Many nerves are given off from the brain and cord and go practically everywhere, so that every part of the body is supplied with them. These nerves are white cords of different sizes; the largest nerve of the body, the one that goes to the leg, called the sciatic, is as large as the little finger. There are really two brains and two cords, as along the central line of the body there is a division of the brain and cord, making two halves exactly alike. These halves are connected together, the division not being complete. Nerves are given off in pairs; for example, from either side of the brain arises a nerve that goes to each eye. So two nerves exactly alike spring from the two sides of the spinal cord, going to each arm. A nerve is composed of a bundle of fibres, microscopic in size. As a nerve passes to the extremities it divides by branching much as does an artery, and thus a bundle of fibres is distributed to a muscle, or a part of the skin, or to an organ, and every part of the body has a direct nerve supply, much as you saw in the microscope it was supplied with blood by means of the capillaries. We cannot prick our finger with the finest needle but nerve, fibres are irritated, and we feel it, and capillaries are injured and we get a drop of blood. Most of the nerves that go to the arms, legs, and organs of the chest and abdomen, arise in and proceed from the spinal cord, but some of the fibres begin in the brain and are continued down the cord, where, joining with fibres that originate in the cord itself, both go to make up the nerve, thus connecting all parts of the body with the great centre. The brain and cord are made up of blood-vessels, nerve cells, nerve fibres, and, holding them all together, connective tissue. The cells are very small, being microscopic in size; there are an immense number of them, and they make up most of the gray matter or outside of the brain, but in the spinal cord the gray matter is in the centre. The fibres that go to make up the nerves begin and spring from the cells, and they also unite them together. The cells are gathered into groups, which have each a separate function to perform. There is a group from which the nerve of the eye proceeds; another for the nerve that goes to the ear; another for the nerve that goes to the arm; and another for the nerve of the heart. There is a group that presides over speech, and other groups that preside over mental action, while all of these are connected together by fibres. Thus it appears that the brain is a true "centre," and the nerves but the means of connection between different parts of the body and the brain, and also between different parts of the brain.