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 to the central 

 nervous system, which must be able to affect the heart in either of two ways, 

 viz. by increasing or diminishing its activity. This subjection to the 

 integrative action of the central nervous system is also necessary for the 

 sake of the organ itself; otherwise the peripheral adaptation of the heart 

 muscle to change in arterial resistance might result in its exhaustion and 

 permanent 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 oblongata along the vagus nerve, 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 somewhat 

 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. 454) the sympathetic fibres leave the spinal cord by the anterior root 

 of the third spinal nerve ; they then pass through the ramus communicans 

 to the corresponding sympathetic ganglion, whence they run up through 

 the second jsm.ulimi and the annulus of Vieussens to the first ganglion; 

 they then pass into the cervical sympathetic strand to the ganglion trunci 

 vagi ; here they join the vagus and pass down with the true vagus fibres 

 to the heart. 



In the dog (Fig. 455) the sympathetic fibres leave the spinal cord 

 by the anterior roots of the second and third dorsal nerves, run in the 



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