THE NERVOUS SYSTEM. 



147 



that the gray part of the brain and spinal cord is partly 

 made of cells of various shapes, with fibers running out 

 of them. The white matter is made of nerve fibers lying 

 side by side. The nerves consist entirely of white nerve 

 fibers. Most of the gray matter with its cells is found 

 in the nerve centers. 



ACTION OF THE NERVOUS 

 SYSTEM. 



SECTION II. 1, If 

 nerves are cut off, we shall 

 find a great change in 

 the parts to which they 

 go. If all the nerves 

 going to a particular part 

 of the skin are cut off, 

 that part of the skin will 

 be insensible. You may 

 prick or pinch it, and it 

 will not feel. If all the 

 nerves going to a muscle 

 are cut off, that muscle 

 will not obey your will: 

 you can no longer use it. 



2, What you have done 

 has not injured directly 

 the skin or the muscle. 

 It has cut off their con- 

 nection with the 

 centers. 



Fig. 57. 



NERVES OF THE SHOULDER AND ARM. 

 1. Artery. 2. Nerves. 



nerve 



We infer, therefore, that it is not really the 

 skin that feels when we prick it. It is something in 

 the nerve centers, and the nerve is the road by which the 



