From most places where there is oxidation, the venous blood comes away hotter than the arterial, and all the hot venous blood mingling, keeps the whole body warm. Much heat is given up, however, to whatever is touching the skin, and much is used in turning liquid perspiration into vapor. Thus, as long as we are in health, we never get hotter than a certain degree. Everywhere this oxidation is going on. Little by little, every part of the body is continually burning away and continually being made anew by the blood. Though it is the same blood, it makes very different things: in the nerves it makes nerve; in the muscle, muscle. It gives different qualities to different parts: out of one gland it makes saliva, another gastric juice; out of it the bone gets strength and the muscle power to contract. But the far greater part of the power of the blood is spent in heat, or goes to keep us warm. One thing more we have to note before we answer the question, why we move. We have seen that we move by reason of our muscles contracting, and that, in a general way, a muscle contracts because a something started in our brain by our will, passes through the spinal cord, through certain nerves, until it reaches the muscle, and this something we may call a nervous impulse. But what starts this? All the nerves do not end in the muscles, but many in the skin. These nerves can not be used to carry nervous impulses from the brain to the skin. By no effort of yours can you make the skin contract. For what purpose then are these nerves? If you prick your finger, you feel the touch, or say that you have sensation in your finger. If you were to cut the nerve leading to the finger you would lose this power of feeling. These nerves, then, ending in the fingers have a different use from those ending in the muscles. The latter carry an impulse from the brain to the muscles, and are called motor nerves. The former carry impulses from the skin to the brain, and are called sensory nerves, or those which bring about sensations. Motor nerves are of but one kind, but there are several kinds of sensory nerves, each kind having a special work to do. The several works which these nerves do are called the senses. By means of these sensory nerves we receive impressions from the external world, sensations of heat, cold, roughness, good and bad odors, taste, sound, and the color and form of things. Thus impressions of the external world are made upon the brain, and it is these impressions that stimulate the brain to action. The brain worked on by them, through ways that we know not of, governs the muscles, sends commands by the motor nerves, and rules the body as a conscious, intelligent will. TO THE NIGHTINGALE. By DRUMMOND OF HAWTHORNDEN.