The brain and nervous system are divided into distinct portions, each having different offices to perform, and each acting independently of the others; as, for example, one portion is employed by the mind in thinking, and in feeling pleasurable or painful mental emotions; another in moving the muscles; while the nerves that run to the nose, ears, eyes, tongue, hands, and surface generally, are employed in seeing, hearing, smelling, tasting, and feeling all physical sensations. The _back_ portion of the spinal marrow and the nerves that run from it are employed in _sensation_, or the _sense of feeling_. These nerves extend over the whole body, but are largely developed in the net-work of nerves in the skin. The _front_ portion of the spinal marrow and its branches are employed in moving those muscles in all parts of the body which are controlled by the _will_ or _choice_ of the mind. These are called the _nerves of motion_. The nerves of sensation and nerves of motion, although they start from different portions of the spine, are united in the same _sheath_ or _cover_, till they terminate in the muscles. Thus, every muscle is moved by nerves of motion; while alongside of this nerve, in the same sheath, is a nerve of sensation. All the nerves of motion and sensation are connected with those portions of the brain used when we think, feel, and choose. By this arrangement the mind _knows_ what is wanted in all parts of the body by means of the nerves of sensation, and then it _acts_ by means of the nerves of motion. For example, when we feel the cold air on the skin, the nerves of sensation report to the brain, and thus to the mind, that the body is growing cold. The mind thus knows that more clothing is needed, and _wills_ to have the eyes look for it, and the hands and feet move to get it. This is done by the nerves of sight and of motion. Next are the nerves of _involuntary motion_, which move all those parts of the head, face, and body that are used in breathing, and in other operations connected with it. By these we continue to breathe when asleep, and whether we will to do so or not. There are also some of the nerves of voluntary motion that are mixed with these, which enable the mind to stop respiration, or to regulate it to a certain extent. But the mind has no power to stop it for any great length of time.