{MUSCLE 1 553 



tinues to pass through a nerve without any sudden or great 

 change in its intensity, there is no stimulation, and the 

 muscle connected with the nerve remains at rest. The 

 same is true of muscle when a weak current is passed 

 directly through it. But in muscle the constancy of the 

 rule is more and more frequently broken by exceptional 

 results as the current is strengthened, a state of permanent 

 contraction being very apt to show itself during the whole 

 time of flow (Wundt) (Fig. 161). Above a certain intensity 

 of current a greater or less degree of permanent contrac- 

 tion is invariably produced. This is sometimes called the 

 * closing tetanus.' It is, however, not a true tetanus, but 

 a tonic contraction, which is strongest in the neighbourhood 

 of the cathode, and does not spread far from it. A similar 

 condition, the so-called galvanotonus, is normally seen in 

 human muscles when they or their motor nerves are traversed 

 by a stream of considerable intensity. Under certain condi- 

 tions, too, e.g., when a strong current is allowed to flow for 

 a comparatively long time through a muscle, the muscle 

 remains contracted after the opening of the current (so- 

 called ' opening tetanus '). 



For nerve, and with these qualifications for muscle, too, 

 we may lay down the law that the voltaic current stimulates 

 at make and at break, but not during its passage. Or, general- 

 izing this a little, since it has been shown that a sudden 

 increase or decrease in the strength of a current already 

 flowing also acts as a stimulus, we may say that the voltaic 

 current stimulates only when its intensity is suddenly and suffi- 

 ciently increased or diminished, but not while it remains constant.* 



When a strong current is closed through a muscle there is 

 an immediate sharp contraction (initial contraction). The 

 muscle then promptly relaxes, but incompletely. When the 

 current is opened, there is another contraction (Fig. 162). 

 The force of the initial contraction, as measured by the 

 resistance necessary to prevent it, is greater than that of the 

 tonic contraction which follows it. 



A second law of great theoretical importance is that/atf] 

 make the stimulation occurs only at the cathode ; at break only 



* This law of du Bois-Reymond has been questioned by Hoorweg and 

 others. It seems to need modification, but the subject cannot be dis- 

 cussed here. 



