320 ADVANCED PHYSIOLOGY 



fluid, but we know now that this is false. Among other 

 ideas which have been held concerning this question are the 

 following: 



1. The chemical theory maintains that when a nerve is 

 stimulated, a chemical disturbance passes along the axis cylin- 

 der of the neuron involved. This may be compared to the 

 change which takes place in a tiny trail of gunpowder when 

 one end of the line is ignited. The difficulty in accepting 

 this theory is that we should have to imagine the train of 

 material in a nerve to be instantaneously replaced, so that 

 the nerve would be ready to transmit another message at 

 once. 



2. The mechanical theory assumes that the molecules 

 of the nerve fibres are in close contact, and that any 

 unusual movement of them at one end of a nerve is 

 transmitted through the whole line until it is felt at the 

 other end. Suppose the molecules are compared to croquet 

 balls placed in contact, in a long, straight line. If one at 

 the end is struck the one at the opposite end bounds away 

 from the others, though the intervening balls do not move 

 appreciably. 



3. The electrical theory looks upon the nerve impulse as 

 an electrical phenomenon. It can easily be excited by an 

 electric shock, it travels very rapidly over the nerve without 

 seeming to produce any changes in it, and in these respects 

 resembles electricity. Then, too, careful study shows that 

 there are electric changes in the nerve when the impulse 

 passes over it. Sometimes an impulse is supposed to jump 

 from one fibre to another as an electric current may jump 

 from one wire to another. On the other hand, the nerve 

 impulse differs from electricity in several important respects. 

 It travels too slowly, 100 feet per second being too slow for 

 electricity. If a string is tied tightly around a nerve it will 

 stop the passage of the nerve impulse, but such treatment of 

 an electric wire will not stop an electric current over it. If 



