Relations between Sweat-glands and Kidneys. - The sweat-glands and the kidneys both remove water and small quantities of salts from the blood, and thus tend to keep it at its normal concentration. Their functions are complementary, so that when much water is excreted by the skin, less is excreted by the kidneys, and vice versa.

This complementary action is to a great extent due to the different distribution of blood under varying conditions, because when both organs are stimulated - as, for example, by salts of ammonium - diuresis will occur, if the blood be driven towards the kidneys by external cold; and diaphoresis if it be attracted to the skin by external warmth.

The quantity of solids contained in the sweat is very small - only a little over one per cent. - three-fourths of these being organic, and one-fourth inorganic. The organic solids are chiefly fats, fatty acids, and small quantities of urea - about one-tenth per cent. When the kidneys are insufficient, however, to excrete urea, the quantity in the sweat becomes greatly increased, and it has even been found crystallised upon the skin.

Action of the Skin in Regulating Temperature. - As I have already mentioned, the skin has an excreting function complementary to that of the kidneys, and it may to some extent relieve them when they are doing their work imperfectly. But its chief function is that of regulating the bodily temperature. The quantity of heat which is changed into potential energy, in converting liquid water into gaseous steam, is very great. Five and a half times as much heat is required to convert boiling water into steam as to raise the same amount of water from the freezing to the boiling point. The immense loss of heat thus occasioned converts the healthy skin under the influence of great heat into an actual cooling apparatus. In negroes on the West Coast of Africa it has been noticed that while the skin is perspiring profusely, it is as cold as marble, and Sir Charles Blagdon observed that in a room with a temperature of 198° Fahr. his side felt quite cold to the touch.

The chief diaphoretics are :-

Stimulating sweat-centres (?)

Ammonium acetate.

Stimulating secreting nerves (?)

Pilocarpine.

,, citrate.

Warmth to surface, as in baths.

Dover's Powder.

Ipecacuanha.

Warm drinks.

Opium.

Alcohol.

Camphor.

Doubtful action

Serpentaria.

Nicotine. Antimony.

Also reflexly through stomach (?) (p. 439).

Sassafras.

Guaiac.

Mezereum.

Senega.

Uses. - Diaphoretics are used in cases of threatened catarrh or inflammation of mucous or serous surfaces, or internal organs after exposure to cold. Their beneficial action in such cases may be partly due to the withdrawal of blood from internal organs to the surface of the body, but it is not improbable that in addition to this the condition of the skin which they induce exercises a favourable action reflexly on internal parts. There seems to be a sort of complementary action between the skin and the internal mucous membranes, as well as between the skin and kidneys. This is sometimes well marked in gouty patients, where the disappearance of an eruption from the skin is followed by asthma, and vice versa. It is also shown by the experiments of Rossbach (p. 252); and the effect of irritation of the stomach and nausea on the secretion of the skin has already been noticed (p. 439).

One of the best diaphoretics to cut short commencing catarrh is compound ipecacuanha powder. In fevers, with the exception of rheumatic fever, the skin is generally dry although the ternperature is high, and diaphoretics are employed to increase the cutaneous secretion, and thus to lower the temperature.

In exanthemata, after the eruption disappears from the skin, there is a tendency to inflammation of internal organs, and in order to prevent this, diaphoretics are used, those which act markedly on the vessels, or stimulating diaphoretics, being especially indicated.

The advantage of a free supply of blood in chronic morbid conditions, such as chronic ulcers, has already been mentioned when speaking of irritants (p. 343); and in chronic morbid conditions of the skin diaphoretics are sometimes employed to promote the cutaneous circulation. In diseases of the kidneys, when it is advantageous to lessen their functional activity, diaphoretics are employed in order to make the skin act vigorously; and they are used also to assist the kidneys in removing the fluid which has already accumulated in the body in cases of dropsy. When the kidneys, though not diseased, are called upon to do excessive work - as in diabetes mellitus, and polyuria - diaphoretics are employed to aid them. Where an unnatural secretion of fluid is taking place from the intestine, as in cases of chronic diarrhoea, diaphoretics are also employed to divert secretion from the intestine to the skin, and thus lessen the diarrhoea.