412 



PRINCIPLES OF GENERAL PHYSIOLOGY 



chain, is merely to show the absence of any direct effect on the vessels of the limb, 

 the fall in the curve being due to the diminution in the blood content, owing to 

 the lowered blood pressure produced by dilatation in the blood vessels of those 

 other parts of the body still under the dominion of the vaso-constrictor centre. 



Experiments on balanciny excitation against inhibition in nerve centres show 



FIG. 120. DEPRESSOR REFLEX ON THE ARTERIOLES OF THE HIND LEG FROM 



STIMULATION OF THE CENTRAL END OF THE VAGUS IN THE DOG. VaSO dilators 



cut. 



Upper curves volume of the leg. Rise indicates vaso-dilatation. 

 Lower curves arterial pressure. Zero is 24 mm. below the time signal. 

 Time in ten seconds. 



The first stimulation shows vaso-dilatation produced by inhibition of tone in the vaso-constrictor centre. 

 The arterial pressure falls. Before the second stimulation the abdominal sympathetic nerve was 

 cut. The fact that this nerve was conveying tonic vaso-constrietor impulses is shown by the rise 

 of the level of the volume of the leg seen in the tracing. 



The second stimulation produces a similar fall in arterial pressure, hut the leg, instead of expanding, 

 merely follows the blood pressure ; decrease in volume is due to the lower pressure at which the 

 blood is driven in and to its being drained away by vascular dilatation in other parts. No inhibi- 

 tion of constrictor tone could be shown by the leg, since the vaso-constrictor nerves, passing to it 

 from the centre by way of the sympathetic, were divided. 



(Bayliss, 1908, 2, Fig. 4.) 



that, by properly choosing the relative strengths of the stimulation applied to 

 the respective nerves, an exact neutralisation can be obtained. I found (1893, 

 p. 318) this to be the case with the vaso-constrictor centre and give an example 

 in Fig. 121. The tracing is that of the arterial pressure in the rabbit. During 

 the rise of the upper signal, the depressor nerve was stimulated continuously, 



