1.1　Structure of the Skin

Skin covers the body surface and directly contacts the external environment.It is the first line of defense of the human body and has very important functions.The skin structure is usually divided into three parts：epidermis，dermis，and subcutaneous tissue.The epidermis and dermis are connected by a basement membrane zone.In addition to various skin appendages（e.g.，hair，sebaceous glands，sweat glands，and nails），skin also contains abundant blood vessels，lymphatic vessels，nerves，and muscles.Skin is the largest organ of the human body，and its total weight accounts for about 16％of an individual's body weight.The total area of an adult skin is about 1.5 m2，and that of newborns is about 0.21 m2.The thickness of the skin varies with the individual，age，and location，and is usually 0.5-4.0 mm.The skin on the eyelids，vulva，and breast is thin，with a thickness of 0.8-1.4 mm，while the palm and the plantar area is thick，with a thickness of 3-4 mm.Skin color varies with the race，age，gender，nutritional status，and location.(https://www.daowen.com)