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. 



We may here remark that a muscle may be thrown into 

 contraction under two different conditions. In the one case it may 

 be free to shorten ; by the lifting of the weight or otherwise, the 

 one end of the muscle may approach the other; and this is the 

 kind of contraction which we have taken, and may take as the 

 ordinary one. But the muscle may be placed under such circum- 

 stances that when it contracts, the one end is not brought nearer 

 to the other, the muscle remains of the same length, and the 

 effect of the contraction is manifested only as an increased strain. 

 In this latter case the contraction is spoken of as an " isometric," 

 in the former case as an " isotonic " contraction. 



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 in- 

 duction-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 



