The penis is an organ of the male reproductive system, serving to deposit sperm into the female body, enabling internal fertilization. Of course, this is essentially the definition of any intromittent organ, and it is not universally agreed which intromittent organs should be called penises – I tend to limit the term to the organs of amniotes, in which the development of organs capable of internal fertilization was an important step in vertebrate development, enabling reproduction outside of a water environment. Ultimately, this is a somewhat anthropocentric definition, encompassing those organs most similar in structure and origin to the human penis, but it will have to do for now. Even with this limited definition, there is significant variation in penis structure and appearance between species, from the familiar and relatively straightforward human penis to the corkscrew penis of ducks and pigs to the dual hemipene of reptiles and the similar structures of marsupials.

At its most basic, the penis is a tube of some length, made rigid by bone, blood, or some combination of the two, which places sperm inside the female, where it fertilizes eggs which are then laid or gestated inside the female. In most mammals, like humans, the penis places sperm in the vagina, which then migrates to the uterus to fertilize eggs. However, in some mammals, like horses, sperm is deposited directly into the uterus, which partially accounts for the reputed length of that species’ penises. Some species’ penises go further than this and perform additional functions to encourage reproduction. The penis and sexual practices of humans and other primates (ideally) stimulate the vagina and clitoris and induce orgasm in the female, which creates muscular contractions and repositions the cervix, both of which appear to make it more likely for sperm to enter the uterus, and from there, fertilize an egg. Looking at two other animals familiar to humans, the male cat has a barbed penis which rakes the vagina during withdrawal, a process that is believed to help incite ovulation, and a section of the canine penis, the bulbus glandis, near the base, swells while in the vagina and prevents full disengagement for up to an hour after ejaculation, ensuring completion of coitus, sealing the male dog’s semen inside the bitch, and preventing her from mating with competitors, at least for a while.

The Human Penis

The external appearance of the penis is that of a fleshy tube emerging from the crotch near the bottom of the pubis and forward of the testicles. When flaccid, the penis usually hangs downwards, although being bound in underwear or clothes may place it in another position. When erect, the penis grows larger in width and girth, becoming closer to horizontal to the body, though some penises may tend to lean or curve upwards or to one side.

The skin on the shaft of the penis is softer than surrounding skin and may be slightly different in coloration. As the skin on the shaft must accommodate the penis' full erect size, when flaccid or semiflaccid it may seem wrinkly or loose. The veins running under the surface of the shaft are sometimes visible and/or prominent, varying with the individual. The shaft contains a higher density of nerve endings than normal skin and is accordingly more sensitive, and stimulation of the shaft may cause erection, which will be discussed later. At the end of the penis is the glans, or "head", which is made of a different kind of tissue from the majority of the shaft, with an exterior that is differently textured and has an even higher nerve density and thus sensitivity. The glans is often said to look like a helmet, at its longest on the top of the penis and curving around on either side to its shortest on the underside. At the tip of the glans is the meatus, a small hole which is the external opening of the urethra; this is where urine and semen are expelled from the penis. When flaccid, the glans is covered to some degree by the foreskin, an extension of the skin that covers the shaft but, again, with higher nerve density, considered to be almost as sensitive or more so than the glans. The foreskin can be drawn back to expose the glans, and when erect, the glans emerges from the foreskin. It is a regular practice among some cultures to surgically remove the foreskin, often shortly after birth; this procedure is called circumcision.

In most embryos with XY chromosomes, the glans develops out of the genital tubercle and the shaft from the genital folds around the 8th week of gestation, in response to male hormones produced by the embryonic testes. In females, these structures would instead go on to become the clitoris and the labia minora, respectively. Average penis length at date of birth is around 1.15 to 1.75 inches, and genetically male or nonstandard babies with penises of less than .6 inches in length and .3 in width are often declared female on birth certificates. The penis grows larger with age, most dramatically during puberty, and by adulthood, though the results of individual reports vary, the average penis length appears to be about 6.1 inches when erect. Flaccid length shows more variation and little correspondence to erect length, but seems to average around 3.9 inches.

When you get down to it, there are more or less three things of note that the penis does - erection, ejaculation, and urination.

Erection can be brought on by a variety of causes, mental and physical, voluntary and involuntary, the most obvious of which is stimulation of the penis. The bulk of the shaft of the penis consists of the corpus cavernosa, two side-by-side cylinders of spongy tissue. When erection occurs, the blood vessels feeding the corpus cavernosa dilate, allowing more blood into the penis, flooding into the corpus cavernosa and filling the holes in the tissue, known as "sinuses". The veins which would drain blood from the penis simultaneously constrict and are pressed against by the swelling corpus cavernosa, decreasing the rate at which blood leaves and trapping it in the penis. As the penis fills with blood, it expands, first in length and then in girth, and the pressure of the blood within makes it stiff.

Ejaculation under normal conditions takes place as part of orgasm, which in turn is usually caused by stimulation of the penis, meaning that the penis is typically erect for ejaculation. Much of ejaculation takes place "upstream" of the penis, with sperm produced by the testes, seminal fluid from the seminal ducts and vesicles and prostatic fluid from the prostate mixed together and propelled onward by contractions of the genital tracts. The penis does its part, along with the perineum, by rhythmically contracting the smooth muscle along the penile urethra, which runs through the penis along the underside, projecting the semen out of the meatus. The initial few contractions occur at roughly .8 second intervals, but later contractions occur at longer intervals and with less force. Depending on the man, ejaculate may be initially be propelled with enough force to travel several feet, or may exit the penis with a relatively low velocity. After ejaculation the blood in the penis begins to drain and the penis shrinks, and the man enters the refractory period, in which erection is difficult and orgasm impossible.

As with all mammals, male humans urinate through the penis. The walls of the bladder contract and the sphincter surrounding the urethra relaxes, forcing urine through the urethra and out of the body through the meatus. Because of the positioning and anatomy of the penis, a male's urine stream while standing will project outwards from the body, and men may use their hands to aim their penises and direct the flow. During urination, sphincters inside the body contract to close off the connection between the reproductive tract and the urethra to prevent urine from traveling in the wrong direction or being mixed with ejaculatory fluids; likewise the connection to the bladder is sealed off during ejaculation.