=11.= In the skull, besides the larger openings we have spoken of, you will find several small holes leading from the outside of the skull into the inside of the brain-case. Some of these holes are filled up during life by blood-vessels, but in others run those delicate white threads or cords which you have already learnt to call nerves. =Nerves are in fact branches of nervous material running out from the brain or spinal cord.= Those from the brain pass through holes in the skull, and at first sight seem to spread out very irregularly. Those which branch off from the spinal cord are far more regular. A nerve runs out on each side between every two vertebræ, little rounded gaps being left for that purpose where the vertebræ fit together, so that when you look at a spinal cord with portions of the nerves still connected with it, it seems not unlike a double comb with a row of teeth on either side. The nerves which spring in this way from the spinal cord are called =spinal nerves=, and soon after they leave the vertebral canal they divide into branches, and so are spread nearly all over the body. In any piece of skin or flesh you examine, never mind in what part of the body, you will find nerves and blood-vessels. If you trace the nerves out in one direction, you will find them joining together to form larger nerves, and these again joining others, till at last all end in either the spinal cord or the brain. If you try to trace the same nerves in the other direction, you will find them branching into smaller and smaller nerves, until they become too small to be seen. If you take a microscope you will find they get still smaller and smaller until they become the very finest possible threads. The blood-vessels in a similar way join together into larger and larger tubes, which last all end, as we shall see, in the heart. =Every part of the body, with some few exceptions, is crowded with nerves and blood-vessels. The nerves all come from the brain or spinal cord--the vessels from the heart. So that every part of the body is governed by two centres, the heart, and the brain or spinal cord.= You will see how important it is to remember this when we get on a little further in our studies. =12.= Well, then, the body is made up in this way. First there is the head. In this is the skull covered with skin and flesh, and containing the brain. The skull rests on the top of the backbone, where the head joins the neck. In the upper part of the neck, the throat divides into two pipes or tubes--one the windpipe, the other the gullet. These running down the neck in front of the vertebral column, covered up by many muscles, when they get about as far down as the level of the shoulders, pass into the great cavity of the body, and first into the upper part of it, or chest.