EXCITATION AND INHIBITION 425 



in which excitation of a particular muscle is associated with inhibition of its 

 antagonist, is shown by McDougall (1903, p. 175) in an ingenious diagram. It 

 shows how the two are always associated and is curiously similar to that of 

 Descartes, which will be found described on page 495. It implies, however, 

 that all cases of inhibition must be associated with excitation somewhere else. 

 In our illustration, " inhibition " of the fountain is associated with " excitation " 

 of the watering can, or of something into which the water runs. In fact, in 

 McDougall's scheme, inhibition of a centre controlling a certain muscle can only 

 take place by stronger excitation of the centre of its antagonist. This does not 

 agree with experimental facts. Sherrington (1906, p. 203), moreover, objects 

 to the theory on the ground that it makes inhibition in nerve centres a different 

 process from peripheral inhibition of smooth muscle, heart, etc. In these latter 

 cases, it does not seem possible to apply the drainage theory. Von Uexkiill, 

 nevertheless, does apply it, in his form, to the case of the claw of the crayfish 

 and in the following way (1909, p. 213, and Uexkiill and Gross, 1913, p. 354). 

 There are two motor nerve tracts, ending at each muscle in a network. These 

 networks are connected by bridges, so that any fresh excitation of either network 

 sucks off the remaining excitation of the antagonist. In front of the network 

 of the closing muscle there is a block of high resistance, so that weak stimuli do not 

 reach the network. The chief evidence for the existence of such bridges seems to 

 lie in the fact that direct stimulation of the opening muscle causes contraction 

 in the extensor of the carpopodite, and in the presence of dividing fibres in the 

 trunk of the nerve. It will be seen that the anatomical facts of Biedermann and 

 of Mangold do not support this view. There is no sign _of anastomosis of nerve 

 fibres to form a network, nor of fibres which could be pointed out as connecting one 

 muscle with the other. It is much simpler to suppose that the two different kinds 

 of fibres which enter the same muscle fibre have opposite functions on account of 

 the difference of the way in which they end in the muscle fibre. If each fibre 

 of the nerve divides, as seems most probable, a branch going to each muscle, it 

 seems that " axone- reflexes " in Langley's sense (1899, p. 388) should cause 

 excitation or inhibition of the antagonist muscle when either muscle is stimulated 

 directly. Which of the two would occur would depend on the strength of the 

 stimulus, as in Richet's experiments. 



Hofmann's work (1914), indeed, gives us definite information on this question. The two 

 axis cylinders, which we have seen to run together, were found, in the case of the opening 

 muscle, to come from two separate nerve trunks, so that they could be excited each apart 

 from the other. One is inhibitory, the other excitatory. There is no nerve network in the 

 neighbourhood of the muscle. It was found that the axis cylinders of the excitatory nerve 

 send branches to the muscle of the preceding joint of the appendage, so that the result of 

 von fJexkiill, described above, turns out to be an axone-reflex, as suggested. Since the two 

 opposing muscles are innervated by the same axis cylinder, which divides, it is clear that both 

 muscles would be stimulated to contraction at the same time, unless the one were provided 

 with means of inhibition from the centre. The balance is peripheral here, instead of central, 

 as in the vertebrate. The opening muscle is innervated merely by two branching axones from 

 the centre, and there are no anastomoses at all. 



Block. It might be thought that a very simple way of putting an end to the 

 excitatory impulses playing on a nerve cell would be to make some synapse in 

 their course impervious to excitation. Sherrington (1906, pp. 100-103) holds that 

 inhibition involves more than this. In the decerebrate cat, the extensors of the 

 knee can be put into contraction by a slight pinch of the opposite foot. The 

 discharge continues for some time, gradually passing off. But if the central 

 end of a branch of the hamstring nerve of the same leg be stimulated for 

 a quarter of a second, the after-discharge is suddenly and completely inhibited. 

 It seems that the efferent neurone is put into a state of excitation which 

 continues after the exciting impulse has ceased, and that this state of excitation 

 can be quelled by inhibitory nerves, although there is no exciting impulse to 

 be blocked off. 



In view of Sherrington's later work on " Plastic Tonus," to be described in Chapter XVIII., 

 it might be suggested that this after-discharge, being itself reflex from receptors in the muscle 

 itself, is subject to the same process as excitation from other sources. But the experiments 

 of Forbes (1912, 1, p. 182) show that a certain amount of after-discharge is still present when 



