] With few exceptions every portion of the skin is provided with sweat glands, but they are not equally distributed over the body. They are fewest in the back and neck, where it is estimated they average 400 to the square inch. They are thickest in the palms of the hands, where they amount to nearly 3000 to each square inch. These minute openings occur in the ridges of the skin, and may be easily seen with a hand lens. The length of a tube when straightened is about 1/4 of an inch. The total number in the body is estimated at about 2,500,000, thus making the entire length of the tubes devoted to the secretion of sweat about 10 miles. 240. Nature and Properties of Sweat. The sweat is a turbid, saltish fluid with a feeble but characteristic odor due to certain volatile fatty acids. Urea is always present in small quantities, and its proportion may be largely increased when there is deficiency of elimination by the kidneys. Thus it is often observed that the sweat is more abundant when the kidneys are inactive, and the reverse is true. This explains the increased excretion of the kidneys in cold weather. Of the inorganic constituents of sweat, common salt is the largest and most important. Some carbon dioxid passes out through the skin, but not more than 1/50 as much as escapes by the lungs. The sweat ordinarily passes off as vapor. If there is no obvious perspiration we must not infer that the skin is inactive, since sweat is continually passing from the surface, though often it may not be apparent.