116 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



action of a galvanic (polarizing) current. This method of sus- 

 pending conductivity temporarily has been frequently employed 

 for experimental purposes, the arrangement being as represented 

 in Fig. 54. 



The Refractory Period. In the case of the heart, the nerve 

 cell, and the muscle it has been shown that for a short period after 

 the tissue enters into a condition of functional activity it is non- 

 irritable toward a second stimulus. This condition of loss of 

 excitability following upon or accompanying functional activity 

 is designated as the refractory period. It is interesting to find that 

 a tissue so irritable as a nerve fiber exhibits the same phenomenon. 

 For a very brief period (0.002 to 0.006 of a second), after it enters 

 into action, as indicated by the electrical response, a second 

 stimulus throw in will be found to be ineffective. As the elec- 

 trical change passes off, that is, as the state of activity subsides, 

 the nerve regains its normal irritability. The refractory period 

 of the nerve fiber may be much prolonged by conditions which slow 

 the processes underlying activity, for example, by low tempera- 

 tures, or by the action of certain drugs, such as yohimbine (Tait). 

 The Question of Fatigue of Nerve Fibers. An important 

 question in connection with the nature of the nerve impulse has 



Fig. 54. Schema to show the method of blocking the nerve impure by means of a polar- 

 izing current: a, The stimulating electrodes; 6, the battery, the current of which is led into the 

 nerve. The depressed irritability at both anode, +, and cathode, , prevents the nerve im- 

 pulse started at a from reaching the muscle. 



been that of the susceptibility of the nerve fibers to fatigue. The 

 obvious fatigue of muscles and of nerve centers has been referred to 

 the accumulation of the products of metabolism of their tissues or 

 to the actual consumption of the energy-yielding material in them. 

 Funtional activity in these tissues implies the breaking down 

 of complex organic material (catabolism) and the setting free 

 of the so-called chemical energy. The internal energy of the 

 compound is liberated as kinetic energy of heat, etc. It has 

 been accepted, therefore, that if the nerve fiber could be dem- 

 onstrated to show fatigue as a result of functional activity, 

 this fact would be probable proof that the conduction of the im- 



