414 GENERAL PHYSIOLOGY 



intensity suddenly falls, performs a second contraction. Further, 

 if a current so weak as to be ineffective be allowed to flow through 

 the preparation and then its intensity be gradually increased, it 

 can be made very strong without the muscle making the slightest 

 contraction. But, if a current of the same intensity be allowed to 

 act suddenly upon the preparation, there is an energetic con- 

 traction at the making. Likewise by induction-shocks a much 

 stronger contraction is obtained upon breaking, when, as has been 

 seen, the electric tension is equalised suddenly, than upon making, 

 when the equalisation takes place more slowly. These and 

 similar phenomena have led to the mistaken idea that only the 

 variation of a current at a certain rate acts as a stimulus, and not the 

 continued constant current, and there has been an inclination tO' 

 transfer this idea to other varieties of stimuli. This error is not un- 

 natural in view of the facts that for a long time muscle was the sole 

 object employed for experiment, and contraction represented the 

 sole obvious expression of the excitation. More careful investi- 

 gation has shown that during the continuance of a constant 

 current the muscle goes into a peculiar condition which du Bois- 

 Reymond has termed electrotonus, and in which its irritability 

 is peculiarly changed. It has also been known for a long time 

 that in the employment of somewhat strong currents the muscle 

 does not extend completely after the making, but continues 

 during the whole duration of the current in a state of feeble 

 closing tetanus. An attempt, involving great effort and trouble, 

 has been made to interpret this latter fact otherwise, and also 

 many other facts that favour the idea that the muscle can be in a 

 state of excitation without showing a sudden twitch or continuous 

 contraction. If experiments had not been limited to the muscle 

 or the nerve, but if other objects, such as unicellular organisms, 

 which express excitation in a greater variety of ways, had been 

 employed, and if the question had been followed comparatively, 

 this error, that stimulation results only from variations in the 

 current, and not from the current itself, would evidently have been 

 avoided. 



The one-sided study of galvanic stimulation in muscle and nerve 

 has led to another incorrect idea, viz., the general law of polar 

 excitation of living substance by the constant current. If a con- 

 stant current be allowed to flow through a living object, it is 

 observed that the whole stretch passed through by the current is 

 not stimulated simultaneously, but that the excitation appears- 

 primarily at the place of entrance or outgo of the current, that is, 

 at the anode or the kathode ; from here it can spread secondarily 

 over the whole object because of the continuity of the living sub- 

 stance. Hence the anode and the kathode are the only places 

 where the current stimulates directly; but under what circum- 

 stances the former is the point of excitation, and under what the-. 



