration 

 of muscarine is it possible to obtain phenomena even superficially 

 analogous to the summation of contractions in skeletal muscle. 



FACTORS MODIFYING THE ACTIVITY OF CARDIAC 

 MUSCLE 



INFLUENCE OF TENSION 



The most important factor in determining the strength and 

 extent of the cardiac contractions is the tension on the muscle fibres. 

 Within certain limits the energy of contraction of the cardiac muscle 

 increases with tension, i.e. with the resistance to the shortening to 

 which the muscle fibre is exposed. This effect can be seen when the 

 resistance to the outflow of blood from the heart is increased, so that the 

 resistance is only experienced during the contraction of the heart- 

 muscle. It is still better marked when the muscle fibres are stretched 

 by the distending force before they begin to contract, i.e. during 

 diastole. In this case, as is shown in Fig. 440, the heart contracts more 

 forcibly the greater its distension during the diastolic period. In the 

 experiment from which this figure was derived, the heart was con- 

 tracting isometrically, i.e. was given a tension which it was unable 

 to overcome, so that the length of its fibres was not altered during the 

 act of contraction. This reaction of the cardiac muscle to tension 

 plays a great part in determining the adaptation of the heart, both in 

 the cold-blooded and in the higher animals, to variations in the load, 

 i.e. variations in the pressure it has to overcome and in the amount cf 

 blood which it has to expel. 



Thus if a ligature be placed round the aorta so as to narrow the 

 vessel to one-third of its normal extent, the arterial pressure, as judged 

 by a manometer connected with the carotid artery, undergoes no change. 

 If, however, the pressure be taken in the intra ventricular cavity each 



