Nervous System. 39 



sation is in the gray matter on the outside of the brain. 

 When we wish to move any part of the body, the first 

 thing is to will to do it. This action is also in the gray 

 matter on the outside of the brain, but in a different part 

 from that which has sensation. The act of willing to do 

 anything sends nerve currents, or inipulses, along nerve 

 fibers to the part that is to be set to work. The white 

 fibers of the inside of the brain connect the cells of the 

 gray matter with the various parts of the body through the 

 base of the brain, the spinal cord, and the spinal nerves. 



The Spinal Cord. The spinal cord is a cylindrical body 

 extending from the brain along the cavity of the spinal 

 column. Its diameter is not uniform throughout. Between 

 the shoulders is an enlargement called the cervical enlarge- 

 ment, where the large nerves are given off to the arms. 

 In the region of the loins is the lumbar enlargement, where 

 the nerves are given off to supply the legs. The outside 

 of the cord is white, but the central portion consists of 

 gray matter. The white portion is made up of fibers, but 

 the gray matter consists of nerve cells as well. 



The Spinal Nerves. These are given off in pairs from 

 the sides of the spinal cord. They pass out through notches 

 between the successive vertebras, so there is no danger of 

 their being crushed, or even pinched, when the backbone 

 bends. In the regions of the shoulders and loins the 

 spinal nerves are large, as they supply the large muscles 

 of the limbs ; but in the middle of the back, where only 

 the muscles of the body wall are supplied, the nerves are 

 small. We have thirty-one pairs of spinal nerves. 



The Roots of the Spinal Nerves. Each spinal nerve 

 arises by two roots, one nearer the back, called the dorsal 



