386 STIMULATION OF NERVES. 



at both withdrawal or reinsertion, or at either, or at neither; 

 but no contraction occurs in the interval during which the 

 plug or plugs remain away from the board or in their place, 

 provided that the current in the battery be constant and the 

 condition of the nerve-muscle normal. 



A nervous impulse it generated in a nerve only when there is 

 a sudden change in the. intensity of a constant current passing 

 through it (including the changes from and to zero, i. e., the total 

 breaking and making of the current). So long as the current 

 remains uniform in intensity, there is no contraction of the 

 muscle, no nervous impulse generated in (he nerve. 



The contractions so obtained are simple contractions, indica- 

 tive of the advent of a single nervous impulse. Very often, 

 especially in working with winter frogs in early spring, the con- 

 tractions thus obtained by variations in the intensity of a con- 

 stant current are not simple, but tetanic. This is an abnormal 

 result, which has not yet been investigated. 



The contractions obtained above are not only variable, inas- 

 much as they come either at a diminution (breaking) or increase 

 (making) of the current, or at both, but also differ in extent, 

 . the nervous impulses differ in intensity. 



These variations depend on the strength of the current 

 (amount of variation of the current), the direction of the cur- 

 rent, and the condition of the nerve. 



II. Law of Contraction. Ob*. II. Arrange in the moist 

 chamber a nerve-muscle preparation as fresh and lively as pos- 

 sible. Place the nerve on a pair of nou-polarizable electrodes, 

 about a centimetre apart. Insert between the electrodes and 

 a battery of two or more cells, first the commutator, and 

 then the rheochord. Let the positive and negative wires have 

 different colors, the same throughout the whole apparatus in 

 each case, and arrange so that when the handle of the commu- 

 tator is raised, the current is ascending in the nerve ; when 

 depressed) descending* 



The handle of the commutator being horizontal, and the plug** 

 of the rheochord all in, withdraw the mercury cups a few de- 

 gnv.s of the scale, and depress the handle of the commutator. 

 If there be any contraction, record it. This is equivalent to 

 the making in the nerve of an extremely feeble descending cur- 

 rent. Then bring the handle of the commutator horizontal, 

 and so break this feeble current, recording any result. 



After wailing a few minutes, repeat the observation, using 

 an ascending current instead of a descending. Thus will be 

 obtained the effects of breaking and making an extremely feeble 

 constant current both ascending and descending. 



Then shift the mercuiy cups several degrees, and repeat the 

 whole observation. This will give the effects of making and 



