

THE CAUSATION OF THE HEART BEAT 999 



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. This however is not the explana- 

 tion 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 contraction 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 FlG - * 45 - Group of pul- 



, ,. . ' f , , sations showing stair- 



on the condition ot the heart muscle may be observed case ' character. 

 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 form an ascending 

 series, each contraction being rather higher than the preceding one. This 

 is often spoken of as the ' staircase phenomenon ' (Fig. 445). 



THE REFRACTORY PERIOD 



At each contraction of the heart muscle there is a sudden decomposition 

 of contractile material which, so far at least as concerns tne 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 con- 

 tractile 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 responds with a contraction to a minimal stimulus. These 

 facts are well shown in Fig. 446. 



When a tracing is being taken from part of the heart, e. g. the ventricle, 

 which is beating rhythmically in consequence of a stimulus communicated 

 to it from some other part such as the sinus venosus, an extra contraction is 

 followed by a ' compensatory pause/ and in certain cases the first contraction 



