Excerpt: 'You: Being Beautiful' on the Biology of Sex

The region is often not that sensitive either, but you never know until you try. The fact that women can be stimulated to orgasm through not only the genitals but also the mouth, nipples, and other parts of the body points to the complexity of the system—and reinforces the fact that the true biology of sex really evolves within the brain. (One theory is that sexual stimuli are carried from the cervix and uterus to the brain through the vagus nerve—one of the nerves stimulated during deep breathing and meditation.)

MEN: If you allow us a few moments to talk about the male anatomy, we think you'll be pretty amazed.

Biologically, men's sexual organs are much different than those of other species. For one, a man's penis doesn't have a bone, unlike those of other species. Why? The bone makes for easy and fast access for males in the animal kingdom (to inseminate their partners quickly); men give up the bone but gain a disproportionately large penis for their body size in return.

The evolutionary implications: One, men use the penis as a tool of attraction, implying that women do place some value in using it as a diagnostic for evaluating potential mates (not so overtly these days). And two, the lack of bone implies that men do equate emotions with sex, since they must be aroused for an erection; instead of easy and fast access, which can be painful to the females, it takes more care to have a sexual relationship between two people.

Another interesting observation: Humans have proportionally smaller testicles than males in other species; that's because other species need to ejaculate more semen to fertilize partners who are in heat prior to other males of the tribe and ensure propagation of their genes.

Human males don't need the size because of the biological drive to be monogamous (at least serially).

Now, it doesn't take a sexologist to know the purpose of the male orgasm: Find the egg, fertilize the egg, begin shopping for Barbies. But what's interesting is that this mad dash to the egg isn't some New York City marathon where all the starter sperm strap on their Nikes with the goal of making it to the finish line. Some of the sperm do that, but others are more like defensive linemen. Their job: to stop other men's sperm from scoring.

Some sperm even have a dual role—blocking other sperm but allowing their own sperm with their genes to penetrate more effectively.

Now, during a man's orgasm, the brain is firing like a lit-up pinball machine, causing contractions in most muscles of the body. The purpose: Like a woman's, these contractions help increase the chance of pregnancy by enabling the penis to penetrate as deeply as possible. The glands that make semen, mostly the prostate, squeeze repeatedly, propelling sperm as deep and as far as possible. The prostate, by the way, is often referred to as the male G-spot, because it's made up of some of the same types of tissues as some of the spots identified around the nerve plexus that is the G-spot in women and can be stimulated in a similar fashion by some adventuresome couples.

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