Try blowing cool air or dumping cold water on your scrotumAs was discussed earlier in this thread, that's the creamerastic reflex. It's the same thing that makes some (espicially, young) guy's testicle quicky retract if the top of the thigh is brushed.The few-seconds-long periodic cylcing is something else. My theory is that it's tied to the peristalsis of the intestines.
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Weird movement in the scrotum?
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Okay, not to make matters worse here, but what about varicoceles or varicose veins in the scrotum. There's another recent thread about this. The veins can move around and make the balls move around too. I might see a dr though if I were this guy just in case.
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There is no connection between the peristalsis of the intestines and the testicles or scrotum, so there's no way they could move together. The scrotum is responding to very slight changes in air temperature surrounding the skin. The scrotal skin is VERY sensitive to temperature change, down to a tenth of a degree Farenheit, and will adjust accordingly. So what you may not feel as a temperature change because it is slight, your scrotum does feel and adjusts the testes position to compensate. It can do this rather quickly. While it appears to be a rythmic movement, it really isn't - it simply a series of small quick adjustments to temperature in the air surrounding the scrotum. Keep in mind that our skin acts like a large radiator, cooling out bodies, and even the skin on the scrotum itself heats the air around it if there is no air movement, so in an environment with little to no air flow, the scrotum and body heat will heat the air around itself, causing the scrotum to loosen and the testes to drop for cooling, then after the scrotum gets too cool, it adjusts again, tightening and bringing the testicles closer to the body for warmth, then it gets too warm again, and it repeats the cycle, creating what looks like a ryhthmic pattern. Try sitting in a bathtub with your scrotum completely submerged in water, and see if it has the same rythmic movement. Your testes shouldn't move as much in the water, because water has much greater resistance to heat changes than air, and the temperature surrounding the scrotum will be much more stable.