The Prostate Gland

The prostate, shown here in red, is a walnut-sized gland surrounding the urethra, located between the bladder and the pubic bone.

While scientists do not know all the prostate's functions, we do know that one of its main roles is to squeeze fluid into the urethra as sperm move through during sexual climax. This fluid, which helps make up semen, energizes the sperm and makes the vaginal canal less acidic.

Stimulation of the prostate also produces pleasureable sensations. (Can you say "understatement", kids? I thought you could..)

As you can see from the next two figures, depending on the angle of entry, the prostate can be either stimulated directly (figure one)

or stroked (figure two). I know that these diagrams make it look like the dick could either get all the way in or make contact with the prostate, but not both. Just remember that everything in there moves around and compresses. It's remarkable that there don't seem to be any good line diagrams of anal sex.

Figure three is my pathetic attempt to show some of that movement, in this case when you've got your legs in the air.

There's been a lot of questions about hygienic issues and anal sex. These diagrams show the rectum, which is where the guy's actually putting his dick. The rectum isn't actually used to store feces, unless you're really full. Feces is produced and stored in the lower colon (not shown here, but located above the rectum), though traces can be left in the rectum after taking a dump.

Here's a more complete diagram of the male reproductive system, showing all the important bits.

The Erection
LotsIof different things cause erections, from the sight of nice pair of tits (male or female, depending on preference) to a certain smell, but what's going on physiologically is pretty simple. Arousal causes blood to flow into the erectile tissue, and little muscles around the blood veins spasm and lock closed so the blood can't flow back out. There's even a specialized enzyme that tells those muscles to relax. Viagra works by blocking production of that enzyme, so once you get an erection you keep it much longer. Cock-rings work by clamping around the base of the cock and balls to prevent the blood from flowing back out.

The Orgasm
SpermIitself, the little wiggly bits that carry the genetic material, are produced in the testes. When orgasm approaches, they travel up through the vas deferens, passing through the seminal vesicle, where they pick up a bunch of fluids to make the trip easier, and the prostate gland, where more fluid is added, and then into the urethra, and finally out to splatter against the wall or all over your partner's face. All sorts of other hormonal and muscular activity is going on at the same time, but they're only the sideshow.

Anal Entry
ItIlooks like you couldn't fit a dick in there, but the human body is pretty flexible. Entry requires that the sphicter muscles around the anus relax. Insertion causes everything to move around, pressure is applied to the bladder and prostate. Pressure on the prostate also comes from outside, as the top presses against the perenium. So changing the angle of penetration causes differing amounts of stimulation to the prostate.

Some interesting facts about dicks
DicksIcome in all sizes and shapes. Some guys are "showers", that is the erectile tissue in their dicks doesn't expand very far, and what you see isn't much smaller than what you're going to get. Others are "growers", who may look like they don't have much, but when engorged become substantially larger. Then there's the phenomenon of "shrinkage". Low temperatures can cause the body to withdraw the blood that normally circulates through the cock, making it shrink significantly. Cold also causes the scrotum to contract, drawing the testes into the body to keep them from freezing off. Sperm is fairly delicate stuff, and can only survive within a narrow temperature window.