MUSCLE AND NERVE 23 



Gently press the point of the scalpel transversely into this opening, 

 dividing the spinal cord. Introduce a seeker through the incision, and, 

 passing it upward into the cranial cavity, quickly destroy the cerebrum. 

 Reverse the direction of the seeker and pass it downward through the 

 vertebral canal, destroying the cord. This operation should consume 

 only a few seconds of time. It is known as pithing, and corresponds to 

 the destruction of the spinal cord of mammals by the excessive flexion 

 of the head upon the spinal column. The odontoid process of the axis 

 then lacerates the cord. 



Amputate one leg. Remove the skin from the thigh and isolate 

 the sciatic nerve. Cut away the muscles of the thigh, but preserve the 

 femur and nerve. Suspend the leg from a clamp fastened horizontally 

 to an iron stand by fixing the femur in its screw clip. Attach the 

 central end of the sciatic nerve to a needle-holder and moisten the 

 preparation with an isotonic solution of sodium chlorid (0.7 per cent.). 



7. Direct and Indirect Stimulation of Muscle. Use the following 

 means to cause a contraction of the calf muscle (gastrocnemius) : 



(a) Mechanical stimulation. Pinch the end of the nerve with the 

 forceps. 



(6) Chemical stimulation. Dip the end of the nerve in a strong 

 solution of sodium chlorid. Cut away the piece of the nerve used. 



(c) Thermal stimulation. Heat a piece of wire and bring it in con- 

 tact with the nerve. 



(d) Electric stimulation. Apply the electrodes from the secondary 

 coil of an inductorium to the nerve. Close, and open the key inserted 

 in the primary circuit. 



(e) Photic stimulation. Under these conditions rays of light do not 

 serve as a stimulus. 



Place your index-finger under the sole of the foot and repeat the 

 electric stimulation. Explain the action of the gastrocnemius muscle. 

 Remove the skin. Stimulate the muscle directly. Which stimulation 

 is more effective? 



Locate the tendo achillis. Carefully dissect the other leg and iden- 

 tify the sartorius and gracilis muscles. Open the abdomen of the frog 

 and trace the sciatic nerve to its point of origin from the spinal cord. 



