636 A MANUAL OF PHYSIOLOGY 



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 striated 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 perma'- 

 nent contraction being very apt to show itself during the whole time 

 of flow (Wundt) (Fig. 222). Above a certain intensity of current a 

 greater or less degree of permanent contraction 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 kathode, 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 conditions, too 

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



FIG. 222. TONIC CONTRACTION OF MUSCLE DURING PASSAGE OF CONSTANT 



CURRENT. 



Two sartorius muscles of frog connected by pelvic attachments. Current from 

 12 small Daniell cells in series passed through their whole length. Current closed 

 at m, opened at b. Time trace, two-second intervals. 



long time through a muscle the muscle remains contracted after 

 the opening of the current (so-called ' opening or Ritter's tetanus ') . 

 Smooth muscle is excited to contraction even when a voltaic current 

 is very gradually passed into it and slowly increased, and again when 

 it is caused very gradually to disappear/ But striped muscle is not 

 stimulated under these conditions. 



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

 la}'' down the law that the voltaic current stimulates at make and at 

 break, but not during its passage. Or, generalizing 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 sufficiently increased or diminished, but not while it 

 remains constant.* 



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

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

 here. 



