The body is an organic whole, like the cell. It is composed of cells, but these cells only develop one of their many powers to its utmost that they may justify their existence in the community; they do not acquire fresh properties. So really the body is merely a mass of protoplasm in which—though in a greater quantity—the same changes are going on that we find in a single cell. Yet how utterly different is the body from the cell! what a wide gulf yawns between a man and an amœba! The body has powers of its own, distinct from those of protoplasm; and the protoplasm may still be alive when the body it helped to compose is dead. A few thousand pounds of protoplasm could not have built Westminster Abbey, but a few hundred men did; for just as out of protoplasm arises the body, so the body gives rise to the mind, a thing as much above it as it is above the cell. So far we have taken the cellular units as our starting-point in discussing life; but a bird’s-eye view of physiology would be incomplete which contained no mention of the major tactics of protoplasm—the life of the body. We must consider the needs of a man as such. A man requires, as we have described before, air and food. The air must contain the proper amount of oxygen, and the food must consist of liquids and solids. The important liquid is water. More than half by weight of the whole body is water, and we are always losing it: through the skin, through the lungs, and through the kidneys. People may drink alcohol with water, but not instead of it. Indeed, the more alcohol they take, the more water they require, for if they take their spirits in an undiluted form, they take water out of the body. If you dip a piece of wet cloth into alcohol for a minute, it dries with remarkable rapidity, because the water has been absorbed out of it. In the same way, if you drink neat spirits, they pass into the blood and stimulate the nervous system, ultimately being excreted by the kidneys; but on their way through the tissues they absorb a great deal of water, which must be replaced. Milk is often spoken of as the ideal food, and so it may be for very young animals; but it lacks one important constituent—iron. A young animal is born with enough iron in its body to do without any in its food until it can take something better than milk. If it is weaned late it becomes anæmic. An adult animal requires iron and also a certain amount of solid food. Its alimentary canal is provided with a good deal of muscle; and this muscle, to be kept healthy, must have something to work upon—in fact, be exercised.