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PHYSIOLOGY 



It was shown by Gotcti that, if each of the three roots which make up the sciatic 

 nerve and send fibres to the gastrocnemius be stimulated in turn, it is often impossible 

 to evoke a maximal contraction of the gastrocnemius, however strongly each root 

 be stimulated. Keith Lucas has shown that if stimuli in gradually increasing strength 

 be applied to the motor nerve (containing only seven to nine fibres), which supplies 

 the dorso-cutaneous muscle of the frog, the contraction of the muscle increases, not 

 gradually, but by a series of steps. This can be explained only by assuming that the 

 smallest effective stimulus excites perhaps four out of the seven nerve fibres, those 

 immediately in contact with the electrodes. With increasing strength of current 

 the stimulus becomes effective for the three fibres lying next to these, and finally still 

 further increase of current may excite all the fibres making up the nerve (Fig. 66). 



SO 100 150 200 



FIG. 66. Curve showing relation of height of contraction of dorso-cutaneous muscle 

 to strength of stimulus. - Ordinates = height of contraction ; abscissa = 

 strength of stimulus. (K. LUCAS.) 



THE REPETITION OF STIMULUS 



SUMMATION. The response of a muscle fibre to a single shock, whether 

 measured by the isotonic or the isometric method, i.e. as shortening or as 

 tension, is independent of the strength of stimulus and varies only with the 

 length of the fibre during the rise of the excitatory condition. If however 

 a second shock is sent in during this period a further evolution of energy is 

 possible, and the effect is still further increased by putting a series of stimuli 

 into the muscle or its attached nerve before the development of the contractile 

 stress due to the first stimulus has feached its maximum. If two shocks at 

 intervals of one hundredth of a second be sent into a muscle, the response, 

 whether shortening or rise of tension, will be greater than that produced by 

 one shock. If a series of shocks be sent in, the excitatory condition is main- 

 tained, so that instead of a simple muscle twitch rising to a maximum and 

 then falling, the muscle lever rises to a given point, which in the muscle con- 

 tracting isometrically may be double that due to a single stimulus, and then 

 remains at this height during the continuation of the repeated excitations. 

 If the muscle be allowed to contract isotonically, the continued contraction 

 produced by a series of stimuli may with a heavy load be three or four times 

 as considerable as that produced by a single stimulus. This condition of 

 apparently continued stimulation brought about by continued application of 

 stimuli is said to be surnmated. 



REFRACTORY PERIOD. If the interval between two stimuli sent into 

 a muscle be successively shortened in a series of observations, we finally 

 arrive at a point at which summation is no longer apparent, i.e. the effect of 



