THE CONTROL OF THE CIRCULATION 



241 



of the vasoconstrictor part; and, conversely, a rise in arterial pressure 

 caused by vasoconstriction may be dependent either on a stimulation of 

 the vasoconstrictor part of the center or on an inhibition of the tone of 

 the vasodilator part. All of these changes have, as a matter of fact, been 

 shown to occur, at least under certain conditions, although the evidence 



Fig. 74. The effect of strong stimulation (heat) of the skin of the foot on the arterial blood 

 pressure and respiratory movements. Upper tracing, thoracic movement; lower tracing, arterial 

 blood pressure. 



for the inhibition of dilator tone- is as yet a little uncertain (see Pig. 75). 

 Without going into the subject in detail, we may nevertheless take 

 as an exa'mple of the methods by which the information has been ob- 

 tained, the experiment performed by Bayliss, 18 showing that the vasodi- 

 lation which results from stimulation of the depressor nerve is owing 

 partly to removal of vasoconstrictor tone and partly to vasodilator 



