THE CAUSATION OF THE HEART-BEAT 1075 



a curve in which, the second contraction is superposed on and rises 

 higher than the first. Such a result, on the explanation given above, 

 would be due to a phenomenon of * block ' limiting the propagation 

 of the first contractile wave, and yielding more to the second, though 

 this is not the explanation given by the original observer. 



SUMMATION OF STIMULI 



If an isolated frog's ventricle which is not beating be stimulated 

 with inadequate shocks, it may be found, on repeating these shocks at 

 short intervals of time, that they become adequate and cause a con- 

 traction of the ventricle. A stimulus therefore which is subminimal 

 may nevertheless cause some change in the heart-muscle, so that the 

 latter responds more readily to subsequent stimuli. 



A similar improving effect of previous stimulation on the 

 condition of the heart-muscle may be observed 

 on the contractions themselves. Thus in a 

 Stannius preparation, if the ventricle be excited 

 with single induction shocks, once in every ten 



seconds, the first four or five contractions FlG - 438 - Group of pul- 

 sations showing stair- 

 form an ascending series, each contraction case character. 



being rather higher than the preceding one. 



This is often spoken of as the ' staircase phenomenon ' (Fig. 438). 



THE REFRACTORY PERIOD 



At each contraction of the heart-muscle there is a sudden decom- 

 position of contractile material which, so far at least as concerns the 

 incidence of an external stimulus, is maximal, i.e. complete. Directly 

 this has occurred a process of assimilation or re-formation of contractile 

 material begins. This lasts throughout the diastolic period, and the 

 store of contractile material is at its maximum just before the next 

 contraction. A mechanical analogy is furnished by a bucket into 

 which a stream of water is constantly flowing, and which tips up 

 automatically and empties out its contents as soon as the' water reaches 

 a certain height. It is evident that the power of the heart-muscle to 

 contract in response to a stimulus (its ' irritability ') must be at a 

 minimum immediately after the automatic discharge or decomposition 

 has taken place, and will continually increase from this point as the 

 store of contractile material grows, until it arrives at such a height 

 that the explosive discharge occurs spontaneously. Hence in each 

 cardiac cycle there is a period, known as the refractory period, in 

 which stimuli applied to the heart have no effect. This will be followed 

 by a period in which a stimulus is followed by an extra contraction, 

 but with a prolonged latent period. Just before the next spontaneous 

 contraction the irritability is at its height, and the heart-muscle 



