THE FROG. 93 



white cord, the optic nerve, connects each of these 

 lobes with one of the eyes; does the optic nerve 

 extend directly from each eye to the corresponding 

 optic lobe? 



3. Extending backward from the under side of the optic 

 lobes is the medulla oblongata. 



4. The medulla oblongata 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 middle of 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. 



Take a live frog, and 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 destroys sensibility, and is 

 essentially the same as cutting off the head. After sever- 

 ing the cord back of the head, run a wire into the brain- 

 cavity and stir it about in order to entirely destroy the 

 brain. In a few minutes hang the frog by a hook through 

 the jaw. 



1. Pinch the toes; what follows? Repeat the experi- 

 ment several times. Pinch the skin near the anus. 



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



