132 APPLIED PHYSIOLOGY 



condition of ' irritable weakness ' is a common occur- 

 rence in disease. 



Alterations in Rhythm. 



Alterations in rhythm may be induced either by 

 abnormal conditions of the muscle substance or by the 

 intervention of nervous influences, and clinically it may 

 be impossible to say which of these factors is at work. 



A discussion of the rhythm of the heart involves a 

 consideration of 



1. Its rate. 



2. The relative duration of systole and diastole. 



3. The regularity of the beat. 



4. The synchronism of the two sides of the heart. 



1. The rate of the heart -beat is due primarily to the 

 excitability of the less differentiated muscle of the sinus 

 venosus, but it is also dependent upon the degree of con- 

 trol exercised by the inhibitory fibres of the vagus. The 

 greater rate of the heart in young subjects is probably 

 due to a feebler influence of the vagus in early life 

 (see p. 125), and the same may be true of the differences 

 in the rate of beat in different individuals. On the 

 other hand, alterations in the heart muscle may be a 

 cause of a more frequent beat. Fever seems to affect 

 the excitability of the heart directly, as well, perhaps, as 

 through the nervous centres, and the same is probably true 

 of the chemical products produced by muscular exercise. 



The initial increase in the heart-rate which exertion 

 induces, however, must be due largely to mechanical 

 influences. 



