Our next question is, What is there in the blood that thus gives to the muscle the power of contracting, or that keeps the muscle alive? The answer is easy. What is the name given to this power of a muscle to contract? We call it _strength_. Straighten out your arm upon the table and put a heavy weight in your hand; then bend your arm. Find the heaviest weight that you can raise in this way, and try it some morning after your breakfast, when you are in good condition. Go without dinner, and in the evening when tired and hungry, try to raise the same weight in the same way. You will not be able to do so. Your muscle is weaker than it was in the morning, and you say that the want of food makes you weak; and that is so, because the food becomes blood. The things which we eat are changed into other things which form part of the blood, and this blood going to the muscle gives it strength. What is true of the relations of the blood to the muscles is true of all other parts of the body. The brain and nerves and spinal cord have a more pressing need of pure blood. The faintness which we feel from want of food is quite as much weakness of the brain and of the nerves as of the muscles, perhaps even more so. The whole history of our daily life is, briefly told, this: The food we eat becomes blood; the blood is carried all over the body, round and round, in the torrent of the circulation; as it sweeps past them or through them, the brain, nerves, muscles and skin pick out new food for their work, and give back the things they have used or no longer want; as they all have different work, some pick up what others have thrown away. There are also scavengers and cleansers to take up things which are wanted no longer, and to throw them out of the body. Thus the blood is kept pure as well as fresh. Thus it is through the blood brought to them, that each part does its work.