CHEMICAL STIMULATION. 151 



about its middle. Divide the sciatic nerve as high up as 

 possible. The preparation consists 

 of the leg, the lower end of the 

 femur, and the sciatic nerve, ter- 

 minating in the leg muscles. Pin 

 a straw-flag to the toes by means 

 of two pins. Fix the femur in a 

 clamp or pair of muscle-forceps sup- 

 ported on a stand, and shown in 

 Fig. 53, taking care that the gastro- 

 cnemius is upwards. The nerve 

 hangs down, as shown in the figure, yig. 53. Straw-flag at- 

 and must always be manipulated tached to a frog's leg 

 with a camel's-hair brush dipped in fixed in a clamp. N, 

 normal saline. Nerve > F fla S' 



(c.) Pinch the free end of the nerve sharply with forceps, 

 the muscles contract and the straw-flag is suddenly raised- 

 Out off the killed part of the nerve, and observe that con- 

 traction also occurs. 



(d.) Prick the muscle with a needle, it contracts. 



5. Thermal Stimulation. 



(a.) To the same preparation apply gently, either to muscle 

 or nerve, a hot copper wire or needle heated to a dull heat, 

 a contraction results in either case. Cut off the dead part of 

 the nerve. 



6. Chemical Stimulation. 



(a.) Place some saturated solution of common salt in a 

 small glass thimble, or place a drop on a perfectly clean 

 glass slide, and allow the free end of the nerve to dip 

 into it. Owing to the high specific gravity of the saline 

 solution, the nerve floats on the surface, but sufficient salt 

 diffuses into the nerve to stimulate it. After a few moments, 

 the individual joints of the toes begin to twitch, and by-and- 

 by the whole limb is thrown into irregular spasms, ulti- 

 mately terminating in a powerful, more or less continuous, 

 contraction or spasm of the whole musculature, constituting 

 tetanus. Out off the part of the nerve affected by the salt, 

 and the spasms will cease. 



