THE PHENOMENON OF CONTRACTION. 



43 



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 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 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 

 nerve into the muscle. The 

 contraction of voluntary muscle 

 has been investigated therefore 

 in various ways, to ascertain 



/ Fig. 21. Schema to show the mnerva- 



Whetner there IS any Obtainable tion of the skeletal (voluntary) muscles: 1, 



. , ,. ,, , .. the intercentral (pyramidal) neuron; 2, the 



evidence Of the number Of spinal neuron; 3, the muscle. 



separate contractions that are 



fused together to make this normal tetanus. In the first place, 

 the normal movements of the muscles have been recorded graph- 

 ically by levers or tambours. The records thus obtained show 

 that our usual contractions are not entirely complete tetani, 

 that is, there is an indication in some part of the curve of 

 the single contractions that are being fused. According to most 

 observers,* these records show that our normal contractions are 

 compounded of single contractions following at the rate of 10 

 per second, or, in other words, the motor neurons discharge 

 about 10 impulses per second into the muscle. The so-called 

 natural muscle-tone has been used for the same purpose. When 

 one places a stethoscope or lays his ear upon a contracting muscle a 

 low tone is heard the pitch of which corresponds with 40 vibrations 

 per second. It is assumed, however, that this note does not represent 

 the actual rate of stimulation of the muscle, since the number 

 * Horsley and Schafer, "Journal of Physiology," 7, 96, 1886. 



