PRACTICAL EXERCISES 653 



the one side, and on the other, through a short-circuiting key, with 

 the secondary coil of an induction machine arranged for tetanus. 



Next pith a frog (cord and brain), and make a muscle-nerve pre- 

 paration. Injure the muscle near the tendo Achillis. Lay the 

 injured part over one unpolarizable electrode, and an uninjured 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 move- 

 ment, and remembering that the real direction is the opposite of the 

 apparent direction, and that when the mercury in the capillary tube 

 is positive to the sulphuric acid, the movement is from capillary to 

 acid, determine which is the positive and which the negative portion 

 of the muscle (p. 629). 



(c) Action Current. Now fasten the muscle to the cork or paraffin 

 plate in the moist chamber, without disturbing its position on the 

 electrodes, 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 opposite to its former movement. 



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

 (crushed, cut, or over-heated) on one electrode, and an uninjured 

 part on the other. Of course the nerve does not need to be pinned. 



Clean the unpolarizable electrodes, and be sure to lower the reser- 

 voir of the electrometer ; otherwise the mercury may reach the point 

 of the capillary tube and run out. 



In 5 a galvanometer (p. 535) may be used with advantage by 

 students, if one is available, instead of the electrometer, the un- 

 polarizable electrodes being connected to it through a short-circuiting 

 key. The spot of light is brought to the middle of the scale by 

 moving the control-magnet; or if a telescope-reading (Fig 149, 

 p. 535) is being used, the zero of the scale is brought by the same 

 means to coincide with the vertical hair-line of the telescope. The 

 short circuiting-key is then opened. 



6. Action-current of Heart. Pith a frog (brain and cord). Excise 

 the heart, and lay the base on one unpolarizable electrode, and the 

 apex on the other, having a sufficiently large pad of clay on the tips 

 of the electrodes to ensure contact during the movements of the 

 heart, or having little cups hollowed in the clay and filled with normal 

 saline, into which the organ dips. Connect the electrodes with the 

 capillary electrometer and open its key. At each beat of the heart 

 the mercury will move (p. 644). 



7. Electrotonus. Set up two pairs of unpolarizable electrodes in 

 the moist chamber. Connect two of them with a capillary electro- 

 meter (or galvanometer), and two with a battery of three or four small 

 Daniell cells, as in Fig. 208. Lay a frog's nerve on the electrodes. 

 When the key in the battery circuit is closed, the mercury (or the 



