It will thus be the wisest policy in the prevention of premature old age and in the interests of a long life, to lighten the work of the kidneys, and avoid their being overstrained by throwing a part of the work on their partner—the skin. This organ, as stated at the close of the preceding chapter, is, in a certain measure, a co-partner with the kidneys. When, through cold weather, for instance, the functions of the skin are diminished, a greater flow of liquid will pass through the kidneys in the form of urine; but when the weather is hot and there is perspiration, to a certain extent, less liquid will be secreted and excreted by the kidneys: that is, they will work less. By perspiration, also, more common salt will pass through the skin, and consequently less through the kidneys. It follows logically from these considerations that if we are anxious to preserve the vitality of the kidneys and also free the blood from noxious elements, we must pay special attention to a good action of the skin, and this is only possible by a rational hygiene. The sudorific glands are abundantly provided with small blood-vessels, which bring a large quantity of warm blood to them, from which they absorb watery and solid parts, and, in all probability, gaseous substances also, and pass them through their tortuous excretory channels to the surface of the skin. The mouths of these channels are the pores, and it is of fundamental importance that they remain open; for if closed these waste products cannot pass out and must remain in the body, while in addition no air can pass into the pores, and so no exchange of materials can take place. These pores can easily be clogged; for example, by the scales—cuticle—which we shed every day, mostly from the superficial layers of the skin, and also by the oily secretion of the sebaceous glands. The fat that these glands secrete is intended by Nature to serve as a protection against liquids, like water, and against the perspiration from the skin. In some persons this fat is secreted in too large quantities and may then, especially with its products of decomposition, clog the pores. In the same way the products of perspiration, and also foreign materials, such as dust, may close the pores. It will, therefore, be necessary to remove all these substances which are preventing the proper aëration of the skin and the elimination of harmful matters by it. It is also not impossible, when the products of decomposition of the sweat remain long in close contact with the skin, that some of those injurious elements may be re-absorbed.