SECTION IX 



THE NERVOUS REGULATION OF THE 

 HEART 



IN order that the activity of the heart may be adapted to the needs 

 of the body as a whole, its automatic mechanism must be subject o 

 the central nervous system, which must be able to affect the heart n 

 either of two ways, viz. by increasing or diminishing the cardiac action. 

 The subjection of the heart's activity to the integrative action c f 

 the central nervous system is also necessary for the sake of the organ 

 itself ; otherwise the peripheral adaptation of the heart-muscle to 

 changes in arterial resistance might result in its exhaustion and per- 

 manent damage. 



The regulation is effected through the intermediation of afferent 

 and efferent nerve fibres connecting the heart with the central nervous 

 system. The importance of these nerves is shown by the behaviour of 

 animals in which they have been extirpated. Thus a dog in whom all 

 the nerves of the heart had been divided survived the operation for 

 eight months, the pulse reading during the time not having appreciably 

 altered and the animal being in a fair condition of health. Although 

 he regained his normal weight after the operation, he was found 

 incapable of carrying out even a moderate amount of work, such as 

 that represented by running, since the mechanism for increasing the 

 action of the heart in response to the needs of the muscles had been 

 lost. 



THE EFFERENT CARDIAC NERVES 



The heart in vertebrates is supplied with nerve fibres from two 

 sources, from the medulla olblongata along the vagus nerves, and from 

 the upper dorsal region of the spinal cord through the mediation of the 

 sympathetic system. 



The fibres which run through the sympathetic system take a some- 

 what different course in the animals on which the regulation of the 

 heart's activity has been chiefly studied, viz. the frog and the mammal. 

 In the frog (Fig. 447) the sympathetic fibres leave the spinal cord by 

 the anterior root of the third spinal nerve ; they then pass through 

 the white ramus cornmunicans to the corresponding sympathetic 



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