244 THE CIRCULATION OF THE BLOOD 



the vagus trunk, to end around the arch of the aorta. In other animals 

 the depressor is bound up with the vagus trunk from which it can some- 

 times be separated by careful dissection. The first prerequisite in inves- 

 tigating the cause of the changes produced by stimulation of these nerves 

 is the elimination of any chance of an alteration in heartbeat as a result 

 of simultaneous stimulation of afferent vagus fibers. This may be done 

 either by cutting both vagi or by administering atropine. Stimulation 

 of the central end of the cardiac depressor nerve after such precautions 

 causes an immediate fall in blood pressure, accompanied by an increase 

 in volume either in the hind limb or in one of the abdominal viscera 

 evidence of general vasodilatation (Fig. 73). 



When the central end of a sensory nerve, such as the sciatic, is acted 

 on by a stimulus of moderate strength, it will usually be found that the 

 arterial blood pressure rises and that the volume of the limb or of some 



Fig. 73. Fall of blood pressure from excitation of the depressor nerve. The drum was 

 stopped in the middle of the curve and the excitation maintained for seventeen minutes. The line 

 of zero pressure should be 30 mm. lower than here shown. (From Bayliss.) 



abdominal viscus becomes diminished evidence of general vasoconstric- 

 tion. But when the sensory nerve is stimulated with extremely weak 

 f aradic shocks, an entirely different result is likely to be obtained ; 

 namely, a fall of blood pressure and an increase in volume of the limb 

 or viscus is usually obtained, indicating that we have stimulated depressor 

 fibers. By careful experimentation with quantitatively graduated elec- 

 trical stimuli, it has been found by Martin and others 17 that on stimu- 

 lating an afferent nerve with weak shocks, a fall in blood pressure is 

 the first effect to be observed, and that this becomes more and more 

 marked as the strength of the stimuli is increased, until a certain opti- 

 mum is reached, after which the fall in blood pressure becomes less evi- 

 dent. When a certain strength of stimulation is exceeded, a rise instead 

 of a fall occurs. After this point additional increase in stimulation causes 



