Things You Didn't Know About Your Penis

1. Use It or Lose ItYou need to have erections regularly to keep yourpenisin shape. "It has to be essentially exercised," says Tobias Kohler, MD, assistant professor of urology at Southern Illinois University School of Medicine.To maintain a healthy tone, the smooth muscle of thepenismust be periodically enriched with oxygen by the rush ofbloodthat engorges the penis and makes it erect, Kohler says.If a guy is physically able to get erect, but never has erections during the day -- maybe he finds himself in very un-erotic circumstances for a long time -- he needn't worry. Thebrainhas an automatic penis maintenance function built in.Impulses from thebraincause erections during thedreamingphase ofsleep, calledthe REM phase. It doesn't matter if you're having a hotsexdreamor a zombie apocalypse nightmare -- your penis gets hard during that period of thesleepcycle.But some men are physically unable to geterections, such as those who've suffered trauma to the nerves involved or who have nerve orbloodvessel damage caused bydiabetes."If they don't do anything to maintain normal erections, they will get shortening of the penis," Kohler says. Without regular erections, penile tissue can become less elastic and shrink, making the penis 1-2 centimeters shorter.A device like a vacuum pump, which forcesthe penis to swell with blood, can help men with physical erection problems maintain a healthy penis, Kohler says.

2. Your Penis May Be a 'Grower' or a 'Show-er'Among men, there is no consistent relationship between the size of the flaccidpenis and its full erect length.In one study of 80 men, researchers found that increases from flaccid to erect lengthsranged widely, from less than a quarter-inch to 3.5 inches longer.Whatever the clinical significance of these data may be, the locker-room significance is considerable. You can't assume that a dude with a big, limp penis gets much bigger with an erection. And the guy whose penis looks tiny might get a surprisingly big erection.An analysis of more than a thousand measurements taken bysexresearcher Alfred Kinsey shows that shorter flaccid penises tend to gain about twice as much length as longer flaccid penises.

3. The Pleasure ZoneMany men consider the underside of the glans (head) of the penis and the underside of the shaft to be most sensitiveto sexual pleasure.Researchers asked 81healthy mento rate the erotic sensitivity of different areas of their bodies, including not only the penis but also zones such as the scrotum,anus, nipples, and neck.The underside of the glans and underside of the shaft had the highest sensitivity rating for a significant majority of men, followed by the upper side of the glans, leftand right sides of the glans, sides of the penis, upper side of the shaft, and foreskin (for the minority of men who were uncircumcised). The study findings were reported in theBritish Journal of Urology Internationalin 2009.

4. Sensitivity Declines With AgeStudies show that the penis steadily loses sensitivity as men age though it's hard to say exactly by how much. That's because different researchers have used different ways to stimulate the penis and measure sensitivity.In general, the sensitivity of the penis is gauged by the least amount of stimulation a man is able to feel. That is called the"sensory threshold."Despite differences between studies, the data show a clear trend when taken together. From age 25 on, sensitivity startsto decline. The sharpest decline in sensitivity is seen between age 65 and 75.What's less clear is whether men really notice a loss of sensitivity as they age.Kohler says that if they are aware of it, his patients seldom mention it."It is a super-rare complaint," he says. "On the other hand, difficulty with erections and difficulty achieving ejaculation are much more common."

5. Vibrators Work on the Penis TooVibrators aren't only for women. They workon the penis, too. In fact, vibration is so effective on the penis that often men with spinal cord injuries can ejaculate with the aid of a special medical vibrator. For this kind of treatment, the vibrator is usually held against the underside of the head of the penis."Medical-grade vibrators aren't necessarilymore powerful," Kohler says. The vibrators are tuned to stimulate parts of thenervous systeminvolved in ejaculation. "They work at frequencies or amplitudes that are morespecific to the [nerve] pathways."Most men don't need a medical vibrator to trigger an orgasm. Kohler says when patients see him about delayed ejaculation-- difficulty reaching orgasm -- he suggests they try a store-bought vibrating personal massager.Although vibrators often help men with ejaculation problems, you don't have to have any kind of medical condition to use one. You could do it just for fun.

6. There's More to the Penis Than Meets the Eye"Most guys would be proud to know that their penis is twice as long as they think it is," Kohler says.That's because half the length of the penisis inside your body. Just like you don't see all of a big oak tree above ground, you don't see the root of your penis tucked up inside your pelvis and attached to your pubic bone.As seen in anMRIpicture, an erect penis isshaped like a boomerang.

7. Your Penis Is a HabitatTheskinof your penis is home to a diversecommunity of bacteria.Lance Price, PhD, and Cindy Liu, PhD, researchers at the Translational Genomics Research Institute, in Flagstaff, Ariz., used genetic tests to identify the bacteria found on men's penises. Their study showed there were a total of 42 unique kinds of bacteria inhabiting the skin of the penis."We see that the human body is essentially an ecosystem," Price says.But uncircumcised and circumcised penises don't have the same variety and abundance of bacteria, the study showed. The researchers first analyzed samples from the penises of 12 men who were planning to get circumcised. Samples were taken and analyzed again after the men were circumcised.