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Although the penis is a rather ordinary, everyday organ (at least for about 50% of the population), it is surprising how little some people know about this part of the body. In some animals, the penis is equipped with a small bone, not so in humans. In the adult male, the penis is about 2 to 3 inches long, although this can differ from day to day. When it is cold, the penis can withdraw into the fatty tissue overlying the pubic bone, which makes it seem shorter, especially in men with a lot of fat there.
There is generally little variation in the 'real' length of the penis between different men; in some races it's somewhat longer, in others a bit shorter. During erection, the penile length varies between 6 and 8 inches.
Three major structures can be found in the penis: the urethra and two long cylinders, the erectile bodies.
The urethra, with a thick, spongy cylinder around it, is located below. The urethra is designed to conduct urine to the outside world, but can only be considered a tube when it does; after the urine is expelled, the elastic fibers in its wall make sure it collapses. The tube ends at the glans of the penis. The skin on the glans is very sensitive and is normally protected by the foreskin, which acts as a cover. The foreskin should be able to slide back and forth over the glans easily, although this is not always the case yet in children. To ease the sliding, small glands produce a kind of grease. When this area of the body is not cleaned regularly, the grease may become noticeable as smegma. In the middle of the lower part of the foreskin a small skin-slip can be found (Latin: phrenulum), running from the foreskin towards the opening of the urethra; the function of this slip of foreskin is unknown, but sometimes it can be painful during erections when it is too short.
The erectile bodies, long cylinders, that are located next to each other at the 'back' of the penis. They contain blood. During erection, the amount of blood inside the cylinders increases, because the inflow increases and the outflow of blood decreases; the cylinders are filled to the limit with blood and will stand upright, causing erection of the penis; it works just like those cylindric balloons. To ensure a good blood supply, several blood vessels run towards the erectile bodies and a dense web of small nerve fibers ensure the opening and closing of the right vessels during erection. Since there are no stiff structures, like bones or something similar, in the penis to give it rigidity, the penis is connected to the pubic bone by strong connective tissue; because of this, the erect penis is still rather stabile and pointing slightly upwards.
The picture below seems to be the one featured on practically every penis enlargement website!
Click Here to view more Penis Diagrams
FUNCTION
As you probably already know, the penis has three functions:
Urination. This is a rather plain and simple function, where the penis serves as a extension of the urethra, thus enabling males to urinate easy anywhere they want without spilling any urine. In women this is more difficult and they will have to find a place to squat.
Sexuality. Simply put: an erect penis, i.e. a stiff one, can give pleasure. Having an erection is, however, a complicated procedure. In order to have an adequate erection three conditions must be fulfilled:
- a certain amount of lust is required (Latin: libido)
- a good inflow and outflow of blood is necessary
- the management, i.e. the nerve supply, must be functioning properly
Getting and maintaining an erection is an automated process. It is mainly run from the nerve cells in the spinal chord. Of course, the owner of the penis does have some say in it, but only up to a limit, for instance by thinking or not thinking about someone/something (porn movies). The effects will generally be limited to erection or no erection; it is pretty hard to want to have only a bit of an erection.
To show that the erection is governed by the spinal chord one can use a vibrator. The vibrations of the machine will excite the nerve endings in the glans of the penis and will elicit an erection via a reflex mechanism in the spinal chord (just like the knee-jerk reflex causes automatic contraction of the muscles in the leg). The 'erectile nerves' that run from the spinal chord to the penis will cause the inflow of blood to the erectile bodies to increase and the outflow to decrease, thus filling these bodies with blood and causing erection of the penis. If there is no sense of lust, the inflow of blood is too low or the nerves to the penis are damaged, then the erection will be limited or even non-existing.
Reproduction. In terms of evolution, erection has proven to be a good method to put male sperm into a womans womb. In ejaculation (coming) an amount of semen (prostatic fluid + sperm cells) is ejected into the urethra at the level of the prostate. The semen is propelled through the urethra by rapid muscular movements in its wall and ejected through the penis, on its way to the female egg-cell; to prevent the semen going the wrong way (to the bladder, which is closer by) the entry to the bladder is closed by a tiny sphincter at that very moment - that is the reason that ejaculation and urinating at the same time is impossible. Ejaculation is not called 'coming' for nothing: according to mother nature the goal is reached the very moment the semen came and erection is not needed any further.
SEE ALSO:
Prostate Gland
Scrotum
Sperm
Testicles
LINKS TO MALE ANATOMY SITES
Sexual Anatomy of Men
The Penis Page
Paul Russo's Anatomy Guide
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