Sexual excitement is caused by physical and mental stimulation. When effective stimulation causes people's sexual desire, it stimulates a series of conditioned and unconditioned reflexes and quickly enters the excitement period. After entering the period of sexual excitement, the corpus cavernosum of penis is congested, the white membrane is tightened, the penis is congested and erect, the scrotum skin is congested and thickened, and then the scrotum becomes hard and fixed. When the plateau period is not reached, or when the excitement period is consciously prolonged, the scrotum continues to be tense and relaxed. The levator testis muscle contracts at will, and the testis can be lifted to the perineum automatically and exclusively.