DEMARCATION CURRENT OF MUSCLE. 191 



way, and observe which is the negative and which the 

 positive pole corresponding to the zinc and copper of the 

 battery. 



(e. ) Prepare a Muscle. Dissect out either the sartorius or 

 semi-membranosus of a frog ; these are selected because 

 they consist of parallel fibres, but avoid touching the muscle 

 with the acid skin of the frog. Lay the muscle on a glass 

 plate or block of paraffin under the moist chamber with the 

 N.P. electrodes. 



(/!) Keep one plug in the shunt at C, so as to short-circuit 

 the electrodes, and the other plug at J. Cut a fresh trans- 

 verse section at one end of the muscle, and adjust the point 

 of one electrode exactly over the centre (equator) of the longi- 

 tudinal surface of the muscle. Apply the other electrode 

 exactly to the centre of the freshly divided transverse 

 surface (Fig. 82). 



(g.) Remove the short-circuiting plug from the shunt, 

 keep one plug in at -J-, so that y 1 ^ of the total current from 

 the muscle goes through the galvanometer. Note the direc- 

 tion and extent of the deflection. By noting the direction, 

 and from the observation already made (d.\ one knows that 

 the longitudinal surface of the muscle is + , and the trans- 

 verse section - . Replace the plug key (0), and allow the 

 needle to come to rest at zero. 



(h.) Bring the N.P. electrode on the longitudinal surface 

 nearer to the end of the muscle, and note the diminution of 

 the deflection of the needle. Replace plug C. 



(i.) Vary the position of the electrodes and note the varia- 

 tion in the deflection. If they be equi-distant from the 

 equator, there is no deflection. The greatest deflection takes 

 place when one electrode is over the equator and the other 

 over the centre of the transverse section of a muscle com- 

 posed of parallel fibres. The deflection, i.e, the electro- 

 motive force, diminishes as the electrodes are moved from the 

 equator or the centre of the tran verse section. In certain 

 positions no deflection is obtained. 



