EXCITATION AND INHIBITION 423 



inhibition is not in the actual motor neurone itself, but in the synapse of the 

 afferent or intermediate neurone with it. This fact, in itself, is difficult to bring 

 into agreement with any recognisable amount of metabolism, a conception foreign 

 to that of a boundary surface. The motor neurones of the flexor muscles of the 

 hind leg can be used for the scratch reflex when inhibited from being used for the 

 ordinary flexion reflex. Of course, strictly speaking, the synaptic membrane is 

 common to both neurones of which it forms the connecting link, but it is con- 

 venient to speak of either one without including the membrane. When fatigue of 

 a particular reflex is brought about by stimulation of a certain afferent nerve, it is 

 found that its motor neurones are not fatigued for a reflex brought about by 

 stimulation of an- 

 other afferent nerve. 

 Similarly with inhib- 

 itory reflexes, we 

 must conclude, then, 

 that the synapses of 

 various afferent (or 

 intermediate) neur- 

 ones with the same 

 motor neurone are 

 practically indepen- 

 dent of one another. 

 But before drawing 

 conclusions as to the 

 irreconcilability of 

 fatigue with increase 

 of anabolism, we ! 



must remember that 

 there are, in all prob- 

 ability, one or more 



intermediate neur- , 



ones between the 



afferent and efferent 

 ones. And although 



the final synapse at . 



the motor neurone is ! 



inhibitory, those pre- MH^ 



ceding it are excita- ...,,,,.,,,.,,,,,,,,,,, L ,;,,.,,,,,,,,,!,,,,,,., i.,, i i . 

 tory. Since all 



nerve impulses are F IG . 127. TYPICAL, FORM OF FALL OF BLOOD PRESSURE PRODUCED 

 of the same excita- BY THE DEPRESSOR NERVE IN THE RABBIT. 



tory nature, that in f^ e tracing shows the beginning and end of a period of stimulation lasting 

 the axis Cylinder seventeen minutes. During the whole of this time the blood pressure remained 



/ . , at the level of the bottom of the curve, 



process Ot the inter- Time in twe lve. S econd intervals. 



mediate neurone, (Bayliss, 1893, Fig. 17.) 



which forms the in- 

 hibitory synapse with the motor neurone, is an excitation, and therefore that at 

 the intermediate synapses is also excitatory, and it may be in these synapses that 

 the apparent fatigue of inhibition is situated. 



In some other cases, there is no indication of fatigue in inhibition. Gaskell 

 (1900, p. 205) has kept the heart of the toad in complete rest for twenty-eight 

 minutes by continuous weak stimulation of the intracranial vagus. In vasomotor 

 reflexes, I found (1893, p. 314) that, by stimulation of the central end of the 

 depressor nerve, the blood pressure was reduced to about half its height, and 

 remained at this level without change for seventeen minutes ; when the stimulation 

 was stopped, the blood pressure returned to its previous level (see Fig. 127). 



In the interpretation of experiments such as those of Forbes, there are two 

 further points to be remembered. The "subsequent augmentation" might have 

 been due to auto-stimulation by the contracting muscle of afferent fibres in its own 



