278 THE BODY AT WOKK 



burning with a heated needle, touching with a drop of glycerin 

 or a strong solution of salt. But of all methods of stimulation, 

 the best is the current from an induction coil. Since it does 

 not injure the nerve, it can be applied as often as may be 

 desked. The amateur provided with an induction coil is in 

 a position to study the relation between stimulus and re- 

 sponse. He can vary the strength of the stimulus and vary 

 the weight which the muscle has to lift. He can observe the 

 progressive onset of fatigue, and otherwise gain much infor- 

 mation regarding the behaviour of muscle as an isolated piece 

 of apparatus. 



It is the ambition of the expert to obtain absolutely correct 

 records of the time-phases and of the changes in electric 

 potential of nerve and muscle under varied experimental con- 

 ditions. For this purpose he needs the finest apparatus which 

 instrument-makers can furnish, and the knowledge and dex- 

 terity requisite for its employment. Consider, for example, 

 the record of the change of form. A nerve-muscle preparation, 

 obtained by the method already described, is arranged so that 

 the point of the lever scratches on a rapidly travelling blackened 

 surface. As the muscle contracts it makes a " tracing." A 

 tuning-fork vibrating at the rate of, say, 400 times a second 

 also scratches a tracing on the same travelling plate. It 

 is easy to time the several phases of contraction and relaxa- 

 tion by comparing them with the undulations made by the 

 tuning-fork. By means of an induction shock a single impulse 

 is generated in the nerve and a single spasm evoked in the 

 muscle. Our tracing shows that the spasm lasts about one- 

 tenth of a second, and that about half this time is occupied by 

 contraction, and half by relaxation. But the ascending curve 

 is usually a little steeper than the descending curve, and the 

 apex a little nearer to the commencement of ascent than to 

 the termination of descent. An electric signal marked the 

 instant at which the current was sent into the nerve. The time 

 taken by the impulse in travelling from the spot where the 

 electric current entered the nerve to its junction with the 

 muscle can therefore be estimated. The contraction begins 

 so much sooner or later, according as the shock is delivered 

 nearer to, or farther from, the muscle. By shifting the electrodes 

 Up and down the nerve, the rate at which the impulse travels 



