The sweat glands, on the other hand, are more frequent on the palms and soles and though sometimes found in the derma are usually situated lower down in the subcutaneous cellular tissue. They are least numerous on the back and neck. Coiled up in the lower layers of the skin, they discharge the sweat through a spiral excretory duct upon its free surface.
The sweat is a clear, colorless, watery fluid with a salty taste, an alkaline reaction, and a characteristic odor that varies with the individual. If very scanty, it may be acid in reaction. Besides water it contains a small percentage of solids, as inorganic salts, especially sodium choloride, fatty acids, neutral fats, and at times, especially in some diseases of the kidneys, urea, that is, the end-products of the metabolism of starches and fats chiefly. There is usually also some carbon dioxide, whence the expression cutaneous respiration.
The sweat serves to keep the skin moist and in good condition, to remove outworn and poisonous or irritating matters, and to regulate the temperature. As a rule it evaporates upon reaching the surface, in which case it is known as invisible or insensible perspiration, but if conditions of the atmosphere are not favorable to prompt evaporation, as when the air is damp, the skin becomes damp and there is visible perspiration.
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 paralyz-
