THE HEART 131 



condition of the muscle wall ; (2) the influence of the 

 nervous mechanism. 



Rhythmieity and Excitability. 



What it is in the fibres of the heart muscle which 

 endows them with the property of regular rhythmical 

 contraction, or rhythmicity, we do not know. In the 

 last resort it must depend upon some peculiarity in their 

 metabolism, possibly the building up of some * explosive ' 

 compound, upon the disintegration of which the starting 

 of a contraction ensues, the normal intervals between 

 contractions representing the time required for the 

 formation of this compound. From this point of view 

 the heart muscle might be compared to a gun, which 

 is always fired off as quickly as it can be loaded. To 

 pursue this analogy, the ' rhythmicity ' of the heart 

 corresponds to the rate of loading ; its ' excitability ' to 

 the ease or otherwise with which the trigger can be 

 pulled.* 



The more excitable the heart, the less the stimulus 

 which is required to produce a contraction. A very 

 excitable heart, therefore, is like a gun with a 'hair- 

 trigger,' and any abnormal stimulus can readily prompt 

 it to produce extra beats. Increased excitability a 



* In the text the myogenic origin of the rhythmical contraction 

 of the heart has been adopted, but the reader is reminded that 

 many physiologists believe that the production of the stimulus to 

 rhythmical contraction resides in the intrinsic nervous mechanism 

 (neurogenic theory). The question is one of no direct clinical 

 interest, but the pros and cons of the two theories are well stated 

 in a paper by Gossage on ' The Automatic Rhythm of the Heart ' 

 (Brit. Med. Journ., 1907, ii. 1818). 



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