THE CARDIAC NERVES. 



185 



of the sympathetic nervous system on page 277. The sympathetic 

 fibers of the heart are found in the first and second spinal nerves 

 of the thoracic region. After connecting with nerve cells situ- 

 ated in the stellate ganglion, they go to the heart, where they 

 end about the cardiac muscular fibers. 



Cutting the sympathetic fibers to the heart causes a slower beat 

 and a prolonged diastole. On the other hand, stimulation of the 

 nerves with an electric current increases the rate of the heart 

 (Fig. 25). For the above reasons the sympathetic nerves to the 

 heart are known as accelerator or augment or y nerves. 



The Inhibitory Nerves. The vagi are a pair of nerves arising 

 on each side of the medulla, and running a course downwards 

 through the neck into the thoracic and abdominal cavities. This 

 pair of nerves supply fibers to the various organs of these regions 



Time in seconds 



Normal Ventricul- 

 ar beet 



Stimulation of I 

 I .Sympathetic I 



Normal 



Fig. 25. Effect of stimulating: vagus and sympathetic nerves on the frog's 

 heart. 



including the heart, which receives branches from both vagi. 

 It is possible by simple experiments to demonstrate the function 

 of these fibers. 



For example, if the vagus on one side be cut, the heart rate 

 will increase a little; if both vagi be cut, the beat is still 

 more markedly quickened, and the increased discharge of blood 

 from the heart produces a rise in the arterial blood pressure 

 (Fig. 26, No. III). By cutting these nerves we remove the influ- 

 ence which the central nervous system exerts through them on 

 the heart rate. Since the heart beats faster after this operation, 

 we must conclude that this organ constantly receives stimuli 

 from the brain through the vagi, and that these stimuli cause 



