76 HUMAN PHYSIOLOGY 



give warning to a particular part of the nervous 

 system situated in the brain of necessary modification 

 to be made in the heart's labours. Thus, if the right 

 side of the heart particularly be somewhat fagged in 

 its work, and if heart fatigue begins to be represented, 

 a message passes along this fourth nerve, to which the 

 name of depressor or relief nerve, may be given. The 

 message is conveyed to the nerve centre which regu- 

 lates the bloodvessels, The result of what may be 

 called an appeal for help or relaxation of its duties 

 on the part of an overworked heart, received by this 

 centre, is at once reflected or sent forth to the blood- 

 vessels of the body and especially to the large blood- 

 vessels in the neighbourhood of the heart itself. The 

 result is that these vessels are made to expand 

 widely, and as it is obviously easier for the heart 

 to propel blood through wide channels than through 

 narrow and contracted ones, relief as in the case 

 of sudden fatigue of the heart is thus obtained. No 

 better example in the whole body of an accurately 

 acting and self-governing mechanism can be obtained 

 than that just described whereby the heart contrives 

 to gain relief under conditions of strain and stress. 



