PRACTICAL EXERCISES 



815 



zinc wire. In the foot of the boot is a hollow (or well) which is filled 

 with physiological salt solution and serves to keep the feet well moist- 

 ened with the salt solution. The nerve is laid on the feet of the boots. 

 When not in use the boots should be kept in physiological salt solution.) 



The zincs are now placed in the tubes, dipping into the zinc sulphate. 

 A piece of clay or blotting-paper moistened with physiological salt solu- 

 tion is laid across the electrodes to complete the circuit between their 

 points, and they are connected with the electrometer to test whether 

 they have been properly set up. There ought tc be little, if any, move- 

 ment of the mercury on opening the side-key of the electrometer. If the 

 movement is large, 

 the electrodes are 

 ' polarized,' and must 

 be set up again. The 

 second pair of bind- 

 ing - screws in the 

 chamber are con- 

 nected with a pair of 

 platinum -pointed 

 electrodes on the one 

 side, and on the other, 

 through a short-cir- 

 cuiting key, with the 

 secondary coil of an 

 induction machine ar- 

 ranged for tetanus. 



Next pith . a frog 

 (cord and brain), and 



8' B 



Fig. 312. Moist Chamber. E, unpolarizable electrodes 

 supported in the cork C; M, muscle stretched over the 

 electrodes and kept in position by the pins A, B, stuck 

 in the cork plate P; B, binding-screws connected with 

 galvanometer or capillary electrometer. The other 

 pair of binding-screws serves to connect a pair of 

 stimulating electrodes inside the chamber with the 

 secondary coil of an induction machine. 



make a muscle-nerve 

 preparation. Injure 

 the muscle near the 

 tendo Achillis. Lay 

 the injured part over 

 one unpolarizable 

 electrode, and an un- 

 injured part over the 

 other. Put a wet sponge in the chamber to keep the air moist, and place 

 the glass lid on it. Focus the meniscus of the mercury, and. open the 

 key of the electrometer; the mercury will move, perhaps right out of the 

 field. Note the direction of movement, and, remembering that the 

 real direction is the opposite of the apparent direction, and that when 

 the mercury in the capillary tube is connected with a part of the muscle 

 which is relatively positive to that connected with the sulphuric acid, 

 the movement is from capillary to acid , determine which is the galvano- 

 metrically positive and which the negative portion of the muscle (p. 796). 



(c) Action Current. Now, without disturbing its position on the 

 electrodes, fasten the muscle to the cork or paraffin plate in the moist 

 chamber by pins thrust through the lower end of the femur and the 

 tendo Achillis. Lay the nerve on the platinum electrodes. Open the 

 key of the electrometer, and let the meniscus come to rest. This 

 happens very quickly, as the capillary electrometer has but little inertia. 

 If the meniscus has shot out of the field, it must be brought back by 

 raising or lowering the reservoir. Stimulate the nerve by opening the 

 key in the secondary circuit ; the meniscus moves in the direction oppo- 

 site to its former movement. 



(d) Repeat (b) and (c) with the nerve alone, laying an injured part 

 (crushed, cut, or overheated) on one electrode, and an uninjured part 

 on the other. Of course, the nerve docs not need to be pinned. 



