APPLIED PHYSIOLOGY 



two influences are separate and travel by different 

 fibres, for on stimulating the sympathetic one some- 

 times produces a mere increase in rate, whilst at other 

 times one gets an augmented output and rise of blood- 

 pressure. Gaskell has described the effects of the 

 sympathetic on the heart, as a whole, as a * katabolic ' 

 one, for it tends to increase those chemical changes 

 which lead to the beat, and he compares it to a vaso- 

 constrictor. The right sympathetic seems to exert more 

 influence than the left. 



Curiously enough, we know much less of the action 

 of the sympathetic, both clinically and physiologically, 

 than of the vagus. It was even long disputed whether 

 or not the sympathetic had a constant tonic action. It 

 is now generally agreed, however, that it has.* Indeed, 

 its action seems to be at least as continuous as that of 

 the vagus. The normal heart rate, in fact, appears to 

 be determined by the influences reaching it through the 

 vagus and sympathetic channels, and between these 

 there is, as it were, a constant struggle for supremacy 

 going on. All the physiological evidence, however, 

 points to the fact that an increase in the rate of the 

 heart is always due to a diminution of vagus tone, and 

 not to an increased action of the sympathetic. It might 

 seem natural to suppose that cases of palpitation 

 characterized by increased force and frequency of the 

 beats were produced by an increase of the sympathetic 

 influence ; but this would appear not to be the case. 

 The mere fact that such attacks may cease very abruptly 

 would negative such a view, for experimental stimulation 

 * See Eeid Hunt, Amer. Joum. of Physiol, 1899. ii. 395. 



