132 PHYSIOLOGY OF MUSCLES AND NERVES. 



present in the nerve for some time, a series of pulsa- 

 tions or an apj^arent tetanus is occasionally observed. 

 This phenomenon has long been known as an opening 

 tetanus, or as Rittefs tetanus. The connection existing 

 between these changes in the excitability, and the fact 

 that the nerve may be excited by electric currents, has 

 led to the adoption of a view of the electric excitement 

 in nerves which we shall not be able to develop until we 

 have more closely studied electric excitement itself. 



3. If a continuous current is passed through a nerve, 

 and is alternately closed and opened, the excitement 

 appears to occur irregularly, somel imes at the closing, 

 sometimes at the opening of the current, and occasion- 

 ally even at both. Closer observation has, however, 

 shown that very definite laws control this, provided that 

 attention is paid to the strength of the current and its 

 direction within the nerve. Let us first examine these 

 phenomena as they occur in fresh nerve, and, as we found 

 that the conditions in the nerve change very rapidly 

 in the neighbourhood of the cut end, let us commence 

 our observations at a low point in a fresh nerve, of 

 which as great a length as possible has been extracted. 

 For this purpose it is especially necessary to possess a 

 convenient means of graduating at will the strength of 

 the applied currents. Various methods have been used 

 for this purpose. The best is that which is based on 

 the distribution of the currents in branching conduc- 

 tors. The electric current, on being made to traverse 

 a conductor which separates at any point into two 

 liranches, divides, the strength of the currents distri- 

 Inited into these two branches not being always equal, 

 but beinof in each branch in inverse ratio to the resis- 

 tance offered in that branch. Supposing that the nerve 



