EXCITATION AND INHIBITION 



411 



inhibition of the centre, although the stimulation causing reflex contraction was 

 continued. 



We may take another example from a different group, the vasomotor reflexes. 

 In Fig. 120 (from an experiment of my own), the uppermost curves represent the 

 volume of one of the hind legs of a dog, a rise in the curve meaning increase of 

 volume due to the presence of more blood in the limb. The vaso-dilator supply 

 had been cut off from 

 the centre by section 

 of the spinal cord 

 in the upper lumbar 

 region. Between the 

 two parts of the 

 tracing, the vaso-con- 

 strictor supply was 

 also cut off by section 

 of the abdominal 

 sympathetic chain. 

 The effect of this, as 

 will be seen, was to 

 cause an increase in 

 the volume of the 

 limb, although there 

 was no change in the 

 blood pressure (the 

 first part of the 

 tracing was inter- 

 rupted before the 

 blood pressure had 

 returned to its nor- 

 mal level, which, at 

 the time of section of 

 the sympathetic, was 

 at its initial height 

 as at the beginning 

 of the figure). The 

 only way in which 

 this vaso - dilatation 

 could have happened 

 was that the vaso- 

 constrictor centre 

 was in a state of 

 tonic excitation, and 

 sending impulses to 

 the limb, keeping its 

 arterioles in a state 

 of tonic contraction. 

 Section of the efferent 

 nerves conveying 

 these impulses re- (After Veszi.) 



leased the blood 



vessels from this tonic excitation, and pressure of the blood inside them caused 

 expansion. At the two marks on the signal line, the central end of the vagus nerve 

 was stimulated. In the dog there are, in this nerve, fibres which excite and also 

 fibres which inhibit the vaso-constrictor centre, but the fall of blood pressure, in the 

 lower curve, shows that in this case the effect of the latter, the " depressor " fibres, 

 was present alone. Owing to inhibition of the tonic state of excitation of the vaso- 

 constrictor centre, the limb dilates in the first stimulation, since the arterioles are 

 freed from the constrictor impulses, just as by the subsequent section of the nerve 

 fibres themselves. The second stimulation, after section of the sympathetic 



FIG. 119. 



INHIBITION OF SPINAL REFLEX IN THE FROG. All 

 sensory roots of the sciatic nerve cut. 



A, Upper tracing contractions of the gastrocnemius muscle. 



Lower signal continuous stimulation of the central end of the 9th dorsal root. 

 Upper signal intermittent stimulation of the central end of the 8th dorsal 



root. 

 The reflex contraction due to the stimulation of the one root is inhibited every 



time that the other root is stimulated. 



B, Similar experiment in which the action of the 10th root is inhibited by 



stimulation of the 8th root. 



The first and last contractions on the myogram show that stimulation of the 

 8th root by itself alone causes reflex contraction, although brief. 



