STIMULATION BY THE CONSTANT CURRENT 87 



of nerve that has been in contact with the fluid. The muscle 

 relaxes. 



Experiment 5. — Next immerse the cut end in a drop of methylated 

 spirit. This, too, causes contraction. 



Experiment 6. — Repeat, using a solution of 15 per cent. NaCl. 

 The same result is obtained. 



There are very many substances which excite a nerve chemically. 

 Of these we may mention glycerine in strong solutions, solution of 

 lactic acid, bile salts, or mineral acids if not too dilute. Substances 

 which kill but do not excite are basic or neutral lead acetate, 

 chromic acid, copper sulphate, ammonia, &c. 



4. Electrical. 



Stimulation by an electrical current has already been frequently 

 employed in the experiments upon muscle, when we used indirect 

 stimulation. 



THE EFFECTS OF THE CONSTANT CURRENT 

 UPON NERVE 



Employing a current of medium strength, e.g. one Daniell, the 

 usual result is that a twitch occurs at make and break of the current, 

 but that during the passage of the current the muscle remains quies- 

 cent, i.e. the nerve is not stimulated. But this is not universally 

 true, for if the nerve be very irritable, there may be produced a tetanic 

 contraction the whole time the current is passing. This can always 

 be produced if the frog from which the nerve muscle preparation is 

 taken has been kept for a day or two at a low temperature, between 

 0° C. and 10° C. A tetanus at make or break is especially liable to 

 occur at the make of a strong descending current, or the break of a 

 strong ascending current, where by an ascending current is meant 

 one in which the direction of the current in the nerve is from the 

 muscle towards the spinal cord, and by a descending the reverse. 

 This condition is spoken of as Bitter's tetanus. A Bitter's tetanus at 

 break of an ascending current may be stopped by closing the current 

 in the same direction, or may be increased by sending in a current 

 in the reverse (descending) direction. Therefore, as a general rule, 

 the nerve is not stimulated during the whole of the time of passage 

 of a constant current, but only when the strength of the current is 

 suddenly varied. It follows that when we were using induced shocks 

 the excitation was in reality double, but the two stimuli followed 

 one another at such a short interval that the effect on the nerve 

 was the same as a single stimulation. Though the nerve is not 



