140 THE HUMAN BODY. 



existing difference is less than it was, and this the galva- 

 nometer shows as the negative variation. 



Secondary Contraction. It is possible to use the action 

 current of one muscle to stimulate the nerve of a second and 

 produce a contraction. For this purpose two frogs' muscles, 

 A and B, are carefully dissected out with their nerves at- 

 tached. The nerve of B is laid over A so that one part of it 

 lies on the belly and another on the tendon. If the nerve 

 of A be stimulated by a single induction shock, for each 

 contraction of A we get a contraction of B, the negative 

 variation of the muscle current of A being the stimulus for 

 the nerve of B. 



Secondary Tetanus. If the nerve of A be given rapidly 

 repeated stimuli so as to throw that muscle into tetanic con- 

 traction, B is also tetanized. This is of importance, as tend- 

 ing to show that the tetanus of A is really a compound con- 

 traction, although to the eye or as recorded by a lever it is one 

 unbroken shortening. If the electrical condition of A 

 remained uniform during contraction, there should be no 

 fetanus of B, but merely a simple contraction due to the set- 

 ting up of the action current or negative variation when A 

 commenced to contract, and a second due to the cessation of 

 this current when A came to rest again. The tetanus of B 

 must be due to rapidly repeated electrical variations in A, and 

 these probably correspond to the potentially separate con- 

 tractile changes going on in A, and fused into its apparently 

 uniform tetanic contraction. 



The Source of Muscular Energy will be more fully dis- 

 cussed in the chapter on nutrition, but a few of the main 

 points may be mentioned here. A muscle where it contracts 

 is able to do work by using energy set free by chemical 

 changes occurring within it, as a steam-engine does work by 

 using the energy set free by the chemical changes occurring 

 in the combustion of its fuel; and as in the steam-engine, 

 so here, the fundamental change is an oxidative one, though in 

 the muscle a very indirect oxidation. A fresh frog's muscle 

 deprived of blood contains no uncombined oxygen; hung up 

 in an atmosphere of pure nitrogen it can be made to contract 

 and do a great deal of work before it dies and passes into 

 rigor mortis. While doing this work it gives off carbon-diox- 

 ide gas and becomes acid from the formation (probably) of 

 sarcolactic acid, but there does not appear to occur any ap- 



