Scrotum, in the male reproductive system, a thin external sac of skin that is divided into two compartments; each compartment contains one of the two testes, the glands that produce sperm, and one of the epididymides, where the sperm is stored. The scrotum is a unique anatomical feature of humans and certain other species of land-dwelling mammals. The function of the scrotum is to protect the testes and to keep them at a temperature several degrees below the normal body temperature. The scrotum thus protrudes from the body wall: moreover, it contracts from cold, exercise, or sexual stimulation and expands and relaxes when warm. When contracted, it conserves heat; while relaxed it is smooth and elongated. permitting the circulation of air that effects cooling. The relatively cool temperature of the scrotum is thought to be important for the production of viable sperm.