122 APPLIED PHYSIOLOGY 



4. Increase or diminish contractility, and so affect the 

 force of the beats. 



If one remembers that each of the physiological 

 properties of the heart muscle can be affected inde- 

 pendently by nervous influences in a positive or negative 

 direction, and reflects upon the different permutations 

 and combinations of disturbances of the heart's action 

 which may be so produced, one will readily understand 

 how complicated the control exercised by the nerves of 

 the heart in health really is. 



Efferent Impulses. 



The paths by which these influences reach the heart 

 are shown in detail in Plate I. 



It will be observed that two sets of efferent fibres pass 

 to the heart from the nervous system : 



1. Inhibitory fibres contained in the vagus. 



2. Accelerator or augmentor fibres which run in the 

 sympathetic system. 



The inhibitory fibres are inimical to the exercise of 

 most of the physiological functions of the heart muscle. 

 They lower its excitability, its conductivity,* and its 

 contractility, but not, apparently, its tonicity . t In other 

 words, the influences which they convey are opposed 

 to those molecular changes which manifest themselves 

 in the discharge of function i.e., they restrain kata- 

 bolism. For this reason the vagus has been described 



* Muskens (Amer. Journ. of Physiol., 1898, vol. i., p. 486) regards 

 as the chief effect of the vagus. 



t Gaskell, however, believes that the vagus does lower tonicity 

 (Schafer's 'Physiology,' ii. 215). 



