<unk>  is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation. Other animal coverings, such as the arthropod exoskeleton, have different developmental origin, structure and chemical composition. The adjective cutaneous means "of the<unk> " (from Latin cutis 'skin'). In mammals, the<unk>  is an organ of the integumentary system made up of multiple layers of ectodermal tissue and guards the underlying muscles, bones, ligaments, and internal organs.<unk>  of a different nature exists in amphibians, reptiles, and birds.<unk>  (including cutaneous and subcutaneous tissues) plays crucial roles in formation, structure, and function of extraskeletal apparatus such as horns of bovids (e.g., cattle) and rhinos, cervids' antlers, giraffids' ossicones, armadillos' osteoderm, and os penis/os clitoris. All mammals have some hair on their<unk> , even marine mammals like whales, dolphins, and porpoises that appear to be hairless. The<unk>  interfaces with the environment and is the first line of defense from external factors. For example, the<unk>  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<unk>  may heal by forming scar tissue. This is sometimes discoloured and depigmented. The thickness of<unk>  also varies from location to location on an organism. In humans, for example, the<unk>  located under the eyes and around the eyelids is the thinnest<unk>  on 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<unk>  on the palms and the soles of the feet is the thickest<unk>  on the body as 4 mm thick. The speed and quality of wound healing in<unk>  is promoted by the reception of estrogen. Fur is dense hair. Primarily, fur augments the insulation the<unk>  provides but can also serve as a secondary sexual characteristic or as camouflage. On some animals, the<unk>  is very hard and thick and can be processed to create leather. Reptiles and most fish have hard protective scales on their<unk>  for protection, and birds have hard feathers, all made of tough beta-keratins. Amphibian<unk>  is not a strong barrier, especially regarding the passage of chemicals via<unk> , and is often subject to osmosis and diffusive forces. For example, a frog sitting in an anesthetic solution would be sedated quickly as the chemical diffuses through its<unk> . Amphibian<unk>  plays key roles in everyday survival and their ability to exploit a wide range of habitats and ecological conditions.
Skin
Soft tissue
Peripheral nervous system
Nervous system
Central nervous system
Heart
Reproductive system
Integumentary system