As you will see by the picture, the spinal nerves, which are given off from the cord in the lower part of the neck and between the shoulder blades, are gathered together into a great loose bundle to form the long nerve-wires needed to supply the shoulders and arms. Those given off from the small of the back just above the hips also run together to form, first a network and then a big single nerve-cord, called the _sciatic_ nerve, which many of you have probably heard of from the frightfully painful disease due to an inflammation of it, called _sciatica_. It is the largest nerve-cord in the body, running down the middle of the back of the thigh to supply the muscles of two-thirds of the leg.[26] The substance of both the spinal cord and the brain is made up of millions of delicate, tiny cells, called _neurons_, most of which, with very long branches, are arranged in chains for carrying messages, forming the white matter; while the others lie in groups, or ganglia, for sorting and deciding upon messages, forming the gray matter. Just at the top of the spinal cord, where it passes into the skull and joins with the brain, it swells out into a sort of knob, about the size of a queen olive or the head of a gold-headed cane, which is known as the _medulla_, or "pith." This is the most vital single part of the entire brain and nervous system; and the smallest direct injury to it will produce instant death, partly because all the messages which pass between the brain and the body have to go through it, and partly because in it are situated the centres which control breathing and the beat of the heart, and another quite important but less vital centre,--that for swallowing. How Messages are Received and Sent. Now to learn how smoothly and beautifully this nerve telephone system of ours works, and how simple it really is, although it has such a large number of lines and so many telephones on each line, and such a large central exchange, let us see how it deals with a message from the outside world. Suppose you are running barefoot and step on a thorn. Instantly the tiny nerve bulbs in the skin of the sole of your foot are stimulated, or set in vibration, and they send these vibrations up the sciatic nerve, into and up the whole length of the spinal cord, through the medulla, which switches them over to the other side of the brain up through the _brain stalk_, and out to the part of the surface (cortex) of the brain which controls the movements of the foot. All this takes only a fraction of a second, but it is not until the message reaches the brain-surface that you feel pain. If you were to cut the sciatic nerve, or even tie a string tightly around it, you could prick or burn the sole of your foot as much as you pleased, and you would not feel any pain at all.