THE AUTONOMIC NERVOUS SYSTEM 117 



nerve known as the vagus, the tenth in the cranial series 

 (see page 73). 



When the vagus nerves of a dog are both cut, it is com- 

 monly observed that the rate of the heart is considerably 

 increased. This is taken to mean that a restraining or 

 inhibitory influence was previously exerted through these 

 nerves upon the heart's activity. This impression is con- 

 firmed when the peripheral end of one of the cut vagi is 

 subjected to electric stimulation. The heart may be made 

 to stand still for some seconds and the circulation is 

 stopped. This would kill the dog in a short time if it 

 could be continued, but it cannot; the heart is said to 

 "escape," that is, to resume its beating in spite of con- 

 tinued stimulation of the vagus. Still, the rate and force 

 of the beat can be kept below the original standard for an 

 indefinite time. The natural conclusion with reference 

 to the share borne by the vagi in the government of the 

 heart is that they contain fibers through which its activity 

 is generally held in check and a reserve for emergencies 

 guaranteed. The organ may be assumed to expend its 

 power unprofitably when these fibers have been cut. 



The heart receives some nerve-fibers in addition to those 

 which reach it by way of the vagi. These others come 

 from the sympathetic ganglia near its own level in the 

 thorax. They are not easily picked up for stimulation 

 in the living animal, but when the experiment is success- 

 ful they are found to have an effect just opposite to that 

 which has been described. They furnish paths through 

 which impulses can be sent to quicken and make more 

 forcible the beating of the heart. The branches coming 

 to the heart from the sympathetic ganglia are called the 

 cardiac accelerators. Simultaneous stimulation of the 

 vagus and an accelerator path brings out the fact that the 

 two sets of fibers are true antagonists; the influence of 

 the one can be neutralized through the other. 



This is a type of nervous organization which we find 

 again and again in the body. Antagonistic nerves are 

 almost as obvious as antagonistic muscles. Being pro- 



