THE VASOMOTOR CENTERS 



237 



mentioned at page 193. The afferent nerves from the muscles have the same 

 influence on the blood pressure (Fig. 98). 



The reftex fall of pressure on stimulation of the depressor appears with both 

 vagi cut; hence, it is independent of changes in the heart frequency and is 

 caused essentially by a vasodilation. This involves primarily the region of the 

 abdominal cavity innervated by the splanchnics, although other parts of the body 

 may take part in it. 



The reflex rise of pressure is produced primarily by a contraction of the 

 vascular region innervated by the splanchnics, even though other regions also 



FIG. 98. Reflex fall of blood pressure in the rabbit produced by stimulation of an afferent 

 muscular nerve (to be read from right to left). The period of stimulation is indicated by 

 the vertical lines. J \ = ten seconds. 



may be concerned. Not all the vascular regions of the body are constricted, at 

 least not to the same extent, when the pressure rises ; for vasodilatation has often 

 been observed in different organs, especially in the muscles. 



It is difficult to decide in many cases whether a given dilator effect is active 

 or passive. It may be that with an increase of pressure produced by an extensive 

 contraction of the splanchnic region, various other regions dilate only because 

 of the high pressure. Or it may be that dilatation is actively produced, either 

 by a decline in the tonus of the constrictor nerves or by stimulation of "the 

 dilator nerves. 



A fall in pressure obtained reflexly is caused by a reduction of tonus in some 

 of the great vascular regions. But as in the case of a rise of pressure, the reduc- 

 tion may be due either to stimulation of dilator nerves, or to diminished activity 

 of the constrictors. After excluding all the vasoconstrictor nerves of the hind 

 limb, Bayliss succeeded in demonstrating vasodilatation in the same region by 

 stimulation of the vagus. In this case, therefore, the dilatation took place 

 through the activity of the vasodilator nerves. 



12. THE VASOMOTOR CENTERS 



We have no positive data as yet for the location of centers for the vaso- 

 dilator nerves. These nerves have been followed far into the central nervous 



