MUSCLE AND NERVE 179 



leading to the nerve is sufficient to excite it when the primary 

 circuit is opened. This may be explained by the assumption 

 of an electrical charge generated in the secondary coil and 

 passing through the nerve to the muscle. In its transit through 

 the nerve it arouses a nerve impulse. If the electrode repre- 

 sented by the wire on which the nerve rests be replaced by a 

 sheet of gold foil which is made to touch the nerve and muscle 

 along their entire length, the charge from the induction coil 

 will reach all points of the preparation at practically the same 

 instant and will not excite it. The charge remains of the same 

 strength, but the electrodes alter the density. 



The duration of a stimulus must be of sufficient length to pro- 

 duce an effect. In the experiments of Tesla, where powerful 

 alternating currents are passed through the body without injury, 

 it is the exceedingly small duration of the changes that prevents 

 harmful effects. It has been found that the results obtained by 

 applying a constant current to a nerve depend upon the direc- 

 tion in which the current flows through the preparation. 



A current, for instance, which in passing through the nerve 

 from the anode (positive electrode) to the kathode (negative elec- 

 trode) flows in the direction of the muscle is called a descending 

 current; while one flowing in a direction away from the muscle 

 is an ascending current. If a current of such strength is chosen 

 that the closing shocks only are effective, its direction is of no 

 consequence. This is true also when the current is increased in 

 strength, so that both opening and closing shocks are capable 

 of causing the muscle to contract. With a strong current, 

 however, it is found that the closing shock in an ascending 

 current and the opening shock of a descending current cause 

 no contraction. In order to understand these results, it is 

 necessary to consider the changes that a constant current pro- 

 duces in a nerve to which it is applied. These are known as 

 electrotonic changes, and they differ in character at the anode 

 and the kathode, so that the condition at the former has been 

 named anelectrotonus, while that at the latter electrode is known 

 as katelectrotonus. While the current is flowing, the irritability 

 of the nerve is raised at the kathode and is lowered at the anode, 

 and this condition is reversed immediately when the current 

 ceases to flow. An excitation is inseparable from|ayrise in 

 irritability. The underlying causes of both are the^same. It 



