160 THE HUMAN MECHANISM 



We express this by saying that the heart beat is automatic, 

 by which we mean that the heart contains within itself a 

 complete mechanism for doing its own work. 



23. The augmentor and the inhibitory nerves of the heart. 

 Nevertheless the heart receives from the central nervous 

 system two pairs of nerves which are able to influence the 

 rate and the force of the automatic beats. One pair of these 

 nerves carries from the spinal cord to the heart impulses 

 which stimulate that organ to beat more rapidly or more 

 forcibly, or both. Hence these are known as the augmentor, 

 or accelerator, nerves. 



The fibers of the other pair of nerves produce exactly 

 the opposite effect. Running from the lower part of the 

 brain, they carry to the heart impulses which slow the beat 

 or lessen its force, or they may produce both effects at the 

 same time. They act, as it were, like a brake on a wheel, 

 checking the activity of the automatic beat. These fibers 

 are known as inhibitory fibers, and their action is a case of 

 inhibition. 



24. Inhibition. In the examples of nervous action which 

 we have thus far studied, the nervous impulse has uniformly 

 thrown some cell into activity. The stimulation of muscle 

 fibers to contract, of gland cells to secrete, and of nerve cells 

 in the execution of reflexes will be readily recalled. To this 

 same class of nervous actions must now be added that of 

 the augmentor nerves of the heart, for they excite the heart 

 to greater activity. 



In the inhibitory nerves, on the other hand, the nervous 

 impulse produces exactly the opposite result. Instead of set- 

 ting organs to work or stimulating them to more vigorous 

 action, they diminish activity and in extreme cases check or 

 stop it altogether. In our subsequent studies we shall meet 

 with many examples of this effect; but we may say at once 

 that inhibition is as characteristic and as important a feature 

 of the nervous system as is excitation (see p. 281). 



