740 A MANUAL OF PHYSIOLOGY 



cuiting key, with the secondary coil of an induction machine arranged 

 for tetanus. 



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

 preparation. 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 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 galvanometrically positive 

 and which the negative portion of the muscle (p. 718). 



(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. Stimu- 

 late the nerve by opening the key iri 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 overheated) 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 

 reservoir of the electrometer ; otherwise the mercury may reach 

 the point of the capillary tube and run out. 



In 4 a galvanometer (p. 617) 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. 206, p. 618) 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. 



5. 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 insure contact during the movements of the 

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

 physiological salt solution, 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. 730). 



6. 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. 278. Lay a frog's nerve on the elec- 

 trodes. When the key in the battery circuit is closed, the mercury 

 (or the needle of the galvanometer) moves in such a direction as 

 to indicate that in the extrapolar regions parts of the nerve nearer 

 to the anode are relatively positive to parts more remote, and parts 

 nearer to the kathode are relatively negative to parts more remote. 



