. Human physiology. h that whichgland, with its escapes by the lungs. Duct. From what has been said it is clear that «, the gland surrounded we must look upon the skin as an excretory by fat-cells; 6, the duct ^ ,, , • L , ,. . ? passing through the organ—that is, one by which waste materialstfernmithroCugh X^ are seParated from the blood circulating in it. Like the lungs, it is a source of loss tothe blood; but while the lungs give off agreat amount of carbonic acid gas and watervapour, the skin excretes but little carbonic acid gas. The excretion of the perspiration is continuous but exceed-
. Human physiology. h that whichgland, with its escapes by the lungs. Duct. From what has been said it is clear that «, the gland surrounded we must look upon the skin as an excretory by fat-cells; 6, the duct ^ ,, , • L , ,. . ? passing through the organ—that is, one by which waste materialstfernmithroCugh X^ are seParated from the blood circulating in it. Like the lungs, it is a source of loss tothe blood; but while the lungs give off agreat amount of carbonic acid gas and watervapour, the skin excretes but little carbonic acid gas. The excretion of the perspiration is continuous but exceed-ingly variable, the average quantity discharged being probablyabout two pounds per day in the adult. The chief use of the perspiration is undoubtedly the protec-tion of the body from too great heat, or, in other words, theequalisation of the temperature of the system. When the surfaceof the body is cold, the supply of blood to the skin is decreasedby the contraction of involuntary muscular fibres which reduce. mucosum, and d,through the upper epi-dermis. THE SKIN 165 the size of the blood-vessels. At the same time the ducts of thesweat glands are contracted, and consequently the secretion ofthe perspiration is slow. But when the body becomes overheatedby exercise, or when the surface of the body is exposed to greatheat, the muscles of the skin relax, the supply of blood increases,the gland ducts open wider, and the perspiration is given off evaporation of this moisture tends to reduce the temperatureof the skin, and consequently that of the body. We now see how necessary it is that the skin should becleansed from any matter which may tend to clog its pores, for ifthe perspiration cannot be freely discharged through the pores, itis reabsorbed into the blood, thus throwing extra work on theother excretory organs, especially the kidneys, which excrete waterand urea. The lungs, skin, and kidneys are all similar in theiraction ; but the work of the skin is more close
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