Skin is the soft outer covering of vertebrates. Other animal coverings such as the arthropod exoskeleton have different developmental origin, structure and chemical composition. The adjective cutaneous means "of the skin" (from Latin cutis, skin). In mammals, the skin is the largest organ of the integumentary system made up of multiple layers of ectodermal tissue, and guards the underlying muscles, bones, ligaments and internal organs. Skin of a different nature exists in amphibians, reptiles, and birds. All mammals have some hair on their skin, even marine mammals which appear to be hairless. The skin is one of the most important parts of the body because it interfaces with the environment and is the first line of defense from external factors. For example, the skin plays a key role in protecting the body against pathogens and excessive water loss. Its other functions are insulation, temperature regulation, sensation, and the production of vitamin D folates. Severely damaged skin may heal by forming scar tissue. This is sometimes discoloured and depigmented. The thickness of skin also varies from location to location on an organism. In humans for example, the skin located under the eyes and around the eyelids is the thinnest skin in the body at 0.5 mm thick, and is one of the first areas to show signs of aging such as "crows feet" and wrinkles. The skin on the palms and the soles of the feet is 4 mm thick and the thickest skin in the body.
Fur is dense hair. Primarily, fur augments the insulation the skin provides but can also serve as a secondary sexual characteristic or as camouflage. On some animals, the skin is very hard and thick, and can be processed to create leather. Reptiles and fish have hard protective scales on their skin for protection, and birds have hard feathers, all made of tough β-keratins. Amphibian skin is not a strong barrier to passage of chemicals and is often subject to osmosis. For example, a frog sitting in an anesthetic solution could quickly go to sleep.
Famous quotes containing the word skin:
“You flicker. I cannot touch you.
I put my hands among the flames. Nothing burns.
And it exhausts me to watch you
Flickering like that, wrinkly and clear red, like the skin of a mouth.”
—Sylvia Plath (19321963)
“Nor skin nor hide nor fleece
shall cover you,
nor curtain of crimson nor fine
shelter of cedar-wood be over you,
nor the fir-tree
nor the pine.”
—Hilda Doolittle (18861961)
“The bottom line on bonding with multiples seems to be that if you see bonding as a static eventa moment in time at which you must have eye contact and skin contact simultaneously with two or more infantsyou may indeed be in trouble.”
—Pamela Patrick Novotny (20th century)