280 THE BODY AT WORK 



of contraction along a muscle, the electric potential of the 

 active part of the structure, whether nerve or muscle, is 

 different from that of the not-acting parts on either side of 

 it. A battery in its commonest form is a glass vessel contain- 

 ing sulphuric acid in which a plate of zinc and a plate of copper 

 are immersed. The zinc is electro-positive as regards the 

 copper. In a muscle the contracted portion is electro-positive 

 as regards the parts uncontracted. The degree of positivity 

 can be measured by connecting the muscle at two spots with the 

 two wires of a galvanometer. When one wire makes contact 

 with the contracted portion, and the other with a part which 

 is not contracted, a current passes through the galvanometer, 

 causing its needle to swing ; and since the wave of contraction 

 is not stationary, but passes down the muscle, the current is 

 subsequently reversed. The wave, as it were, first tilts up one 

 end, and then, passing on, tilts up the other, letting down the 

 first. The contracted spot is electro-positive to the spot not 

 contracted, and then the latter, contracting, becomes electro- 

 positive to the former, which has relaxed. The needle of the 

 galvanometer swings first to the left, then to the right. The 

 importance of this method of investigation lies in the fact 

 that the electric variation exactly represents, both in time 

 and in intensity, the change which is occurring in nerve and 

 in muscle. By following it, we can ascertain the rate at which 

 an impulse travels down a nerve. We can determine its length 

 and its " form." Represented on paper, it is a wave. This 

 wave travels in warm-blooded animals with the rapidity of 

 35 metres in a second. When it reaches a muscle, its rate 

 that is to say, the rate at which the wave of contraction in- 

 vades the muscle is 6 metres in a second. The time during 

 which any particular level in the muscle remains contracted 

 in a single spasm, under the influence of an artificial stimulus, 

 is about 0-05 second. The length of the wave is 300 to 400 milli- 

 metres. These measurements give us a very clear idea of the 

 events which occur in a nerve-muscle. An impulse picked up 

 by a motor-cell in the spinal cord runs down its axon termed 

 later a nerve-fibre with great rapidity. Even the most distant 

 muscle is reached in less than one-thirtieth of a second. From 

 the end-plate of the nerve it travels in both directions along the 

 muscle-fibre or group of fibres, since each nerve divides into 



