THE NERVOUS SYSTEM 297 



mass which in cross section has a certain resemblance to 

 the letter H. The white matter that surrounds the gray 

 mass is composed almost entirely of fibers running length- 

 wise of the cord. The spinal cord is lodged in a canal 

 within the bones of the vertebral column and sends of! a 

 pair of nerves between each of the vertebrae. Each spinal 

 nerve arises from the cord by two roots, a dorsal and a 

 ventral, which penetrate the white matter and extend 

 into the gray. The two roots soon unite and pass out of 

 the spinal column as a single nerve. Each dorsal root 

 is furnished with a ganglion, the cells of which give origin 

 to the nerve fibers of the dorsal root and to others which 

 proceed outward and form a part of the spinal nerve. 



Experiment has shown that the fibers of the two roots 

 have different functions. If the dorsal root is cut and the 

 end in contact with the cord is stimulated a sensation of 

 pain is felt that is referred to the part to which these 

 particular nerve fibers are distributed. If the other cut 

 end is stimulated no effect is apparent, and the part 

 which the nerve supplies may be cut or burned without 

 producing the least sensation. It is evident, therefore, 

 that the nerves producing sensation pass into the cord 

 through the dorsal root. Cutting a ventral root destroys 

 all power of voluntary movement in the part supplied by 

 the nerve in question, but the sensibility in that part 

 remains undiminished. Stimulating the cut end of the 

 root in contact with the cord produces no marked result 

 while stimulating the outer cut end produces vigorous 

 movement in the part to which the nerve is distributed. 

 The ventral root contains the motor fibers, or those having 

 to do with the production of motion. 



The spinal cord is the center of many reflex acts. If 

 the spinal cord of a frog is cut across near the middle and 

 one of the hind toes is pinched, the leg is quickly withdrawn. 



