46 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



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 contracture. 



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- 

 ployed. The contractions of the muscle have been recorded by 

 means of levers or tambours, so as to give a curve which can be 

 analyzed; the vibrations of the muscle have been estimated on the 

 principle of sympathetic resonance, and the musical tone emitted 



by the muscle during contrac- 

 tion has been determined. 

 The estimates arrived at by 

 these several methods all indi- 

 cated a relatively slow rhythm 

 of stimulation approximating 

 a rate of 20 stimuli per second. 

 The whole subject has been 

 reinvestigated more recently 

 by employing the "string 

 galvanometer" (see p. 99) to 

 record the number of electrical 

 variations occurring during a 

 voluntary contraction. Since 



Fig. 21. Schema to show the mnerva- J 



tion of the skeletal (voluntary) muscles: 1, each Separate stimulus to a 

 the intercentral (pyramidal) neuron : 2, the i -,. , . 



spinal neuron; 3, the muscle. HlUSCle CaUSCS a distinct 



trical variation, it is evident 



that if we can record the number of such variations per second we 

 shall have almost conclusive evidence as regards the number of 

 simple contractions which enter into the production of voluntary 



