Amphibia. 89 



4. The spinal bulb narrows and becomes the spinal cord. Trace 

 the spinal cord back into the spinal column, cutting away the part 

 of the backbone that covers it. 



5. In the body cavity find nerves emerging from the sides of 

 the spinal column, hence called the spinal nerves. Find that 

 several of these, after running backward, unite to form one large 

 nerve. Trace the nerve down between the muscles of the thigh ; 

 this is the sciatic nerve. 



REFLEX ACTION OF THE FROG'S SPINAL CORD. 



Chloroform a frog as directed on page 83. As soon as it becomes 

 insensible find, by bending the head, the joint between the head 

 and the backbone ; lay the frog on a board, and quickly thrust 

 the blade of a knife through the body at this joint, and completely 

 sever the spinal column and spinal cord. This is essentially the 

 same as cutting off the head. Run a wire into the brain cavity and 

 stir it about in order to destroy the brain. In a few minutes hang 

 the frog by a hook through the jaw. 



1. Pinch the toes; what follows? Repeat the experiment sev- 

 eral times. Pinch the skin near the anus. 



2. Slit the skin along the back side of the thigh ; tear apart the 

 muscles, and find the sciatic nerve ; with a sharp pair of scissors 

 (while watching closely the foot) sever this nerve ; what takes 

 place ? 



3. Hang up as before, and pinch the toes of each foot; what 

 difference is now observed? 



4. With the forceps alternately pinch the two ends of the 

 severed sciatic nerve ; what takes place as these two ends are 

 pinched? 



5. Run a wire down the spinal column, twisting it about to 

 destroy the spinal cord ; what occurs while this is doing? 



6. Pinch the toes as before ; what results ? 



7. Again pinch the end of the sciatic nerve, still connected 

 with the parts below, being careful to pinch a little lower than 

 before. 



