SEC. 4. THE MUSCLE-NERVE PREPARATION AS A 



MACHINE. 



79. 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 influence of the nature and mode of application of the 

 stimulus. When we apply a weak stimulus, a weak induction- 

 shock, to a nerve we get a small contraction, a slight shortening of the 

 muscle ; when we apply a stronger stimulus, a stronger induction- 

 shock, we get a larger contraction, a greater shortening of the 

 muscle. We take, other things being equal, the amount of 

 contraction of the muscle as a measure of the nervous impulse, 

 and say that in the former case a weak or slight, in the latter case 

 a stronger or larger nervous impulse has been generated. Now 

 the muscle of the muscle-nerve preparation consists of many 

 muscular fibres and the nerve of many nerve fibres ; and we may 

 fairly suppose that in two experiments we may in the one 

 experiment bring the induction-shock or other stimulus to bear 

 on a few nerve fibres only, and in the other experiment on many 

 or even all the fibres of the nerve. In the former case only those 

 muscular fibres in which the few nerve fibres stimulated end will 

 be thrown into contraction, the others remaining quiet, and the 

 shortening of the muscle as a whole, since only a few fibres take part 

 in it, will necessarily be less than when all the fibres of the nerve 



