430 THE FUNCTIONS OF CROSS-STRIATED MUSCLES 



between contractions, whereas it keeps the red muscle almost continuously con- 

 tracted (Ranvier, Kronecker and Stirling; cf. Fig. 167). 



Everything which tends to make the single contractions occupy more time 

 operates to reduce the frequency of stimulation necessary to evoke complete 

 tetanus. Thus fatigued muscles are thrown into tetanus with a lower frequency 

 than unfatigued, because their contractions are slower. 



The more the frequency is reduced below that which is just sufficient to 

 produce tetanus, the more distinctly do the contractions produced by the indi- 

 vidual stimuli stand out from one another, until finally below a certain fre- 



FIG. 167. Tetanus curves of the white (lower tracings) and of the red (upper tracings) muscles 

 of the rabbit, after Kronecker and Stirling. To be read from right to left. A. ten stimuli 

 per second. B, six stimuli per second. 



quency there is no fusion whatever. We have therefore all possible gradations 

 between the isolated contractions and complete tetanus. This suggests that teta- 

 nus itself, notwithstanding the continuous curve by which it is represented 

 graphically, is really a discontinuous process, and complete proof of this is 

 furnished by the electrical variations accompanying tetanus (page 433). 



How are we to conceive of the processes going on in the muscle in tetanus ? 

 One significant fact is that by artificially supporting the muscle, so that it does 

 not lift its weight until it has contracted some distance, the single contractions 

 can be made to reach the same height as tetanus with the same strength of cur- 

 rent (v. Frey). We may say, therefore, that in tetanus the muscle contracts 

 to its utmost, because to a certain extent it is supported on itself. In addition to 

 this the irritability of both nerve and muscle is increased by a previous stimu- 

 lation i. e., if the excitation is not too strong or does not continue so long as 

 to involve much fatigue. Hence not infrequently it happens that stimuli, which 

 of themselves are ineffective, become effective merely by being repeated with 

 sufficient frequency. 



Tetanus may be looked upon therefore as a sort of heaping up of small con- 

 tractions due to the rapidity of the stimuli and to increased irritability. 



F. VOLUNTARY CONTRACTIONS 



If we compare voluntary contractions on the same drum with the rapid, 

 twitchlike muscular contraction produced by a single artificial stimulus, we 

 discover that the former are both slower and less abrupt. Comparing them 

 with the contractions obtained by rapidly repeated shocks, we find more in 

 common. Many other circumstances strongly support this resemblance, the 

 most important of them being, that the voluntary contraction as well as the 

 contractions which appear reflexly with strychnine poisoning are accompanied, 



