THE PHENOMENON OF CONTRACTION. 



43 



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, wath 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. 



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 move- 

 ments a neuromuscular me- 

 chanism 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 ordinary voluntary 

 movements the muscular con- 

 tractions are tetanic, we must suppose that the motor nerve 

 cells discharge a series of nerve impulses through the motor 



ing. 21. Schema to show the mnerva- 

 tion of the skeletal (voluntary) muscles: 1, 

 the inter-central (pyramidal) neuron; 2, the 

 spinal neuron; 3, the muscle. 



