134 APPLIED PHYSIOLOGY 



result of nervous shock. A soldier in South Africa, for 

 instance, was suddenly startled by the explosion near 

 him of a 4*7 gun. His heart immediately began to beat 

 very rapidly, and continued to do so for some months. 

 Vagus acceleration, however, can never, as we have seen, 

 lead to a rate of more than 150. Attacks of ' paroxysmal 

 tachycardia,' in which the beat runs up temporarily to 

 200 per minute or more, must have another origin than 

 that, and seem to be due to a sudden loss of tone in the 

 heart leading to temporary dilatation, in which so little 

 blood is expelled at each systole that the total number of 

 beats must be greatly increased if the circulation is to go 

 on.* The opposite condition of bradycardia, or diminu- 

 tion in the heart's rate, may also be due either to 

 diminished excitability of the muscle itself or to in- 

 creased vagus inhibition. The former is the cause of 

 the very slow pulse sometimes observed during conva- 

 lescence from enteric fever or pneumonia, t The latter 

 occurs, as we have already seen, when the vagus is 

 stimulated by a high blood-pressure, and also occasion- 

 ally under emotional influences. A slow beat from 

 change in the heart muscle can easily be distinguished 

 clinically from one due to increased vagus inhibition by 

 the administration of a dose of atropine. In the former 

 case the rate will not be accelerated ; in the latter case 

 it will. 



* Mackenzie has suggested (' The Study of the Pulse,' p. 126) 

 that paroxysmal tachycardia is really due to a long-continued series 

 of premature systoles. 



Martius, ' Tachycardie ' (Stuttgart, 1895). 



f Dehio, 'Ueber die Bradycardie der Eeconvalescenten,' Deut. 

 Arch.f. Klin. Med., 1894, lii. 74. 



