The detrusor (or detrusor urinae) is a sac shaped muscle that forms one of the layers of the scrotum. It can contract or relax in order to move the testicles closer or further away from the groin. This movement allows the testicles to be warmed or cooled to keep them at the correct temperature. The detrusor may also contract with fear or the release of adrenaline. It is an involuntary muscle and a man normally cannot willingly contract it to shrink the scrotum.