PRACTICAL EXERCISES 743 



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 commutator 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 con- 

 traction 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 contrac- 

 tions 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. 686. 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. 



n. Ritter's Tetanus. Lay the nerve of a muscle-nerve prepara- 

 tion 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 contracted (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 immediately 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 con- 

 tinues. Divide it between 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. 727). 



