Penis General Anatomy

This page includes penis general anatomy notions, either about external organs and internal ones.

The male genitals organs are divided into two main categories, external and internal.

External Genital Organs

The external genital organs are the penis and the scrotum. We will begin to discuss about the penis.

The Penis

The penis and all its external partsThe penis, even if it just appears as an inflatable balloon, is a complex organ. It's made up of tissues, muscles (unstriated), arteries, veins, corpus spongiosum, corpora cavernosa and nerves. It begins inside the body, near the prostate and it ends with the glans, a few inches outside the body. The penile sizes are considered to begin from the pubic bone to the tip of the glans.

The elements of the head of the penis: glans, prepuce, corona, etc.Externally, the penis is constituted of the following elements: base, trunk, corona and glans. The penis is represented by a cylindroid shape, the trunk or "the body" and by a conoid part, the glans. The main structure of both is vascular, that is, a network of vessels communicating between them and in which the blood volume and the covering sheath firmness (the tunica albuginea) are the fundamental conditions for erection.

The penis trunk is essentially constituted by three columns made up by erectile tissues:

  • The two corpora cavernosa
  • The corpus spongiosum

Internal view of the penisThe corpora cavernosa has an elastic fibrilar tissue which may be enlarged up to four times its basal size during an erection. The corpus spongiosum is found in the middle of the two corpora cavernosa and it surrounds the urethra. As the penis surface is approaching to the body, the two corpora cavernosa will be divided and fixed in the pubic bone through a hard membrane. The corpus spongiosum is located below the corpora cavernosa and surrounds the urethra. At the tip of the penis, the corpora cavernosa expands to form the head of the penis, or glans.

The corpora cavernosa is wrapped in a membrane that is called tunica albuginea. The glans is "the head" of the penis and it constitutes the tip. At the base, there is a margin called corona. In the lower part of the glans where the corona and the trunk join, there is a very sensitive part, called fraenum or frenulum. And, finally, the penis is wrapped by a layer of retractile skin which is normally called penis cutis but which, near to the glans, it's called prepuce. Some infants (and even some adult men) undergo circumcision, which is the operation consisting in removing some or all of the prepuce from the penis, through a medical instrument called thermocautery, letting the glans permanently uncovered. The prevalence of circumcision varies mostly with religious affiliation, and sometimes culture.

But there is another problem called phimosis, that is, when the prepuce is not enough elastic to discover the glans easily. There is also another kind of phimosis, called not closed. When we are unable to cover the glans easily, the condition is called paraphimosis. In any case, the patient, who is often a child, is submitted to a partial circumcision or to an surgery whose goal is to cut the fraenum, when this latter is responsible to the problem.

The Erection

When aroused, the tissues constituting the corpora cavernosa and the corpus spongiosum of the penis are irrigated by many blood vessels, swelling and hardening it. The muscles in the root of corpora cavernosa - called unstriated muscles, because they cannot be controlled voluntarily - will relax, inducing a forced blood afflux inside these corpus, causing the size changes and the hardening of the penis during an erection. When this occurs, the arterial blood will fill the cavities constituting the corpora cavernosa and the corpus spongiosum. At the same time, the exit of the blood is blocked by the contraction of microscopic vasal sphincters. This way, the arterial blood which has reached the penis is "trapped" inside the corpora cavernosa and the corpus spongiosum, determining the erection characteristic turgescence. It's the tunica albuginea that ensures this turgescence, similar to the surface of a ball which is kept hard when filled by air. At the end of sexual intercourse, the vasal sphincters will relax, allowing the exit of blood through the veins of the penis.

This transition from a flaccid penis to a hard one is called erection. The penis doesn't have any bones or cartilages and its only muscles are unstriated. After an ejaculation or a prolonged interruption of sexual stimulus, the penis will return to its flaccid state.

Finally, erection may suffer from troubles, either in terms of erectile dysfunction (impotence), and in terms of priapism, that is, an excessive erection which will not decrease and that is generally caused by hormonal disorders or the abuse of hormones and other substances intake.

The Scrotum

The scrotum is the skin tissue wrapping the testicles and taking their form. In other words it's the external and visible part of the testicles. However, in its physical conformation, the scrotum is much more complex than it appears; in reality, it's like a soft muscular pocket, with thin walls, it's constituted by two compartments where testicles are found. Under the scrotum tissue the cremasteric muscle is located; this muscle is responsible for the testicles raising and descending. Cold and other stimulus will cause the raising of the pocket, in order to keep the testicles in an ideal temperature state. Whereas the warm and the relaxation let the scrotum completely flaccid.

Internal Genital Organs

The internal genital organs are obviously more complex and numerous than external ones.

The Testicles

Internal view of the testiclesThe testicles, having an ovoid and irregular shape, have a vertical axis of about 3 centimeters and an average diameter of about 2cm. They are wrapped by the scrotum and they have the role to produce sperm and hormones. In the adult man, they have a weight of about 30 grams and they are slightly in disparity. The testicle which hangs lower than the other one is generally the left for reasons not clearly known. We think that this disparity is useful to prevent painful shocks when a man is walking. The testicles have two functions: sperm production and sexual male hormones production, called androgens (don't confuse with oestrogenics which are female hormones). The most important of these hormones is the testosterone. Hormone production by the testicles is evident since birth, but it increases enormously during puberty and it keeps at high levels during all the adult age, until a slight decrease in the old age. Sperm production will not manifest before puberty and it's clearly influenced by hormonal production.

Spermatozoons are produced in the testicles, inside special little ducts, called sperm storage tubules, located in the center of each testicle; they are connected to many other ducts which move sperm to other important organs and, finally, outside the penis if necessary. In each testicle, near the sperm storage tubules, there are numerous cavities called interstitials or Leydig's cells. They are responsible of testosterone production, which is directly secreted in the surrounding blood vessels. Most of the changes occurring during puberty are the result of major testosterone quantity in the body.

During sexual arousal, the testicles expand themselves because the blood fills the vessels inside them. Upon the ejaculation, the testicles return to their normal dimensions. When a man is about to ejaculate, the testicles move closer to the body. Then they return back to their usual position in the scrotum. As we have already seen, the testicles also come near the body in conditions of intense fear, anger or cold. This run-in and run-out is a fertility protection system. The testicles need to keep a temperature of one or two degrees lower than the body one, because at the bodily temperature, the testicles will not be able to produce spermatozoons. Under various conditions - cold, in particular - the scrotum wrinkles up, drawing the testicles close up against the body. Under other conditions - like relaxation or warmth - the scrotum is very loose and soft, and the testicles then hang farther from the body.

The Epididymis

The epididymis: shape and structureThe epididymis are the small reliefs that may be appreciated in the upper part of each testicle when consulting any testicles anatomic image. Internally, the epididymis have the form of tubes firmly enlaced, which adhere to the surface of each testicle. The epididymis have the role to store spermatozoons and make them mature. After they are produced, they are stored in the epididymis until they are ejaculated, destroyed or absorbed by the surrounding tissues.

The Deferent Ducts

A global view of the male genital apparatus and all its elementsThe deferent ducts have the role to move spermatozoons from the testicles (where they are produced) to the other male genital organs. Directly connected to the epididymis, they pass through the scrotum, where they conduct spermatozoons to seminal vesicle and prostatic gland, where they are mixed with others fluids produced by these organs, forming the sperm (or the semen). The length of each deferent duct is approximately 40-45 centimeters.

The Vasectomy

The surgical intervention of vasectomy which is a surgical procedure for male sterilization and/or permanent birth control is precisely performed on the deferent ducts. These are severed and joined inside the scrotum; the effect could also be produced by eradicating a part of them. In this way, we can prevent spermatozoons to reach other organs, like seminal vesicles and prostatic gland. The sterilized man is still able to ejaculate but his fluid doesn't contain spermatozoons. Once these cannot exit the testicles, they are destroyed and absorbed as all other spermatozoons that are not ejaculated. The semen of a sterilized man has the same look as the one of a fertile man. The difference can only be noticed microscopically. If the sterilized man regrets the vasectomy and he wants to have children again, he can undergo another operation, where the deferent ducts are restored once more to their original state.

The vasectomy has no effects on the libido or potency in general. But still it is a surgical operation with all its involved risks. As the procedure is considered a permanent method of birth control (not easily reversed), men are usually counseled/advised to consider how the long-term outcome of a vasectomy might affect them both emotionally and physically.

The Seminal Vesicles

They have a size of about 8 centimeters and they are folded in themselves. They are located above the prostate and respectively at the sides. The seminal vesicles secrete a seminal fluid similar to glucose (the fructose) which will mix with spermatozoons and the prostatic fluid in the ejaculatory ducts, after these liquids have passed through the deferent ducts. This seminal fluid has the function of nutrient energy for the spermatozoons making them flow quickly. It seems that the fructose concentration in a normal ejaculation is so high that it reach the value of six calories, approximately.

The Cowper's Glands

The Cowper's glands are small glands located beneath the prostate gland and at each side of the urethra. These glands are responsible of the secretion of a seminal fluid which is normally produced during a sexual arousal and which is discharged from the urethra in small quantities. It's the typical fluid discharged from the penis when a man is sexually aroused. This small quantity of fluid is directly secreted on the urethra and it may contain a little amount of spermatozoons and other seminal fluids. This means that the fluid secreted from Cowper's glans may potentially fecundate a woman, even if the man ejaculates outside the vagina. The risk is relatively small, but real. So, the fact of "ejaculating outside" is not a perfect contraceptive method, furthermore it's not recommended neither from a psychological point of view (it's not good for sexual experience), nor from the physical one, since this practice may inflame the prostate.

The fluid produced by Cowper's glands is alcaline. It's function is help to neutralize the urethra acid environment and to delay spermatozoons life after ejaculation.

The Prostate

The prostate is located immediately under the bladder. It is an anatomical formation of the urogenital apparatus, commonly described in terms of a flattened inverted cone. It has four walls which continue one above the other with rounded corners. The prostate average size is about 40 millimeters in length, 30 millimeters diagonally and 25 millimeters anteroposteriorly. It has a weight varying between 15 and 20 grams.

The prostate stands at the base of the bladder, to which it adheres with very vigorous support structures; furthermore it's ensured in its position thanks to multiple ligaments which fasten it either to pelvic bones (previous to the pubis), and to other anatomical support structures (lower urogenital diaphragm).

The prostate is composed by some number of sections called lobes or acinus. Its main function is to produce and store inside its acinus the prostatic fluid which joins the fluid secreted by seminal vesicles. This liquid also serves to feed spermatozoons found in the sperm, when it's stored in the deferential ampullas. Even if it's commonly said that it's the prostate which ejects sperm during ejaculation, in reality this is done by the periprostatic muscles in combination with the pubococcygeus muscle. The prostate is only an intermediary vehicle, like a sling pocket. The surrounding muscles are the real responsible of the ejaculation process and they stimulate the semen.

More than 95% of seminal fluid is composed of: the prostatic liquid (more than 35%) and the liquid produced by seminal glands (about 60%). Only less than 5% is actually composed by spermatozoons.

The prostate is an organ which easily tends to become inflamed, either spontanously or caused by bacterias; some men suffer from chronic prostatitis, that is, a prostate inflammation which is difficult to cure and which may last very long time or is recurrent. If the inflammation is too severe, the prostate may swell causing serious problems to the whole urogenital apparatus. Prostatitis may be one of the causes of impotence.

The Ejaculatory Ducts

The ejaculatory ducts are the connection canals between the deferent ducts and the seminal vesicles. These are located inside the prostate, they are about two and a half centimeters long and conduct directly to the urethra. It's there that the semen is collected during sexual intercourse. When a man is about to have an orgasm, he goes through the famous "point of no return", the stage where the orgasm is unavoidable and it's usually accompanied by an ejaculation. Even if orgasm and ejaculation seem to be the same thing, they actually are two different processes which occur simultaneously. It's by a spinal reflex that the semen, accumulated in the ejaculatory ducts, is expelled through the urethra, by means of squirts that can be between three and eight in a few seconds. Thus, ejaculation is the ejecting of the semen usually carrying whereas the orgasm simply corresponds to neuromuscular tensions releases, accumulated during sexual intercourse.

The Urethra

The male urethra is the tube which begins in the bladder, crosses the prostate and the whole penis length up to the glans tip. It's about 20 centimeters long and it has mainly two functions: eliminating the urine and expelling the semen during ejaculation. Thus we can say that it connects the urinary bladder to the genitals for the removal of fluids out of the body.

A. Langueduc



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