Each of your muscles is made up of thousands of thin, long, cylindrical cells called muscle fibers. The muscle fibers' highly specialized structure enables the muscles to relax and contract to produce movement. Muscles vary greatly in their shape and size, depending on their function.
The skin — the largest organ of the body — is made up of a thin outer layer (called the epidermis) and a thicker outer layer (called the dermis). Below the dermis is the subcutaneous tissue, which contains fat. Buried in the skin are nerves that sense cold, heat, pain, pressure, and touch. Sebaceous glands secrete a lubricating substance called sebum. Deep within the skin are your sweat glands, which produce perspiration when you are too hot.
