THE NEURO-MUSCULAR JUNCTION 311 



contraction of the corresponding half of the muscle, which does not 

 extend to the other hah . On snipping the muscle a little higher up 

 at B, where nerve- endings are present, the resulting contraction involves 

 the whole of the muscle, owing to the fact that the excitation started 

 in the nerve-endings spreads in both directions through the branching 

 nerve fibres. It is possible that this irreciprocity of conduction may 

 be of comparatively late appearance in evolution. So far as we know, 

 an excitatory process in a sheet of muscle and nerve fibres, such as we 

 find in lower invertebrata, e.g. in medusa, may travel with equal 

 facility in all directions. We are probably not warranted from our 

 experiments on skeletal muscle in concluding that the contraction of 

 a cardiac muscle- cell may not set up an excitatory process in the 

 surrounding network of nerve fibres. It is impossible, however, to 

 put such a suggestion to experimental test, since in the heart there is 

 no portion of muscle fibre sufficiently removed from nerves to allow of 

 an excitation being applied which might not at the same time affect the 

 nerve fibres. 



There is evidence that the transmission of the excitatory condition 

 across the end- plate, from nerve to muscle, involves a special excitatory 

 process and the expenditure of energy. Thus there is a period of delay 

 between the arrival of an excitatory impulse at the terminations of 

 the motor nerve and the beginning of the electrical change which 

 marks the moment of stimulation of the muscle fibre. If we compare 

 the latent period of a muscle stimulated directly with its latent period 

 when excited through the nerve, we find that there is an increased 

 period of delay in the latter which is not wholly accounted for by the 

 time taken for the impulse to travel from the stimulated spot down the 

 nerve fibres to the muscle. The extra delay is due to the processes 

 occurring in the end-plate. This end-plate delay has been found to 

 amount to -0013 sec. We may take it roughly at a thousandth 

 of a second. The end- plate seems to be the weakest point in the neuro- 

 muscular chain. We have already seen that, when a nerve of a 

 nerve- muscle preparation is stimulated repeatedly, the muscle very 

 soon shows signs of fatigue, and that the seat of this fatigue is not in 

 the nerve, nor in the muscle, but in the end-plate. 



It has been suggested that the excitation of muscle through nerve 

 depends on an electrical change or discharge at the nerve- ending. 

 This discharge must originate in the terminations of the axon and 

 must influence, in the first instance, the substance which forms the 

 intermediary between the axon and the contractile material of the 

 muscle. We have indeed direct evidence of the existence of a third 

 substance, neither nerve nor muscle, at the point of junction of these 

 two tissues. Thus, curare is generally said to paralyse the end- 

 plates. Evidence for this statement has been given in the 



