250 NUTRITIONAL PHYSIOLOGY 



would seem to explain themselves. The inhibitory fibers 

 restrain the heart much of the time from beating at the 

 rate which it would exhibit if nervous regulation were 

 entirely withdrawn. An exaggerated inhibitory influence 

 may weaken its working to the point where the circulation 

 becomes quite inadequate and faintness is produced. In 

 laboratory trials actual arrest of the heart of a dog may be 

 caused, though the beat is always resumed within a minute, 

 so that death cannot be made to result. We ought not to 

 lay much stress upon these extreme possibilities. It 

 would be absurd to say that the function of the inhibitory 

 cardiac nerves is to stop the heart; that could never be for 

 the best good of the animal. We can say with more reason 

 that their service is to economize the strength of the heart 

 and to provide a reserve for emergencies. This has been 

 described as a "brake action." 



The accelerator nerves of the heart have a stimulating 

 effect which extends to the vigor as well as to the rate of 

 its beating. When we observe a specific increase of heart 

 action we can hardly say whether it has been produced by 

 positive accelerator influence or by the abatement of the 

 habitual inhibitory control. It may, of course, represent 

 the combined result of both. The government of organs 

 by means of the balanced action of two opposing sets of 

 nerves is repeatedly met with in the body. It can be de- 

 monstrated for the stomach and for other sections of the 

 alimentary canal. A singular example is afforded by the 

 nervous relations of the iris, the colored ring surrounding 

 the black pupil of the eye. Stimulation of one nerve causes 

 contraction of the pupil, while widening or dilatation fol- 

 lows the application of stimuli to an entirely different 

 strand of fibers. 



The heart may quicken the circulation by beating at a 

 more rapid rate and with increased power, but it cannot 

 send the blood to a particular part of the body at the ex- 

 pense of another part. The total blood-flow thus depends 

 upon the heart acting under the balanced sway of inhib- 

 itory and accelerator nerves, but the distribution of blood, 



