THE CONTROL OP THE CIRCULATION 239 



disappears, although it immediately reappears when the alcohol effect 

 has passed off, and there is no evidence of a change in tone. The tonic 

 and the reflex mechanisms of the vasomotor center can not therefore be 

 identical. 



At the present stage of our knowledge, it is only possible for us to 

 study the effect of stimulation of pressor and depressor fibers on the 

 vasoreflex center. Such fibers are contained in practically every sen- 

 sory nerve of the body, and it would appear that a fairly equal mixture 

 of both kinds of fiber exists in most of these nerves. 



Pressor and Depressor Impulses. Depressor impulses are alone present 

 in the cardiac depressor nerve. Sometimes as in the rabbit, this exists 

 as an independent nerve trunk, originating by two branches, one from 

 the superior laryngeal, the other from the vagus, and descending close to 



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.) 



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 in such 

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

 increase in volume which can be demonstrated 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 



