Some 500 years later, researchers know the male member doesn't exactly have a mind of its own. But what we do know about men's sexual circuitry may surprise you. 

First of all, men might be surprised to know the default state of the penis is erect. It's the brain that brings erections down to earth. 

How can this be? 

There's a constant battle in men's bodies between the forces of erection and flaccidity, says Dr. Irwin Goldstein of Boston University, author of a recent paper in last month's Scientific American. This ongoing struggle between the excitatory and inhibitory systems, as scientists call them, determines when the penis is erect. 

The battle takes place in a circuit of nerves that loop through the brain, penis and spinal cord. Excitatory or inhibitory signals — seeing a pretty woman, smelling an alluring perfume or spilling a glass of ice water in your crotch — can occur anywhere within the loop. 

 
Without signals from the brain, the default state of the penis is erect 
 

In this sexual civil war, an area of the brain called the para-gigantocellular nucleus (PGN) works to suppress erections. But when the mind is at rest, during REM sleep, for example, the brain stops firing neurons and men experience spontaneous erections — as many as four or five a night. 

Oh, and for ladies: the same process takes place in your body. Go back to the beginning of this article and substitute "clitoris" for "penis." The two organs are made from the same tissue, and erections are basically unisex. 

 
'Although the penis does not "think" for itself, it keeps the brain and spinal cord well appraised of its feelings' — 
Dr. Irwin Goldstein 
 

While the prospect of permanent erections may sound like fun at first, consider the consequences. They would doubtless be a source of great embarrassment and discomfort. They can also be unhealthy: if freshly oxygenated blood isn't brought to the penis, the condition can permanently damage sensitive tissue. 

Too few erections can be unhealthy, too. "It's a requisite for penis health to have a few erections each day," says Goldstein in an interview with Fox News. 

So who's in the driver's seat, especially if part of the brain is constantly working against erections? 

"Sometimes sexual impulses override my intellect," said Bruno, a graduate student from Boston who asked his real name not be used. "I meet a girl, and all of a sudden it's sex, sex, sex." Thinking about it, that is. 

When men "think with their penises," it's really the libido at work, not the actual penis itself — and sex drive is undoubtedly controlled by the brain. 

 
The penis and clitoris are made from the same tissue, and erections are basically unisex 
 

The contradiction is that the brain inhibits erections but is also the source of our most sex-crazed thoughts. Even more confusing, the brain isn't even necessary for some erections. 

Those who suffer from paralysis can have erections even though nerve impulses can't travel between the brain and penis. So who or what is in control? The answer lies in a tiny, yet-to-be-named area in the spine just above the tailbone which has the ability to form erections all on its own. 

When you touch a hot stove, the nerve impulse travels from your hand to your spine and back before the signal ever reaches your brain. Something similar happens with excitatory impulses, creating what experts call a "spinal-mediated erection." Touching a paralyzed person's genitals can produce an erection, even if the recipient can't feel anything at all. 

So this kind of erection is a reflex, not a thought. But the interesting thing is that when the loop of nerves is intact, the penis, brain and spine are constantly talking to each other. 

"Although the penis does not 'think' for itself," Goldstein writes, "it keeps the brain and spinal cord well appraised of its feelings." 

Does this help men who find themselves thinking with the wrong head? Perhaps not, but most men already know the score. Whether they choose to listen to their better judgment, of course, is another matter. 

"Me and 'Mr. Happy' usually work well together until 'Mr. Alcohol' takes over," says Jeff McKinney, a public relations executive from New Jersey. "Then, neither of us are in control."