132 THE SENSES OF ANIMALS 



of ions — at the end of the nerve fibre which causes a similar 

 change in the nearest part of the fibre ; that in turn causes a 

 change in the next bit, and so on, with the result that the 

 change runs along the fibre to the body of the cell, from which 

 it is sent along the shorter fibres. The physico-chemical 

 change is repeated at the ends of the short fibres and so is 

 passed over the gap to the fibres of other nerve cells. 



Any chemical change is accompanied by electrical changes, 

 and consequently the passage of a message in a nerve can 

 conveniently be recorded in terms of the electrical change 

 that takes place; the potential of each part of the fibre 

 changes successively, and the "action potential" is easily 



MV 



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MICRO-SECONDS 



Fig. I. Tracing recorded by an oscillograph of the "action potential" when the 

 impulse travelling along a nerve fibre passes a pair of closely spaced electrodes 

 in contact with the fibre. The potential first rises and then falls to a lower value 

 and returns gradually to its original level. 



recorded with an oscillograph. The impulse or message 

 travelling along the nerve is not like one of those ingenious 

 boxes that brings you your change when it is blown along a 

 pipe by compressed air in a large shop; nothing discrete 

 travels along the nerve. The process is much more like the im- 

 pulse that travels along a goods train when the engine starts 

 and all the trucks one after the other are jerked forward 

 with a noisy bumping of buffers until, an appreciable time 

 after the engine has started, the guard's van at the end is 

 jerked into motion. Nothing tangible has passed along the 



