818 ELECTRO-PHYSIOLOGY 



10. Formula of Contraction for (Human) Nerves in Situ. Connect 

 eight or ten dry cells in series.* Connect one terminal of the battery 

 to a large plate electrode, and the other to a small electrode, both 

 covered with cotton, flannel, or sponge, moistened with salt solution. 

 Include in the circuit a simple key for making or breaking the current, 

 and a. commutator for changing its direction at will. Leave the key 

 open. Place the large electrode behind the shoulder (or on the back 

 of the neck), and the small electrode over the ulnar nerve at the elbow 

 between the internal condyle and the olecranon. Arrange the com- 

 mutator so that the small electrode shall be the kathode. Close, and 

 then open the key. If no contraction occurs at closing, the battery 

 is too weak, and more cells must be added. If contraction occurs at 

 closing, but not at opening, reverse the commutator, making the small 

 electrode the anode, and observe whether contraction now occurs at 

 closing, at opening, or at both. Note also the relative strength of the 

 various contractions. If the current is ' weak,' the only contraction 

 will be a closing one when the kathode is over the nerve. If the current 

 is of ' medium ' strength, a closing kathodic contraction and both 

 opening and closing anodic contractions will be obtained. With ' strong ' 

 currents contractions will occur at closing and at opening, whether the 

 kathode or the anode is over the nerve. The contractions will vary 

 in strength, as described on p. 763. To work out the different cases 

 of the formula summarized in the table, the number of cells must be 

 increased or diminished. 



The abbreviations KCC, ACC, are used respectively for kathodic 

 closing contraction and anodic closing contraction; KOC, AOC, for 

 kathodic opening contraction and anodic opening contraction. KCC 

 is stronger than KOC, and ACC than AOC. KCC is stronger than 

 ACC, and AOC than KOC. Therefore, as the strength of the current 

 is increased, in the case of normal tissues, KCC is first obtained, then 

 ACC, then AOC, and finally KOC. 



ii. Ritter's Tetanus. Lay the nerve of a muscle-nerve preparation 

 on a pair of unpolarizable electrodes connected through a simple key 

 with a battery of three or four small Daniells. Connect the muscle 

 with a lever. Pass an ascending current (anode next the muscle) for 

 a few minutes through the nerve, and let the writing-point trace on 

 a slowly-moving drum. When the current is closed there may be a 

 single momentary twitch, or the muscle may remain somewhat con- 

 tracted (galvanotonus) as long as the current is allowed to pass, or it 

 may continue to contract spasmodically (' closing tetanus '). When 

 the current is opened the muscle will contract once, and then immedi- 

 ately relax, or there may be a more or less continued tetanus (Ritter's 

 or ' opening tetanus '). If opening tetanus is obtained, divide the 

 nerve between the electrodes: the tetanus continues. Divide it be- 

 tween the anode and the muscle: the tetanus at once disappears. This 

 shows that the seat of the excitation which causes the tetanus is in 

 the neighbourhood of the anode (p. 804). 



* If the laboratory possesses a battery (with rheostat), such as is used by 

 neurologists, the experiment is more conveniently performed with this. 



