Secondly, clothing should be as easy as possible; there should not be pressure, except for definite medical purposes of support, on any part of a properly developed body; for pressure not only prevents the free flow of blood to the capillary vessels of the skin, but checks the play of any muscle which is being used, forbidding its expansion at the moment of its energy, and thus cramping the freedom of its movement. Thirdly, clothing should be as light as is possible consistently with reasonable warmth. What is wanted, therefore, next the skin (in cold weather at any rate) is some material like wool, which is porous and therefore holds in its interstices innumerable little air-chambers that when once warmed by the heat of the body form between it and the outer clothing and air a layer of protection. It is exactly this plan that nature has adopted in the covering of birds, and we find the lower part of the feather-quill clad, not in the hard plumage of its tip, but in soft down which interposes a cushion of air between the body and the atmosphere. This is especially and markedly the case in aquatic birds, part of whose body is incessantly in water; the natural oil of the stiff part of the feathers is absolutely waterproof, while the down near their base prevents, by its air and warmth-holding capacities, the chill of the water reaching the body. Wool also has this advantage, that it absorbs moisture, and thus a man clad in under-garments of wool will be less liable to take cold if he gets chilled after violent exercise, since the sweat is to a large extent drawn away from complete contact with the skin. On the other hand, cotton or silk next the skin, while it is less warming and has not the full protective advantages of wool, gives more air to the skin, since it does not cling so closely, and thus facilitates the breathing functions of the skin, and also allows more light to pass through it. Much must depend on the constitutional vigour of the skin in individual cases, and on its reacting powers. For a person naturally liable to catch cold, wool is certainly the safer clothing. Finally, all clothing worn next the skin should be very frequently washed, for it is absorbing all day and every day the waste products of the skin; it should also be well aired before use and kept if possible, not in hermetically sealed drawers, but where air can get to it.