GENERAL PHYSIOLOGY OF MUSCLE-TISSUE. 85 



to relax, and ultimately returns to its normal condition, notwith- 

 standing the continued stimulation. If the stimulation be with- 

 drawn, the muscle does not at once return to its original length but 

 remains more or less contracted for a variable time. This contrac- 

 tion after stimulation is known as the contraction-remainder. 



If the stimulation be still further increased in frequency, the 

 individual contractions become fused together and the curve described 

 by the lever becomes a continuous line. (See Fig. 33.) Notwith- 

 standing the fact that the individual contractions are no longer visible, 

 it can be shown by other methods that the muscle is undergoing a 

 series of slight alternate contractions and relaxations or vibrations 

 at least. After a varying length of time the muscle becomes fatigued, 

 relaxes, and returns to its natural condition even though the stimu- 

 lation continues. The number of stimuli per second necessary to 

 develop tetanus will depend under normal circumstances on the 



FIG. 33. CURVES SHOWING THE ANALYSIS OF TETANUS or A FROG'S MUSCLE (GAS- 

 TROCNEMIUS). The numbers under the curves indicate the number of shocks per 

 second applied to the muscle. There is almost complete tetanus with twenty- 

 five per second, and it is a little lower than the previous one because the muscle 

 was slightly fatigued. (Stirling.) 



period of duration of the individual contractions. The longer this 

 period, the less the number of stimuli required, and the reverse. 

 Hence the number of stimuli will vary for different classes of animals 

 and for different muscles in the same animal, e. g., 2 or 3 for the 

 muscles of the tortoise, 10 for the muscles of the rabbit, 15 to 20 for 

 the frog, 70 to 80 for birds, 330 to 340 for insects. 



Voluntary Tetanus. The voluntary contractions as they occur 

 in the living body are to be regarded as states of tetanus more or less 

 complete; for the simplest voluntary contraction, however rapidly 

 it takes place, has always a longer duration than a single con- 

 traction caused by a single induction shock. As tetanus experi- 

 mentally produced is the result of a certain number of successive 

 stimulations per second, it is assumed that a voluntary tetanus is the 

 result of the transmission to the muscles of a certain number of nerve 



