NERVE. 51 



nerve attached is fastened to a lever so that its contraction can 

 be recorded. The nerve is passed through the two openings 

 in the bottom of the gas chamber. These holes are made air- 

 tight with a paste made of kaolin and isotonic NaCl solution. 

 One pair of electrodes is set so that the nerve within the chamber 

 can be stimulated, another touches the nerve at the end farthest 

 from the muscle. Minimal break shocks are to be used for 

 stimulation. Carbon dioxide is passed through the chamber, 

 stimuli being applied from time to time at both electrodes in 

 order to discover changes in the response. 



Wash out the carbon dioxide by passing air through the 

 chamber. When the nerve is again normal blow ether vapour 

 through until it affects the nerve, or if necessary paint the 

 nerve with ether. Do not prolong anaesthesia too long or it 

 will be difficult for the nerve to recover. In a like manner 

 study the influence of chloroform. 



Electrical. When a direct current is passed through a nerve 

 the excitability around the kathode is increased, while around the 

 anode it is decreased. The changed condition produced by the 

 direct current is called electrotonus katelectrotonus in the first 

 case and anelectrotonus in the second case. Midway between the 

 two electrodes there is a point where the excitability of the nerve 

 has not been changed. This indifferent point moves toward the 

 kathode as the current is increased and in the opposite direction 

 as the current is decreased. This can be shown by stimulating 

 the nerve along its course during the passing of the direct current. 

 Experiment 23. Electrotonus in Nerve. When a' direct 

 current is to be passed through a tissue for any length of time 

 non-polarizable electrodes must be used as the ordinary elec- 

 trodes are easily polarized, thus altering the efficiency of the 

 current. The former consists of two boot-shaped porous 

 receptacles which are partly filled with a 10% ZnSO 4 solution. 

 A piece of pure zinc attached to the source of current is inserted 

 into each boot. The boot is kept moist on the outside by 

 isotonic NaCl solution, the porous material acting as a barrier 

 to the passage of zinc sulphate during the time usually con- 

 sumed in an experiment. 



The boots must be kept clean. Each time after using they 



