Penis

The penis (plural penises or penes) or phallus is an external male sexual organ. The penis is the male reproductive organ and for mammals additionally serves as the external male organ of urination. The human penis differs from those of some other mammals. It has no baculum, or erectile bone; instead it relies entirely on engorgement with blood to reach its erect state. It cannot be withdrawn into the groin, and is larger than average in proportion to body mass.

Structure

The human penis is made up of three columns of erectile tissue:

·        The two corpora cavernosa (singular: corpus cavernosum) and

·        One corpus spongiosum

The corpus spongiosum lies on the underside (known also as the ventral side) of the penis; the two corpora cavernosa lie next to each other on the upper side (dorsal side).

The end of the corpus spongiosum is enlarged and cone-shaped and forms the glans penis. The glans supports the foreskin or prepuce, a loose fold of skin that in adults can retract to expose the glans. The area on the underside of the penis, where the foreskin is attached, is called the frenum (or frenulum).

The urethra, which is the last part of the urinary tract, traverses the corpus spongiosum and its opening, known as the meatus, lies on the tip of the glans penis. It is both a passage for urine and for the ejaculation of semen. Sperm is produced in the testes and stored in the attached epididymis. During ejaculation, sperm are propelled up the vas deferens, two ducts that pass over and behind the bladder. Fluids are added by the seminal vesicles and the vas deferens turns into the ejaculatory ducts, which join the urethra inside the prostate gland. The prostate as well as the bulbourethral glands add further secretions, and the semen is expelled through the penis.

The raphe is the visible ridge between the lateral halves of the penis, found on the ventral or under side of the penis, running from the meatus (opening of the urethra) across the scrotum to the perineum (area between scrotum and anus).

Erection

Erection is the stiffening and rising of the penis which occurs in the sexually aroused male, though it can also happen in non-sexual situations. The primary physiological mechanism that brings about erection is the autonomic dilation of arteries supplying blood to the penis, which allows more blood to fill the three spongy erectile tissue chambers in the penis, causing it to lengthen and stiffen. The now engorged erectile tissue presses against and constricts the veins that carry blood away from the penis. More blood enters the penis than leaves until an equilibrium is reached (equal volume of blood flowing into the dilated arteries and out of the constricted veins). A constant erectile size is achieved at equilibrium.

Erection facilitates sexual intercourse though it is not essential for some other sexual activities. Although many erect penises point upwards (see illustration), it is common and normal for the erect penis to point nearly vertically upwards or nearly vertically downwards, depending on the tension of the suspensory ligament that holds it in position. Stiffness of erectile angle also varies.

Disorders affecting the penis:

Edema (swelling) of the foreskin can result from sexual activity, including masturbation.

Paraphimosis is an inability to move the foreskin forward over the glans. It can result from fluid trapped in a foreskin which is left retracted, perhaps following a medical procedure, or accumulation of fluid in the foreskin because of friction during vigorous sexual activity.

In Peyronie's disease, anomalous scar tissue grows in the soft tissue of the penis, causing curvature. Severe cases can benefit from surgical correction.

A thrombosis can occur during periods of frequent and prolonged sexual activity, especially fellatio. It is usually harmless and self-corrects within a few weeks.

Pudendal nerve entrapment is a condition characterized by pain on sitting and loss of penile (or clitoral) sensation and orgasm. Occasionally there is a total loss of sensation and orgasm. The pudendal nerve can be damaged by narrow hard cycle seats and accidents.

Penile fracture can occur if the erect penis is bent excessively. A pop or cracking sound and pain is normally associated with this event. Emergency medical assistance should be obtained. Prompt medical attention lowers likelihood of permanent penile curvature.

In diabetes, peripheral neuropathy can cause tingling in the penile skin and possibly reduced or completely absent sensation. The reduced sensations can lead to injuries for either partner and their absence can make it impossible to have sexual pleasure through stimulation of the penis. Since the problems are caused by permanent nerve damage, preventive treatment through good control of the diabetes is the primary treatment. Some limited recovery may be possible through improved diabetes control.

Erectile dysfunction or impotence is the inability to have and maintain an erection sufficiently firm for satisfactory sexual performance. Diabetes is a leading cause, as is normal aging. A variety of treatments exist, including drugs such as sildenafil citrate (marketed as Viagra®) which works by vasodilation.

Priapism is a painful and potentially harmful medical condition in which the erect penis does not return to its flaccid state. The causative mechanisms are poorly understood but involve complex neurological and vascular factors. Potential complications include ischaemia, thrombosis, and impotence. In serious cases the condition may result in gangrene, which may necessitate amputation.

Developmental disorders of the penis

Hypospadias is a developmental disorder where the meatus is positioned wrongly at birth. Hypospadias can also occur iatrogenically by the downward pressure of an indwelling urethral catheter.[2] It is usually corrected by surgery.

A micropenis is a very small penis caused by developmental or congenital problems.

Diphallia, or penile duplication (PD), is the condition of having two penises.

Penis replacement

The first successful penis allotransplant surgery was done on September 2005 in a military hospital in Guangzhou, China (Guangzhou Daily, source.) A man at 44 sustained an injury after an accident and his penis was severed. Urination became difficult as his urethra was partly blocked. A newly brain-dead man, at 23, was tracked down and his penis selected for the transplant. Despite atrophy of blood vessels and nerves, the arteries, veins, nerves and the corpora spongiosa were successfully matched. It remains to be observed if infection or rejection has occurred after the transplant and how much sexual function the man would regain.

Sperm & Seminal Fluid

A mature sperm, or spermatozoan, has 3 distinct parts: a head, a mid-piece, and a tail. The tail is made up of microtubules that form cilia and flagella, and the mid-piece contains energy-producing mitochondria. The head contains 23 chromosomes within a nucleus. The tip of the nucleus is covered by a cap called the acrosome, which is believed to contain enzymes needed to breach the egg for fertilization. A normal human male usually produces several hundred million sperm per day. Sperm are continually produced throughout a male's reproductive life, though production decreases with age.

During ejaculation, sperm leaves the penis in a fluid called seminal fluid. This fluid is produced by 3 types of glands, the seminal vesicles, the prostate gland, and Cowper's glands. Each component of a seminal fluid has a particular function. Sperm are more viable in a basic solution, so seminal fluid has a slightly basic pH. Seminal fluid also acts as an energy source for the sperm, and contains chemicals that cause the uterus to contract.

Ejaculation

As sexual stimulation becomes intense, sperm enter the urethra. Sperm and seminal fluid together are called semen. Once semen is in the urethra, rhythmical muscle contractions cause it to be expelled from the penis in spurts. During ejaculation, the bladder is closed off so that no urine enters the urethra. The contractions that expel semen are also a part of male orgasms. There may be more than 400 million sperm in 3.5 milliliters of semen expelled during ejaculation. However, fertilization may still occur even if the sperm count is lower.

Following ejaculation and/or loss of sexual arousal, the penis returns to its normal flaccid state. After ejaculation a male typically experiences a period of time, called the refractory period, during which stimulation doesn't result in an erection.

Chapter III - Male Reproduction