MUSCULAR ACTIVITY 103 



all voluntary muscular movements, even the briefest, are tetanic 

 in character. They owe this special character to a peculiarity 

 of the nervous system, through which the muscles are excited to 

 contract. When nerve-cells discharge their impulses into muscles 

 the discharge is never a single one but always a series in rapid 

 succession. The result in the muscle is, of course, a tetanus. 



The Electrical Phenomena of Muscle. When a living muscle 

 is carefully exposed and suitable electrodes connected with a 

 sensitive galvanometer or electrometer are applied to its surface 

 the entire surface is found to be isoelectric, i. e., having a uniform 

 electric potential. If, however, an injury such as cutting or 

 burning is inflicted upon any part of the muscle the injured sur- 

 face is found to possess a different potential from the surround- 

 ing uninjured surfaces. This difference of potential is shown by 

 movements of the indicator of the galvanometer or electrometer. 

 These movements are usually in such a direction as to indicate 

 that the injured region has a lower potential than uninjured parts 

 of the same tissue. This difference of potential existing between 

 injured and uninjured living tissue is often referred to as the 

 current of injury, although no current actually flows unless the 

 two regions are connected by an electrical conductor. No cur- 

 rent of injury can be obtained by connecting living tissue with 

 dead tissue. Only while the injured tissue is in act of dying does 

 it exhibit the altered potential which may give rise to an injury 

 current. 



The explanation of the change of electric potential accompany- 

 ing an injury to living tissue is found in the fact that the death 

 process which follows injury involves extensive chemical changes 

 in the tissue. This disturbance in chemical relationship brings 

 about corresponding disturbance in the electric equilibrium which 

 finds expression in an altered electric potential in the part where 

 the chemical activity is going on. 



Just as the chemical changes which follow injury to the tissue 

 give rise to the change of electrical potential which we call the 

 current of injury, so the chemical changes which accompany 

 normal activity in the tissue give rise to electrical changes which 

 are designated currents of action. Action currents cannot easily 

 be demonstrated in an ordinary contracting muscle because the 

 whole muscle goes into contraction at once and so the electric 



