What is the name of the sac of skin that contains the testis?

The sac of skin that contains the testis is known as the scrotum. This structure plays a crucial role in the male reproductive system by providing an environment for the testis that is slightly cooler than the body's internal temperature. This temperature regulation is vital for the production of healthy sperm, as sperm development requires a temperature that is optimal for spermatogenesis, which is lower than the body's usual temperature.

The scrotum also has the ability to expand and contract, which helps in maintaining this ideal temperature as it can move the testis closer to or further away from the body according to external conditions. Other options, such as the perineum, vulva, and labia, refer to different anatomical structures associated with human anatomy but do not relate to the function or structure of the male reproductive system as the scrotum does.