THE CONTROL OF THE CIRCULATION 247 



Mechanism of Action of Pressor and Depressor Impulses. When we 

 consider the exact mechanism by which these afferent impulses operate, 

 we have to bear in mind four possibilities: the reflex fall in blood 

 pressure produced by stimulation of a depressor afferent fiber may 

 be due either to a stimulation of the vasodilator part of the cen- 

 ter or to an inhibition of the tone of the vasoconstrictor part ; and, con- 

 versely, a rise in pressure 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 

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



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

 as an example 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 

 stimulation. The volume of the hind limb of a curarized and vagotomized 

 rabbit increases when the central end of the cardiac depressor nerve is 

 stimulated. In order to determine whether this dilatation is due solely 

 to the removal of vasoconstrictor tone, the above experiment was repeated 

 on a rabbit in which the sympathetic chain had been cut below the level 

 of the second lumbar spinal roots. By such an operation all the vaso- 

 constrictor fibers to the vessels of the hind limb are severed, but the 

 vasodilator fibers, since they emanate through the sacral sensory roots, 

 are left intact. It was nevertheless found on stimulating the depressor 

 nerve that dilatation of the hind limb still occurred, thus indicating 

 that stimulation through vasodilator fibers must have taken place. Con- 

 versely, in another experiment, instead of the sympathetic chain, the 

 spinal cord was cut below the level of the second lumbar segment, thus 

 severing the dilator but not the constrictor path, and again depressor 

 stimulation caused the volume of the limb to increase, indicating that 

 an inhibition of constrictor tone must have occurred. 



Reciprocal Innervation of Vascular Areas 



It must not be imagined that changes in the caliber of the blood ves- 

 sels occurring in one vascular area are necessarily occurring all over 

 the body. On the contrary, a most important reciprocal relationship 

 exists in the blood supply to different parts. After food is taken, for 

 example, more blood is required by the digestive organs than when they 

 are at rest, and this is insured by dilatation of their own vessels along 

 with reciprocal constriction of those of other parts of the body. On 

 account of the relatively great capacity of the abdominal vessels, their 



