An emulsion is a mixture of two substances that are not supposed to be mixed. In medicine, it is often called an emulsion in the case of a fatty mass suspended in saline. For example, the hydro-lipid film is an emulsion composed of a mixture of sweat, dead skin cells, and sebum, an oily substance on the skin. It is present on the entire surface of the body and protects it from external chemical or microbial aggressions by forming a natural barrier. It also helps regulate skin hydration. Emulsions called lipid emulsions are also used as part of parenteral nutrition, meaning the supply of nutrients by routes other than the digestive tract, often intravenously.