422 



THE FUNCTIONS OF CROSS-STRIATED MUSCLES 



the more irritable it is, the more do the visible phenomena of continuous 

 excitation remain in the background, whereas the effects of variation in the 

 current become the more apparent (Biedermann). 



For all irritable tissues there is a minimum duration of the electric cur- 

 rent necessary to give a stimulus. Other things being equal, the more this 

 time is shortened, the less becomes the stimulating effect until finally it fails 

 altogether. The length of time necessary to produce the maximal effect de- 

 pends primarily on the strength of the current. The greater the strength 

 the shorter the time may be. A constant current of medium strength re- 

 quires 0.016 second to produce its maximal effect on motor nerves ( J. Konig) ; 



FIG. 160. A convenient arrangement of the apparatus for sending induction shocks through 

 a muscle is shown in this Figure. EC, the battery cell; K, key for closing and opening the 

 primary current. When the wires are connected with binding posts 1 and 2 of the induction 

 coil, single make and break shocks are obtained from the secondary coil and are conveyed 

 by the wires connected therewith to the muscle. When the wires are connected with binding 

 posts 1 and 3 of the induction coil the automatic interrupter is brought into play and a series 

 of rapidly repeated (tetanic) shocks is obtained. By means of hand electrodes connected with 

 the secondary end of the induction coil, stimuli may be applied in various other ways. 



no contractions are obtained if the time be reduced below 0.002 second. In- 

 duction shocks are still shorter than this; nevertheless, because of their high 

 tension they are the most effective stimuli for the nerves. 



So far we have considered only the effects of currents suddenly turned on 

 in their full strength. It is possible also by means of special apparatus to arrange 

 the experiment so that starting from nil the current will increase gradually 

 and only reach its maximum after a certain measured time. When this meas- 

 ured time does not exceed roVfrth of a second the effect is the same as that of a 

 current of equal intensity suddenly opened in full force. When it is more than 

 ^inrth of a second the effect varies with the intensity of the current, being for 

 a weak current less effective than the sudden opening, and with a strong current 

 more effective (Gildemeister). But the most characteristic thing about these 



