Moreover, what I have said of the relations of muscle to blood is also true of all other parts of the body. Just as the muscle cannot work without a due supply of blood, so also the brain and the spinal cord and the nerves have even a more pressing need of pure blood. The weakness and faintness which we feel from want of food is quite as much a weakness of the brain and of the nerves as of the muscles,--perhaps rather more so. And other parts of the body of which we shall have to speak later on need blood too. The whole history of our daily life is shortly 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 rather through them, the muscle, the brain, the nerve, the 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 works, some use up what others have thrown away. There are, besides, scavengers and cleaners to pick up things no longer wanted anywhere and to throw them out of the body. Thus the blood is kept pure as well as fresh. Through the blood thus ever brought to them, each part does its work: the muscle contracts, the brain feels and wills, the nerves carry the feeling and the willing, and the other organs of the body do their work too, and thus the whole body is kept alive and well. THE NATURE OF BLOOD. § IV. =21. What, then, is this blood which does so much?= Did you ever look through a good microscope at the thin transparent web of a frog’s foot, and watch the red blood coursing along its narrow channels? If not, go and look at it at once; you will never understand any physiology till you have done so. There you will see a network of delicate passages far finer than any of your own hairs, and through those passages a tumbling crowd of tiny oval yellow globules hurrying and jostling along. Some of the passages are wider than others, and through some of the wider ones you will see a thick stream of globules rushing onwards towards the smaller channels, and spreading out among them. The globules which you see are floating in a fluid so clear that you cannot see it. Some of the smaller channels are so narrow that only one globule or =corpuscle=, as we may call it, can pass through at a time, and very frequently you may see them passing in single file. Watching them as they glide along these narrow paths, you will note that at last they tumble again into wider passages, somewhat like those from which they came, except that the stream runs away from instead of towards the narrower channels; and in the stream the corpuscle you are watching shoots out of sight. The finest passages are called =capillaries=; they are guarded by delicate walls which you can hardly see; they seem to you passages only, and how fine and small they are will come home to you when you recollect that all you are looking at is going on in the depths