Skin
<p><b>Skin</b> is the soft outer covering of vertebrates. Other <a href="http://en.wikipedia.org/wiki/Cuticle" title="Cuticle">animal coverings</a> such as the <a href="http://en.wikipedia.org/wiki/Arthropod_exoskeleton" title="Arthropod exoskeleton">arthropod exoskeleton</a> or the <a href="http://en.wikipedia.org/wiki/Seashell" title="Seashell">seashell</a> have different <a href="http://en.wikipedia.org/wiki/Cellular_differentiation" title="Cellular differentiation">developmental origin</a>, structure and chemical composition. The adjective <b>cutaneous</b> means "of the skin" (from Latin <i>cutis</i>, skin). </p>
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In <a href="http://en.wikipedia.org/wiki/Mammal" title="Mammal">mammals</a>, the skin is the largest <a href="http://en.wikipedia.org/wiki/Organ%28anatomy%29" title="Organ (anatomy)">organ</a> of the <a href="http://en.wikipedia.org/wiki/Integumentary_system" title="Integumentary system">integumentary system</a> made up of multiple layers of <a href="http://en.wikipedia.org/wiki/Ectodermal" title="Ectodermal" class="mw-redirect">ectodermal</a> <a href="http://en.wikipedia.org/wiki/Tissue%28biology%29" title="Tissue (biology)">tissue</a>, and guards the underlying <a href="http://en.wikipedia.org/wiki/Muscle" title="Muscle">muscles</a>, <a href="http://en.wikipedia.org/wiki/Bone" title="Bone">bones</a>, <a href="http://en.wikipedia.org/wiki/Ligament" title="Ligament">ligaments</a> and <a href="http://en.wikipedia.org/wiki/Organ_%28anatomy%29" title="Organ (anatomy)">internal organs</a>.<sup id="cite_ref-Hcare_0-0" class="reference"><a href="http://en.wikipedia.org/wiki/Skin#cite_note-Hcare-0">[1]</a></sup> </p>
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Skin of a different nature exists in <a href="http://en.wikipedia.org/wiki/Amphibian" title="Amphibian">amphibians</a>, <a href="http://en.wikipedia.org/wiki/Reptile" title="Reptile">reptiles</a>, and <a href="http://en.wikipedia.org/wiki/Bird" title="Bird">birds</a>.<sup id="cite_ref-1" class="reference"><a href="http://en.wikipedia.org/wiki/Skin#cite_note-1">[2]</a></sup> All mammals have some hair on their skin, even <a href="http://en.wikipedia.org/wiki/Marine_mammal" title="Marine mammal">marine mammals</a> which appear to be hairless. </p>
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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. </p>
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For example, the skin plays a key role in protecting the <a href="http://en.wikipedia.org/wiki/Body" title="Body">body</a> against <a href="http://en.wikipedia.org/wiki/Pathogen" title="Pathogen">pathogens</a><sup id="cite_ref-Proksch_2-0" class="reference"><a href="http://en.wikipedia.org/wiki/Skin#cite_note-Proksch-2">[3]</a></sup> and excessive water loss.<sup id="cite_ref-Madison_3-0" class="reference"><a href="http://en.wikipedia.org/wiki/Skin#cite_note-Madison-3">[4]</a></sup> </p>
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Its other functions are <a href="http://en.wikipedia.org/wiki/Thermal_insulation" title="Thermal insulation">insulation</a>, <a href="http://en.wikipedia.org/wiki/Temperature" title="Temperature">temperature</a> regulation, sensation, and the production of <a href="http://en.wikipedia.org/wiki/Vitamin_D" title="Vitamin D">vitamin D</a> folates. </p>
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Severely damaged skin may heal by forming <a href="http://en.wikipedia.org/wiki/Scar" title="Scar">scar tissue</a>. This is sometimes discoloured and depigmented. </p>
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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.</p>