THE CIRCULATION OF THE BLOOD 391 



former for the fall of pressure occurs equally well after division of all the 

 cardiac nerves. For this reason the nerve was termed the depressor nerve 

 of the vaso-motor center. 



On exposure of the abdominal cavity, it is observed during stimulation of 

 the depressor that there is a notable dilatation of the intestinal vessels. 

 From this fact it was assumed that the action of the depressor nerve was to 

 lower the general pressure through reflex dilatation of these vessels. It has 

 been shown by Porter and Beyer that if the splanchnics are divided and the 

 peripheral end stimulated so as to maintain the tonus of the intestinal vessels, 

 and hence the general pressure, stimulation of the depressor nerve will 

 nevertheless be followed by a fall of the blood-pressure almost as great as 

 when the splanchnics are intact. From this it is evident that the depressor 

 nerve is related to centers which influence the vascular apparatus in its 

 entirety. It has been supposed that through it the heart can protect itself 

 from injurious results of an excessive rise of arterial pressure. 



Thus, when the intra-cardiac pressure or the mtra-aortic pressure 

 rises beyond a normal amount from increased resistance, the peripheral 

 terminations of this nerve are stimulated with the result that the vaso-motor 

 center is inhibited and the arterioles relaxed. Through this means the 

 pressure falls and the work of the heart is lessened. 



The Traube-Hering Waves. Under certain experimental conditions 

 the arterial blood-pressure tracing exhibits, in addition to the usual respira- 

 tory variations, certain longer rhythmic variations more or less wave-like in 

 character, which are known as Traube-Hering waves. They can be devel- 

 oped on a blood-pressure tracing by injecting magnesium sulphate or mor- 

 phine into the circulation, by tying the cerebral arteries, etc. These waves 

 indicate a periodic contraction and dilatation of the blood-vessels, the result 

 of a periodic stimulation of the vaso-motor center. 



