Though an abundant supply of blood increases the action of the sweat glands, they are regulated by definite secretory nerves rather than by the vasomotor nerves. In a cold sweat the action is probably due to some disturbance of the nerve supply without increase of the blood supply. Ordinarily perspiring is a reflex act due to the stimulation of the afferent cutaneous nerves, as by the application of heat, but sometimes, as in cases of strong emotions, involuntary impulses are sent from the brain to the spinal centers and so arouse the action of the glands. Atropin has the power of preventing the secretion of sweat by paralyzing the terminations of the secretory nerves, while pilocarpin produces an opposite effect in a similar way. On account of these sweat glands the skin becomes next in importance after the kidneys in the excretion of waste products. The quantity of sweat excreted varies greatly and is hard to measure. It is influenced by the temperature and humidity of the surrounding air, by the nature and quantity of food and drink consumed, by the amount of exercise, the relative activity of other organs, especially the kidneys, and by certain mental conditions. The hotter it is, the greater the amount of perspiration. In damp weather there may be less perspiration, but it does not evaporate and is therefore more in evidence. Ordinarily man has a temperature of 98.6°. The source of this body heat or temperature is the general body metabolism, muscular activity, and activity of the glands, especially of the liver, which is constantly active, the blood in the hepatic vein being warmer than that in any other part of the body. The tissue of the brain also is said to be warmer than the surrounding blood, and the heart and respiratory muscles, which are in constant activity, are responsible for much of the body heat. The amount of heat generated in the body, therefore, varies at different times, according as a person is awake or asleep, quiet or active.