sists of a single tube with a blind end, coiled in a sort of ball about
1/60 of an inch in diameter. From this coil the tube passes upwards
through the dermis in a wavy course until it reaches the cuticle, which it
penetrates with a number of spiral turns, at last opening on the surface.
The tubes consist of delicate walls of membrane lined with cells. The coil
of the gland is enveloped by minute blood-vessels. The cells of the glands
are separated from the blood only by a fine partition, and draw from it
whatever supplies they need for their special work.

[Illustration: Fig. 101.--Concave or Adherent Surface of the Nail.

A, border of the root;
B, whitish portion of semilunar shape (the lunula);
C, body of nail. The continuous line around border represents the free
edge.
]

[Illustration: Fig. 102.--Nail in Position.

A, section of cutaneous fold (B) turned back to show the root of the
nail;
B, cutaneous fold covering the root of the nail;
C, semi lunar whitish portion (lunula);
D, free border.
]

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 th