EXTERNAL PENIS STRUCTURES
The exterior features of the penis can be divided into two main parts – the glans (tip) and the shaft. The tip and shaft come together at the “neck” of the penis. A third portion of the penis begins deep inside the male body and is called the root. The interior of the penis is made up of three columns of erectile tissue, blood vessels, nerves, and connective tissues know as fascia.
The penis serves three main functions. First, it provides a route to eliminate urine from the bladder through the urethra. Second, it functions as a sexual organ by becoming rigid and erect in order to allow sexual intercourse. Finally, it serves as not only the pathway for ejaculating semen, but its muscular contractions propel semen out of the urethra.
The bell-shaped tip or head of the penis is called the glans. The rounded base of the glans where it flares out is called the corona. The glans is hairless and very sensitive to touch and stimulation. This sensitivity is due to a network of nerves that surface in the glans, especially on the back side and the corona. Unlike the rest of the penis, the skin here is not loose but firmly attached to the tissue below. In uncircumcised men the glans can often appear bright pink and be markedly different in color than the rest of the penis. In circumcised men the glans is typically a slightly different color than the shaft of the penis. The glans is actually an extension of one of the columns of erectile tissue – the corpus spongiosum, which is why it swells during an erection (and is often the last part of the penis to stop swelling when an erection goes away). The urethra exits the penis at the tip of the glans as a small slit called the external urethral meatus – sometimes simply called the meatus. Urine and sexual fluids (pre-ejaculate and semen) are released from the penis through the urethral meatus.
Just below the corona of the glans is a smooth indentation called the coronal sulcus or neck of the glans. On the underside of the penis the neck curves upwards into a small triangular indentation called the median sulcus of the glans. A piece of tissue called the frenulum also appears at the neck of the penis and connects the median sulcus of the glans to the foreskin on the shaft. Some men who have been circumcised will still have this small band of frenulum tissue.
The neck of the penis appears differently in circumcised versus uncircumcised men. In uncircumcised men, the foreskin or prepuce grows from the neck of the penis and drapes over part or all of the glans. More about the foreskin can be found by clicking HERE. In men who have been circumcised, the neck of the penis will have a band of darker skin at the neck.
The largest portion of the penis is called the shaft. It extends from the base of the penis as it exits the body up to the glans. A ring of a darker more textured skin appears on the upper portion of the shaft – this is often considered part of the neck of the shaft of the penis (which sits directly below the neck of the glans penis). The remainder of the shaft is skin colored and can grow some sparse pubic hair.
As stated earlier, the lower portion of the shaft makes up the majority of the penis and is covered in skin that is similar in color to other skin found on the body. This skin is not firmly attached to the three columns of erectile tissue beneath it. This allows the penis to go between flaccid (or limp) and erect. A raised ridge that looks like a scar, called the raphe, is visible on the underside of the penis and extends from the scrotum, up the underside of the shaft of the penis, through the frenulum, and ends right at the glans. The raphe extends onto the scrotum and divides the left and right sacs of the scrotum before continuing behind the scrotum until it reaches the anus.
The lower shaft may have varying amounts of sparse pubic hair which increases near the base of the penis and may run up the underside of the penis near the raphe. In some men, pubic hairmay even grow onto the sides and upper portions of the penis near the neck. For more information on penile hair, click HERE.
Penis Size & Appearance
There are actually only small variations in the size of the human penis from one man to the next. In fact, it is rare for a man to have a penis size that is outside of the “normal” range. Nearly 90% of men are within ½ inch (1cm) of “average.” The shape of the penis when erect should be relatively straight and point upright and forward. However, many men may notice the penis twists, curves, or may not stand completely upright when erect. Most often these are slight; however, in some cases these can be significant. Click to learn more about average erection size, small erection size, large erection size, erection curvatures, and erections that are not upright/centered.
Disorders of the penis
There are different types of disorders that can affect the penis. Many of these occur in the foreskin area and only affect uncircumcised men (click HERE), while some affect skin surfaces such as the shaft and glans (click HERE). Causes of pain in the penis will also be discussed (click HERE). Disorders that involve erections such as difficulty attaining or maintaining an erection, inability to stop an unwanted erection, or erection pain are discussed elsewhere on this site.
INTERNAL PENIS STRUCTURES
The tip (glans) and interior of the penis are composed of erectile tissue. Two columns-- called the corpora cavernosa-- run along the back side of the penis from the root through the shaft. The third column runs along the underside of the penis and is called the corpus spongiosum. The end of the corpus spongiosum swells as it exits the shaft, forming the glans.
Two of the three erectile columns run along the back side of the penis – one on the left and one on the right side. These two columns are known as the corpora cavernosa and comprise the vast majority of the erectile tissue in the shaft of the penis that becomes rigid when erect.
The third column of erectile tissue, known as the corpus spongiosum, runs the full length of the underside of the penis as a smaller column and expands at the tip to form the glans. The spongiosum swells in size during an erection, but does not become as rigid or hard as the cavernosa. The urethra is a tube that extends from the bladder through the middle of the corpus spongiosum penis. Because the urethra is surrounded by swollen rather than erect tissues, semen can more easily be carried from the reproductive glands through the erect penis during ejaculation.
Each cavernosa is surrounded by fascia (membrane) called the tunica vaginalis. The spongiosum is surrounded by a membrane called the trabeculae of the spongiosum. All three erectile columns are surrounded by two more sheets of fascia. The Buck’s fascia (or deep fascia) surrounds the erectile columns. The dartos fascia (or superficial fascia) is an extension of the dartos facia in the scrotum and surrounds the Buck’s fascia. The dartos fascia lies just beneath the skin of the penis. These fascia are necessary in allowing the penis to become erect and rigid.
Root of the Penis
The root of the penis is found inside the body. The three columns of erectile tissues attach to muscles (IC and BS muscles), facia, and ligaments in a pouch-like area on the pelvis. The corpora cavernosum originate in the root of the penis as two tapered ends that flare outward. These flared portions are called the crura or crus – a left crura and a right crura. The corpus spongiosum originates in the root of the penis and has a bulb-like enlarged end that is called the “bulb of the penis.”
The crura attach to the pelvic bone and the two ischiocavernosal (IC) muscles. The bulb of the penis is surrounded by the bulbospongiousus (BS) muscle.
The ischiocavernosal (IC) muscles function in part to stabilize erections (preventing the penis from falling left or right). The IC muscles also allow a man to “contract” or pull an erection into a more upright position. However, it is difficult for these muscles to stay contracted for more than a few seconds at a time.
Bulbospongiousus (BS) muscles assist with erections by compressing the erectile tissue in the bulb, as well as compressing the deep dorsal vein of the penis which traps blood in the erectile tissues assisting with the rigidity of an erection. The BS muscles also contract repeatedly during ejaculation and orgasm to force semen out of the urethra in waves or spurts. The contractions of the BS muscles account for the pulsing motion of the penis during ejaculation. Image below copyrighted by Veisland Medical Images and used by permission.
The root of the penis is also supported by two ligaments – the fundiform ligament and suspensory ligament.
The fundiform ligament (shown in dark blue) is actually a sling. It connects to the linea alba (the middle portion of the "six-pack" in the abs). It attaches to the linea alba just below the belly button then runs down the lower abs. As it reaches the penis, it splits apart forming a sling that wraps around the base of the penis. This sling pulls the erect penis toward the body, thus helping an erection point in an upward direction.
The suspensory ligaments (shown in light blue) attach to the pubic bone, as well as the abdomen. It attaches to the upper side of the base and root of the penis to prevent an erection from moving too far downward. This ligament can sometimes be felt (especially during an erection) at the very base of the penis in the pubic hair area.
Combined, these ligaments support the penis when erect by preventing the base of the penis from pulling away from the body, thus helping the penis remain more upright and stable during an erection. With age and after repeated erections over the years, these ligaments can become more stretched which is why men notice their erections becoming less upright as they become older.
Penile Blood Vessels
Blood enters and exits the penis through different sets of arteries and veins.
The first group of vessels run along the dorsal, or back side, of the penis, as shown below. They supply the skin and glans of the penis with blood. There are two dorsal veins. The first is the superficial dorsal vein and it is just under the surface of the skin (see in blue below).
A second vein lies deeper beneath the Buck’s facia (seen in image below). When an erection occurs, this deep dorsal vein gets pinched against the Buck’s facia which decreases the amount of blood leaving the penis, thus creating a rigid erection.
The second group of vessels supply and drain the two corpus cavernosa. The cavernosal arteries (shown as black dots in the image at the left) provide blood flow to the two cavernosum. The cavernosa are then drained by the cavernosal veins. Other vessels inside the penis supply and drain the corpus spongiosum.
There are two main nerve tracts in the penis – the cavernosus nerves and the dorsal penile nerves. The cavernosus nerves run deep within the penis to the corporal cavernosa and are responsible for sending the nerve signals that cause an erection. The dorsal penile nerves branch off the pudendal nerve near the prostate. The posterior scrotal nerve and dorsal penile nerves are the two main branches of the pudendal nerve and carry sexual sensations. The dorsal penile nerves run along the back of the penis then branch near the neck of the penis. Multiple nerve endings surface at the glans and concentrate on the corona of the glans. The glans, especially the corona and upper/back surface of the glans, is an area where sexual sensitivity and stimulation is concentrated.
A branch of the dorsal nerve called the frenular nerve can wrap around the penis and surface near the frenulum. This is most easily illustrated from a profile (side) position. Nerve endings from the frenular nerve can then fan creating another area of possible sexual sensitivity in many men.
Not all men’s penises have nerve endings in the same place and in the same concentrations; however, the back side of the glans and the frenulum are typically two of the most common areas for sexual stimulation. For some men, the back of the glans is the most sexually sensitive area. In others, the frenulum is the most sexually sensitive area of the penis. It is estimated that one in three men who are circumcised may have had the frenulum nerve removed during circumcision. However, if the frenulum nerve is removed during circumcision (especially circumcision in infancy/childhood), the other nerves on the dorsal side of the penis often become more sensitive to sexual stimulation. Even in adult men who undergo circumcision, sexual stimulation often increases in other areas over time. So, it’s thought that most men have similar levels of sexual sensitivity in the penis, whether or not they have an intact frenulum. In addition, nerve endings that branch off the dorsal nerve along the shaft of the penis can also transmit sexual sensations. So, sexual stimulation occurs along the length of the penis during intercourse. However, not all nerve endings transmit sexual pleasure – there are also nerves that conduct messages such as pain, temperature, pressure, moisture, and friction.
Images on this page from top to bottom include:
Sergeant Illustration, Owned
Sergeant Illustration, Owned
Veisland Medical Images and used by permission.
Elen Bushe/Shutterstock.com (modified)
Elen Bushe/Shutterstock.com (modified)