448 MANUAL OF PHYSIOLOGY. 



ready for action, and have to act against a gentle spring-like 

 resistance, so that the motions are even, and there is no jerking, 

 as would occur if the attachments of the inactive muscles were 

 allowed to become slack. 



Electric Phenomena. In a living muscle electric currents may 

 be detected, having a definite direction, and certain relations to 

 the vitality of the tissue. As they seem to be invariably present 

 in a passive muscle, they have been called natural muscle currents. 



They are generally studied in the muscles of cold-blooded ani- 

 mals after removal from the body. The muscle is spoken of as if 



FIG. 181. 



cA.c 



Non-polarizable Electrodes. The glass tubes (a a) contain sulphate of zinc solution 

 (z. s ), into which well amalgamated zinc rods dip. The lower extremity is plugged with 

 china clay (cA.c.), which protrudes at (c') the point. The tubes can be moved in the 

 holders (h h), so as to be brought accurately into contact with the muscle. (Foster.) 



it were a cylinder with longitudinal and transverse surfaces cor- 

 responding to its natural surface and its cut extremities. In such 

 a block of frog's muscle the measurement of the electric currents 

 requires considerable care, because they are so difficult to detect 

 that a most sensitive galvanometer must be used ; and such an 

 instrument can easily be disturbed by currents due to bringing 

 metal electrodes into contact with the moist saline tissues. Spe- 

 cially constructed electrodes must be used to avoid these currents 

 of polarization taking place in the terminals touching the muscle. 

 These are called non-polarizable electrodes, and may be made on 



