656 A MANUAL OF PHYSIOLOGY 



trodes are placed in a moist chamber. A muscle-nerve preparation 

 is arranged with the nerve on the electrodes and the muscle attached 

 to a lever. The effects of make and break of a weak current, ascend- 

 ing and descending, can be worked out with the simple rheocord. 

 The effects of a medium current will probably be obtained with 

 a single Daniell connected directly with the electrodes through a 

 key. The effects of a strong current will be got when three or four 

 Daniells are connected with the electrodes. Care must be taken to 

 keep the preparation in a moist atmosphere, and more than one 

 preparation may be needed to verify the whole formula. 



11. Bitter'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 same- 

 what 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 continues. 

 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. 640). 



12. Galvanotropism. Place at each end of a rectangular trough 

 filled with tap-water a metallic plate, or a plate of carbon, connected 

 through a commutator and key with the poles of a Grove or bichro- 

 mate battery of several cells, or, if the laboratory is provided with a 

 current from the street, with the switch through one or more incan- 

 descent lamps. Put into the water a number of tadpoles, which 

 should not be too young. When the current is closed, the tadpoles 

 will arrange themselves in a definite way with their long axes in the 

 direction of the lines of flow, the head being turned towards the 

 anode. Reverse the current, and they turn their heads in the 

 opposite direction. If the current is taken from the laboratory 

 supply, the anode may be known as the electrode at which least 

 gas comes off, or at which a mixture of potassium iodide and starch 

 becomes blue (p. 650). 



