276 PHYSIOLOGY 



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 conduc- 

 tion 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 amounts 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 previous chapter. Kiihne has shown that when the 

 irritability of the frog's sartorius is tested at different points it is greater in 

 the situation of the end-plates. This might be ascribed to the presence 

 of the more irritable nerve-fibres passing into the muscle-fibres at these 

 points. The unequal distribution of irritability is not, however, changed 

 when the muscle is fully poisoned with curare, so as to block entirely the 

 passage of any impulse from the nerve to the muscle. We must therefore 

 regard curare as acting, not on the axon terminations, but on the substance 



