THE HUMAN MOTOR 



Sanderson,! 1 ) an "ascendant period " generally longer than the 

 following period which is " descendant." When the excitations 

 succeed each other at average intervals of 15 hundredths of 

 a second a series of impulses resembling fig. 120 is produced. 



iTVJ 



Tetanic blending of impulses. 



When they follow each other at short intervals they tend to 

 blend themselves into a straight line (fig. 121), which shows a 

 curve for " physiological tetanus." 



The number of impulses producing a sustained tetanic contrac- 

 tion varies in differ- 

 ent muscle- accord- 

 ing to the age, the 

 temperature, the 

 weight under which 

 the muscle contracts, 

 and its initial state. 

 In man it corres- 

 ponds to 20 or 30 

 impulses, that is, 20 

 or 30 excitations per 

 second. Up to 60 

 impulses per second 



the muscle remains tetanised for 4 to 5 minutes, then the curve 

 descends ; there is, from that moment, fatigue ; normal contrac- 

 tion has ceased. Above a frequency of 60, fatigue is more rapid, 

 as is shown in the diagram (fig. 122) from Hofmann.( 2 ) Physio- 

 logically, the muscle receives a real nervous or voluntary excita- 

 tion instead of an electric stimulus, the number ot impulses being 

 always sufficient to cause contraction ; this leads directly (fig. 

 122) to a true state oi tetanus. If a contraction takes place volun- 

 tarily, experiment shows that the muscle gives a sound corres- 

 ponding to a determined frequency, which is clearly perceptible 



(*) It seems, according to the researches of Piper on the nerve of the biceps 

 of the arm, that the latent period may be a little longer, about l6 4 Qg of a 

 second (Pfltieger's, Archiv. vol. cxxvii, p. 474, 1909). 



(*) F. B. Hofmann (Pflitegers Archiv., vol. xciii., p. 197, 1902). 



