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EXPERIMENTAL PHYSIOLOGY. 



small tray to avoid spilling the mercury. From two of 

 the binding screws connect wires with two N.P. electrodes 



Fig. 90. Scheme of Electrotonic Variation of Excitability in a Nerve. 

 K, Cathode; A, anode; N n, nerve. The curve above the line indi- 

 cates increase, and that below the line decrease, of excitability. 



or the platinum electrodes of du Bois, introducing a short- 

 circuiting key in the electrode circuit (Fig. 91). 



Fig. 91. Scheme of Electrotonic Variation of Excitability. D, Drop of 

 strong solution of salt on the nerve, N ; F, flag on the muscle. 



(6.) Make a nerve-muscle preparation, attach a straw flag 

 to the foot, as directed in Lesson XXVII., 4, and clamp the 

 femur in a clamp on a stand, as in Fig. 53. Lay the nerve 

 over the electrodes. Trace the direction of the current, 

 and make a mark to guide you as to when the current in 

 the nerve is descending or ascending i.e., whether the 

 negative or positive pole is next the muscle. 



(c.) Place a drop of a saturated solution of common salt 

 on the nerve between the electrodes and the muscle. In a 

 minute or less the toes begin to twitch, and by-and-by all the 

 muscles of the leg become tetanic, so that the flag is raised 

 and kept in the horizontal position. 



