256 PRACTICAL PHYSIOLOGY. [U. 



the distance of the secondary from the primary coil to obtain a 

 minimal stimulus, i.e., response. Apply a drop of saturated solution 

 of common salt to the nerve between tlie electrodes and the muscle. 

 Almost at once the excitability of the nerve is increased, as shown 

 by the height of the contraction, so that the excitabihty increases 

 at once. 



(/>.) After several minutes the muscles begin to twitch, the salt 

 acting as a chemical stimulus. It is thus evident that the excita- 

 bility is early increased, but before muscular response to chemical 

 stimulation is elicited a considerable time elapses. 



4. Effect of Section on the Excitability of a Nerve. 



{a.) Arrange a coil for single shocks, expose the sciatic nerve 

 in a pithed frog, and under it, near its central end, place insulated 

 electrodes, using single break shocks. Ascertain the distance of 

 the secondary from the primary coil at which the break shock is 

 just too weak to cause the muscles to respond (sub-minimal). 



(A.) With a sharp pair of scissors divide the sciatic nerve on the 

 central side of the electrodes. The stimulus (previously sub-minimal) 

 now causes a stro^ig contraction. 



{c.) Ascertain the distance (perhaps several cm.) to which the 

 secondary coil must be pushed away from the primary in order to 

 obtain again a sub-minimal stimulus. The condition of increased 

 excitability lasts for some time. 



5. Excitability of Flexors and Extensors (Rollett). 



Arrange a coil for repeated shocks. Expose either the sciatic 

 nerve or the sciatic plexus in a pithed frog. Select a weak 

 current, and flexion of the leg muscles is obtained ; on pushing up 

 the secondary coil, the extensors prevail 



6. Functions of Different Motor Nerves (Sciatic Plexus). 



Strip off the skin from the hind-legs of a pithed frog. Open 

 the abdomen and expose the sciatic plexus, the frog being placed 

 on its back. Stimulate with faradic electricity — selecting a 

 strength of current just adequate to yield a muscular response — 

 each of the three cords forming the sciatic plexus. The upper cord 

 supplies muscles acting chiefly on the hip-joint, the lowest acts 

 chiefly on the muscles moving the ankle and toes, and the middle 

 one on the muscles acting on the knee-joint. 



7. Conductivity v. Excitability (Grlinhagen's Experiment). 

 (a.) Pass the nerve of a frog's leg through a glass tube (fig. 179), 



sealing the ends with clay, but not compressing the nerve. The 

 tube is supplied with an inlet and outlet, to which elastic tubes can 



