THE NERVES AND SPINAL CORD 



news of the cut to the brain, and you do not feel pain 

 when you are cut. In a little while the blood washes away 

 the cocaine, and you can feel again. 



221. How fast nerve messages travel. The nerves can 

 carry messages about one hundred feet each second, or a 

 little faster than an express train. In the time between 

 two ticks of a watch, news of a pin prick can travel from 

 the foot to the brain, and the mind can send word back 

 for the muscles to move the foot away from the pin. If 

 your arm were long enough to touch the sun, you would 

 die of old age before the feeling of the burn could reach 

 you. 



222. The spinal cord. As we follow the nerves back- 

 ward, we can trace them into the inside of the backbone, 

 where they seem to come 



from a white string of 

 flesh called the spinal 

 cord. 



The spinal cord is a 

 soft cord about eighteen 

 inches long, and about 

 the size of the end of 

 the little finger. It is 

 hung in the middle of 

 the rings of bone which 

 make up the backbone. 

 When it is cut across, its end looks like a white ring 

 around a gray, butterfly-shaped center. In the gray 

 matter are cells in which a part of the mind lives. The 

 white matter is made of nerve threads like those in the 

 body. Some stop at the cells of the spinal cord; others 

 go on to the brain ; and still others start at the cells of the 

 cord and end in the brain. 



A thin slice from the spinal cord with the 

 cells and nerves magnified 200 diameters. 



a cells in the gray matter. 



b fibers in the gray matter. 



c nerve threads in the white matter. 



