SEC. 4. THE MUSCLE-NERVE PREPARATION AS A 

 MACHINE. 



The facts described in the foregoing sections shew that a 

 muscle with its nerve may be justly regarded as a machine which, 

 when stimulated, will do a certain amount of work. But the 

 actual amount of work which a muscle-nerve preparation will do is 

 found to depend on a large number of circumstances, and conse- 

 quently to vary within very wide limits. These variations will be 

 largely determined by the condition of the muscle and nerve in 

 respect to their nutrition ; in other words, by the degree of irrita- 

 bility manifested by the muscle or by the nerve or by both. But 

 quite apart from the general influences affecting its nutrition and 

 thus its irritability, a muscle-nerve preparation is affected as 

 regards the amount of its work by a variety of other circumstances, 

 which we may briefly consider here, reserving to a succeeding 

 section the study of variations in irritability. 



The nature and mode of application of the stimulus as affecting 

 the amount and character of the contraction. 



We have seen that a nervous impulse is a molecular disturbance 

 travelling along the nerve in the form of a wave. We saw further 

 that the velocity with which this wave travels is in the frog about 

 28 inches per sec., and in the mammal somewhat higher, but that 

 it varies according to circumstances, being especially dependent on 

 temperature. The wave-length, that is, the total length of nerve 

 along which the disturbance is at any one instant taking place, 

 from the point nearer the muscle which the disturbance has just 

 reached, to the point farther from the muscle which the disturbance 

 has just left, may we have seen be put down (in the frog) as 18 mm.; 

 but possibly this too varies somewhat. The greatest and most 

 important variations however are those of the energy of the nervous 

 impulse, of the amount of disturbance which takes place in the 

 nerve or in the nerve fibre as the wave of the nervous impulse 

 passes over it ; this we might designate as the height of the wave. 



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