The sebaceous gland lies just below the skin adjacent to the |hair follicles| and connected by a short duct. It secretes sebum (oil) into the |hair follicle|, providing a lubricant for the hair and skin. Sebum is a semifluid substance composed of waxes, fatty acids, cholesterol, and debris from skin cells. By coating the hair and the dead keratin cells of the |stratum corneum|, sebum sequesters moisture, keeping hair glossy and skin pliable. Sebum is important for many other reasons. It contains a precursor to |vitamin D| that produces the mature vitamin when struck by the ultraviolet rays of the sun. It also kills certain forms of harmful bacteria.
