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_.