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62 is in motion: and all these motions are necessary to a proper and healthy secretion of the fluids and solids taken into the stomach. We may therefore say that life is dependent upon voluntary as well as involuntary motion for its existence, and above all it may be affirmed, that a healthy and sound body is never seen, unless there has been proper daily exercise. For, as every function that we have named must be maintained in order that there may be a state of perfect health, so also the whole congeries of organs must be exercised together, in order that there may be nothing superfluous, and nothing wanting, in the entire fabric. If we take each part separately we shall arrive at the same result as if we speak of the whole together. No organ is of greater importance to life than the lungs; and hence it is necessary not only to breathe a pure atmosphere, but also that we take in enough of it at every respiration to inflate the whole chest. The lungs may be said to commence with the soft lining of the trachea, and to extend in two huge lobes down into the chest; and so thin and delicate is the substance of which they are composed, that enough of it is folded into a single thorax to cover one hundred and sixty square yards. This great extent of surface has been given for the