[Illustration:  THE BRAIN.]

4.  Brain Cells.—­The brain is a curious organ of a grayish color outside and white inside.  It is soft, almost like jelly, and this is why it is placed so carefully in a strong, bony box.  If we should put a little piece of the brain under a microscope, we should find that it is made up of a great number of very small objects called nerve or brain cells.  In the illustration you can see some of these brain cells.

[Illustration:  BRAIN CELLS.]

5.  The Nerves.—­Each cell has one or more branches.  Some of the branches are joined to the branches of other cells so as to unite the cells together, just as children take hold of one another’s hands.  Other branches are drawn out very long.

6. The long branches are such slender threads that a great number of them together would not be as large as a fine silk thread.  A great many of these fine nerve threads are bound up in little bundles which look like white cords.  These are called nerves.

7. The nerves branch out from the brain through openings in the skull, and go to every part of the body.  Every little muscle fibre, the heart, the stomach, the lungs, the liver, even the bones—­all have nerves coming to them from the brain.  So you see that the brain is not wholly shut up in the skull, because its cells have slender branches running into all parts of the body; and thus the brain itself is really in every part of the body, though we usually speak of it as being entirely in the skull.

8.  The Spinal Cord.—­There are a number of small holes in the skull through which the nerves pass out, but most of the nerves are bound up in one large bundle and pass out through an opening at the back part of the skull and runs downward through a long canal in the backbone.  This bundle of nerves forms the spinal cord.  The spinal cord contains cells also, like those of the brain.  It is really a continuation of the brain down through the backbone.

[Illustration:  BRAIN AND SPINAL CORD.]

9.  Nerves from the Spinal Cord.—­The spinal cord gives off branches of nerves which go to the arms, the chest, the legs, and other parts.  One of the branches which goes to the hand runs along the back side of the arm, passing over the elbow.  If we happen to strike the elbow against some sharp object, we sometimes hit this nerve.  When we do so, the under side of the arm and the little finger feel very numb and strange.  This is why you call this part of the elbow the “funny” or “crazy bone.”  The cells of the spinal cord also send out branches to the body and to other cells in the brain.

10.  How we Feel.—­If we cut or burn ourselves we suffer pain.  Can you tell why it hurts us to prick the flesh with a pin, or to pinch or burn or bruise it?  It is because the flesh contains a great many nerve-branches from the brain.  When we hurt the skin or the flesh, in any way, these nerves are injured.  There are so many of these little nerves in the flesh and skin that we cannot put the finest needle into the flesh without hurting some of them.