The cell layers of the epidermis grow down
around the papillae. On the fingertips, the papillae form the lines and whorls
called finger- prints. The dermis carries the skin's blood supply. Through
microscopically thin walls of tiny capillary vessels in this layer of skin, the
blood supply of the body can be brought close to the surface.
When the body
becomes overly warm, these blood vessels dilate. When dilated, the capillaries
have more surface area, permitting an increase in the rate of evaporation and
cooling. When the body becomes cool, the capillaries constrict, reducing heat
loss through the skin. Along with blood vessels, bundles of nerve fibers enter
the skin and branch out in profusion. The supply of nerve endings makes the
dermis highly responsive, especially in such areas as the fingertips, where the
nerve endings are in particularly heavy supply.
There are involuntary muscles in the skin
which dilate and contract the capillaries. There is also an erector muscle
connected to the side of each follicle, or hair pocket. When erector muscles
contract, hairs stand upright. It is this that makes furred animals appears
larger when they are in danger; it also provides an insulating air layer
between the hairs as protection against cold. Some investigators believe that
the action of erector muscles in man, which produces "goose pimples,"
stems from the days when our ancestors had hairy bodies.
The subcutaneous
layer under the skin is attached loosely to inner body structures such as bones
and muscles. Along with blood vessels and nerves, the subcutaneous layer
contains fat globules which serve both to insulate the body against heat and
cold and to cushion inner organs against bumps and jolts. If fatty tissues
become too thick, graceful movement of muscles may be hindered. As people grow
older, the fatty tissue in the subcutaneous layer may be absorbed, causing the
outer skin layers to form uneven folds, or wrinkles.
