588 



PHYSIOLOGY. 



activity. To study animal electricity, it is necessary to use the 

 instruments employed in the physical laboratory,, but they have to be 

 made very sensitive, since the electric- potential is feeble in animals. 

 There are usually employed three methods of revealing animal elec- 

 tricity: (1) the physiological rheoscope; (2) the galvanometer; (3) 

 the capillary electrometer. 



Physiological Rheoscope. This name has been given to the 

 nerve-muscle preparation of the frog where the greatest possible 



length of the sciatic nerve attached 

 may be used. The preparation of 

 the nerve requires special care, for 

 the nerve must be removed by a 

 little seeker of glass or bone. No 

 metal must touch it. It is removed 

 from below upward, and if properly 

 done there should be no contraction 

 of the muscle during the operation. 

 If the nerve of this preparation be 

 brought into contact with a segment 

 of separated muscle so as to touch 

 simultaneously the longitudinal and 

 transverse surfaces, a contraction 

 instantly follows. 



Galvanometer. The instru- 

 ment usually employed is Thomp- 

 son's astatic, high-resistance, re- 

 flecting galvanometer. In this in- 

 strument a pair of suspended mag- 

 nets nearly astatic are surrounded 

 by many windings of fine, insulated 

 wire with a resistance equal to 10.,- 

 Fig. 223. Thompson Galvanometer. to 20,000 ohms, which explains 



the name of high-resistance galvano- 

 meter. Because it has on the upper magnet a slightly concave 

 mirror by which a ray of light can be reflected on a scale, it is also 

 called reflecting. By placing the point of the unpolarizable elec- 

 trode on the center of the longitudinal surface of the muscle, and 

 the other electrode over the center of the freshly divided transverse 

 surface of the muscle, and connecting the electrodes with the gal- 

 vanometer, with a shunt interposed between the electrodes and the 

 galvanometer, it will be seen that the needle of the galvanometer 





