The whole body is covered and protected by skin. It consists of two layers, the epidermis, or scarf skin, and the dermis, or true skin. The former is composed of layers of cells which are being continually worn away in the form of powder, and as continually replaced ; it contains no bloodvessels and few nerves ; it is full of little holes or pores, which are the openings of the sweat-ducts.

The dermis, or true skin, consists of fibres of connective and elastic tissue, interlaced with bloodvessels and nerves. The surface has numerous minute elevations called papillae. The sensibility of the skin as the organ of touch is due to the nerve-endings in these elevations; they also contain bloodvessels. Many of the papillae have small bodies called touch corpuscles ; they are most numerous in the tips of fingers and palms of the hand. Each papilla is the rudiment of a hair. The deep portion of the dermis is looser in texture and contains some fatty tissue.

The dermis contains two secreting organs, the sweat or sudoriferous glands—fat or sebaceous glands.

The sweat glands consist of a coiled tube and a duct which is twisted like a corkscrew in passing through the epidermis. Perspiration is being continually secreted and carried off in invisible vapour ; when more is poured out than can at once be removed by evaporation, it appears as drops on the skin.

The sebaceous glands secrete an oily substance called sebum; the ducts open into hair follicles, into which the oil is discharged ; it also exudes over the whole surface of the skin and keeps it soft.

The nails are composed of cells of the epidermis joined so as to form a solid plate.

The hairs are growths of the epidermis developed in little pits or follicles. The hair follicles extend down to and below the dermis. The hair grows from the bottom of the follicle ; connected with each follicle are small muscles ; when these muscles contract the little hairs are pulled straight and are said to ' stand on end.' ' Goose skin ' is produced in the same way.

The skin as an excretory organ is nearly related to the kidneys ; when free action of the skin is interrupted, its work is usually taken up by the kidneys ; both excrete organic and saline matter in solution.

The skin absorbs oxygen and exhales carbonic acid and. water as the lungs do.