EXCITATION AND INHIBITION 429 



and that the presence of inhibitory fibres is not a satisfactory explanation. 

 Changes in the sign of the movement of ions to or from the membrane is suggested, 

 a process which would be equivalent to reversal of excitation into inhibition. 



The fact, discovered by Dale (1906), that ergotoxin converts the normal vaso- 

 constrictor effect of the abdominal sympathetic nerve on the hind leg into a 

 vaso-dilator one is, to my mind, more consistently explained by peripheral reversal 

 than by paralysis of vaso-constrictors leaving unmasked vaso-dilator fibres. I have 

 been quite unable to find any evidence of the existence of such fibres in the 

 abdominal sympathetic chain. 



But the whole question as to the mechanism of these various reversal phenomena 

 cannot be said to be capable of decision as yet. 



Pearce (1913) has described experiments in which the normal vasoconstrictor action of 



FIG. 129. THREE STIMULATIONS OF THE CENTRAL END OF THE MEDIAN 



NERVE IN THE RABBIT. 

 Upper tracing volume of the kidney. 

 Lower tracing arterial pressure. Zero is 23 mm. below the upper signal. 



The first stimulation is under ether alone. There is a rise of blood pressure, with vaso- 

 constriction in the kidney. 



The second is under chloroform. There is a fall of blood pressure, with vaso-dilatation in 

 the kidney. 



Third stimulation after partial recovery from chloroform under ether. Preliminary vaso- 

 dilatation, followed by a larger vaso-constriction. 



(Bayliss, 1908, 2, Fig. 24.) 



adrenaline on the arterioles of the frog appeared to be converted into a dilator one in the 

 absence of calcium. These experiments have been already referred to (page 217) and I regret 

 to say that I have been quite unable to confirm them. I found constriction produced by 

 adrenaline even after prolonged perfusion with Ringer's solution free from calcium, although 

 addition of calcium increased the effect to a small degree. On the heart, also, I found the 

 usual augmentation and acceleration to be produced on the auricle when the calcium was 

 reduced as far ( as possible without causing complete cessation of the beats. Of course, under 

 these conditions, conduction is bad, so that one is apt to find the ventricle following only 

 each alternate auricular beat, when they are accelerated by adrenaline. 



EXCITABILITY IX PLANTS 



The mechanism by which movements are produced in plants, especially in 

 the higher plants, will be discussed in the next chapter. The nature of the 

 excitation process concerns us here, 



