THE PHENOMENON OF CONTRACTION. 45 



conditions become ineffective. It should be added that the high 

 figures given above for the correspondence between the stimuli and 

 the muscle-tone hold good only for entirely fresh preparations. 

 The lability of the muscle quickly becomes less as it is fatigued ; so 

 that in the frog, for instance, the correspondence in long-continued 

 contractions is accurate only when the rate of stimulation does 

 not exceed 30 per second. 



The Number of Stimuli Necessary for Complete Tetanus. 

 The number of stimuli necessary to produce complete tetanus 

 varies, as we should expect, with the kind of muscle used and in 

 accordance with the rapidity of the process of relaxation shown 

 by these muscles in simple contractions. The series that may be 

 arranged to demonstrate this variation is quite large, extending 

 from a supposed rate of 300 per second for insect muscle to a low 

 limit of one stimulus in 5 to 7 seconds for plain muscle. The frog's 

 muscle goes into complete tetanus with a rate of stimulation of 

 from 20 to 30 per second. Inasmuch as the rapidity of relaxation 

 of the muscle is much retarded by certain influences, such as a 

 low temperature or fatigue, it follows that these same influences 

 affect in a corresponding way the rate of stimulation necessary to 

 give complete tetanus. A frog's muscle stimulated at the rate of 

 10 stimuli per second may record an incomplete tetanus, but if the 

 stimulus is maintained for some time the tetanus finally becomes 

 complete in consequence of the slowing of the phase of relaxation, 

 or, another way of looking at the matter, in consequence of the 

 development of that condition of maintained contraction which 

 has been spoken of above as contract ure. 



Voluntary Contractions. After ascertaining that muscles may 

 give either simple or tetanic contractions one asks naturally 

 whether in our voluntary movements we can also obtain both 

 sorts of contractions. In the first place, it is obvious that most 

 of our voluntary movements are too long continued to be simple 

 contractions. The time element alone would place them in the 

 group of tetanic contractions, and this is the usual conclusion 

 regarding them. In voluntary movements a neuromuscular 

 mechanism comes into play. This mechanism consists, on the 

 motor side, of at least two nerve units or neurons and the muscle, 

 as indicated in the accompanying diagram (Fig. 21). If in ordi- 

 nary voluntary movements the muscular contractions are tetanic, 

 we must suppose that the motor nerve cells discharge a series of 

 nerve impulses through the motor nerve into the muscle. The 

 contraction of voluntary muscle has been investigated, therefore, 

 in various ways to ascertain whether there is any objective indica- 

 tion of the number of separate contractions that are fused together 

 to make this normal tetanus. Various methods have been em- 



