Penis Anatomy & Physiology
ANATOMY OF THE PENIS

Penis anatomy is remarkably complex: Following is the definitions for the most important parts.

The Corpora Cavernosa 

The corpora cavernosa are two chambers that fill most of the penis. The chambers are filled with a spongy tissue that includes muscles, open spaces, veins and arteries. An erection occurs when the corpora cavernosa become engorged with blood and expand.

The Tunica Albuginea
 A membrane called the tunica albuginea surrounds the corpora cavenosa. This membrane helps keep blood  in the penis during an erection.

Urethra
The urethra is the tube through which urine travels. Ejaculate also travels through the urethra. It runs down the underside of the penis, beneath the corpora cavernosa and widens at its opening, called the meatus. The meatus is located at the glans (the head of the penis).

Corpus Spongiosum
The corpus spongiosum is a chamber that surrounds the urethra. It becomes engorged with blood during an erection.

The Prostate
The prostate is a small gland located in the pelvis. It surrounds the urethra and plays an important role in ejaculation. Sperm, which is produced in the testicles and stored in the seminal vesicles, is mixed with prostatic fluid and secretions from the bulbourethral gland to form semen. During ejaculation, semen is expelled through the urethra from the ejaculatory ducts.

STRUCTURE OF THE PENIS

This illustration represents an average normal adult human penis. The head of the penis (glans) has a covering, called the foreskin (prepuce). This covering folds in on itself, forming a double layer. The foreskin is not a `flap' of skin on the end of the penis, and it is not `useless' or `redundant' skin.

There is some natural variation in the length of the foreskin, which often covers a bit more or less of the glans than illustrated.

Structure

• The outer foreskin layer is a continuation of the skin of the shaft of the penis.
• The inner foreskin layer is not properly `skin', but mucocutaneous tissue of a unique type found nowhere else on the body.
• The frenar band is the interface (join) between the outer and inner foreskin layers. When the penis is not erect, it tightens to narrow the foreskin opening. During erection, the frenar band forms a ridge that goes all the way around, about halfway down the shaft.
• The reddish or purplish glans or glans penis (head of the penis) is smooth, shiny, moist and extremely sensitive.
• The frenulum, or frenum, is a connecting membrane on the underside of the penis, similar to that beneath the tongue.

The foreskin has twelve known functions.
They are:

1. To cover and bond with the synechia so as to permit the development of the mucosal surface of the glans and inner foreskin.          
2. To protect the infant's glans from feces and ammonia in diapers.
3. To protect the glans penis from friction and abrasion thoughout life.
4. To keep the glans moisturized and soft with emollient oils.
5. To lubricate the glans.
6. To coat the glans with a waxy protective substance.
7. To provide sufficient skin to cover an erection by unfolding.
8. To provide an aid to masturbation and foreplay.
9. to serve as an aid to penetration.
10. To reduce friction and chafing during intercourse.
11. To serve as erogenous tissue because of its rich supply of erogenous receptors.                          
12. to contact and stimulate the G-spot of the female partner

PHYSIOLOGY OF ERECTION

The physiological process of erection begins in the brain and involves the nervous and vascular systems. Neurotransmitters in the brain (e.g., epinephrine, acetylcholine, nitric oxide) are some of the chemicals that initiate it. Physical or psychological stimulation (arousal) causes nerves to send messages to the vascular system, which results in significant blood flow to the penis. Two arteries in the penis supply blood to erectile tissue and the corpora cavernosa, which become engorged and expand as a result of increased blood flow and pressure.

Because blood must stay in the penis to maintain rigidity, erectile tissue is enclosed by fibrous elastic sheathes (tunicae) that cinch to prevent blood from leaving the penis during erection. When stimulation ends, or following ejaculation, pressure in the penis decreases, blood is released, and the penis resumes its normal shape.

HOW YOU GET AN ERECTION

An erection starts when you become sexually stimulated.

Your nervous system sends chemical messages to your pelvic area. The smooth muscles inside your penis relax. The arteries widen. The corpora cavernosa, acting like a sponge, fill with blood. In fact, the corpora absorb up to eight times more blood than when your penis is flaccid.

As your penis swells and lengthens, the filled corpora cavernosa press against the veins. The pressure greatly reduces the blood flow from your penis. This keeps your erection firm enough for intercourse.  When everything works the way it should, you will keep your erection for as long as you are sexually aroused.

ERECTION PROCESS

PENILE STIMULATION

The most important components of the physical erogenous stimulation of the penis during foreplay and intercourse are the sensations from the foreskin, frenulum/frenar band, and glans. These structures each have their own feeling, and each contributes in its own way to the man's total experience of lovemaking. It must be emphasized that emotional excitement is an extremely important component of sexual enjoyment, and intensifies the man's perception of any physical sensations from his penis.

The foreskin has an inner and outer layer. The outer foreskin layer contains nerve endings which respond to gentle touching during the early stages of sexual arousal. This helps to trigger an erection. The nerves of the inner and outer foreskin contribute to the experience of penile stimulation, up to and including orgasm. These receptors are stimulated by stretching, or when the foreskin rolls over the surface of the glans during intercourse or masturbation.

The foreskin contains sensory receptors called Meissner corpuscles. We believe that these nerves, similar to nerve endings in the fingertips, are there to provide pleasure, as well as fine sensory perception. This seems to help a man to enjoy sex longer without ejaculating prematurely, because he can more easily tell when he is approaching the threshold of orgasm.

Stimulation of the frenulum and frenar band results in intense pleasurable feelings during arousal. The frenar band consists of a number of "ridges". Sensations from these structures during intercourse or masturbation are thought to be the primary trigger of orgasm in the intact male.

THE ANATOMY OF MALE AROUSAL

Our bodies go through a number of changes during sex. Understanding these changes helps us understand both our sexuality and that of our spouse. Most scientific folk divide these sexual responses up into four phases - arousal, plateau, orgasm, and resolution.

AROUSAL -  This phase starts with some kind of sexual stimulation - a touch, a sight, or a thought. Blood begins to move to various parts of the body.  The lips, nipples (sometimes nipples become erect), earlobes and genitals become fuller and more sensitive.  The penis begins to fill with blood and become erect, and the scrotum starts to thicken, pulling the testicles closer to the man's body.

PLATEAU - With continued stimulation, the penis becomes slightly firmer, may darken in color, and fluid containing sperm begin to seep out.  Over long periods of foreplay, the penis may become flaccid and then harden again.  This is perfectly normal.  Even if the penis stays erect, firmness will wax and wane to some degree.  The scrotum continues to thicken and the testes start to enlarge.  The right testicle will rise and slightly rotate first, then the left will do the same.  Heart rate and blood pressure will increase, and a skin flush may appear on the chest, neck or face.  Breathing and heart rate increase, and moans or other sounds may be made.  As climax approaches the skin of the glans darkens and becomes taught, and the urethral opening may slightly open.  Immediately before ejaculation, at the "point of no return", the sphincter from the bladder closes so that semen can't move into the bladder, and the prostate and seminal vesicles contract to move seminal fluid to the urethra bulb near the base of the penis.

ORGASM - The PC muscles contract to expel the seminal fluid.  Much of the seminal duct system contracts, as does the rectal sphincter.  There may be foot spasms or contracting facial muscles. Muscles tense, and breathing, heart rate and blood pressure rise further.  Because the penis becomes hypersensitive at or immediately after orgasm, a reduction of stimulation during orgasm is desired by most men.  This varies from the man who wants all stimulation to end as soon as orgasm starts, to men who want heavy stimulation through orgasm.  Men's orgasms are much shorter than women's.  

RESOLUTION - This is when the body returns to normal after orgasm.  A fine perspiration may cover the body.  The penis becomes flaccid in two stages: partial erection is lost immediately, the remainder is lost gradually, possibly over several minutes.  Hypersensitivity of the penis decreases gradually, though some men may find any touch or ongoing thrusting very uncomfortable for some time.  After a strong orgasm the penis may spontaneously jerk for a minute or two.  The scrotum thins and the testes descend and return to normal size.  Breathing, heart rate and blood pressure return to normal, and muscles relax. Continued stimulation may prevent full loss of erection, or resumed stimulation may result in a new erection, but there is period of time known as the refractory period during which it is impossible to have another ejaculation, even if another erection is achieved.  The refractory period ranges from minutes to hours or even a day or more.  As a man ages his refractory period increases, but two men the same age can have vastly different refractory periods.

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