A report on Penis and Erection

Mallard pseudo-penis
Three columns of erectile tissue make up most of the volume of the penis.
Females have corkscrew vaginas with many blind pockets designed for difficult penetration and to prevent becoming pregnant. This reduced the likelihood of fertilization by unwanted aggressors in favor of fitter mates.
8 stages of erection
External male genitalia of a Labrador Retriever
Various erection angle and shape of penises.
Penises of minke whales on display at the Icelandic Phallological Museum
An erect horse penis
Genitorinary system of a raccoon (Procyon lotor)
Penis of a human, with pubic hair removed to show anatomical detail
The spine-covered penis of Callosobruchus analis, a bean weevil

An erection (clinically: penile erection or penile tumescence) is a physiological phenomenon in which the penis becomes firm, engorged, and enlarged.

- Erection

In many mammals, the size of a flaccid penis is smaller than its erect size.

- Penis
Mallard pseudo-penis

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Overall

Human glans penis (dorsal view)

Glans penis

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Bulbous structure at the distal end of the human penis that is the most sensitive erogenous zone and primary anatomical source of male sexual pleasure.

Bulbous structure at the distal end of the human penis that is the most sensitive erogenous zone and primary anatomical source of male sexual pleasure.

Human glans penis (dorsal view)
The male anatomy showing the location of the glans penis
Internal anatomy of human glans penis: 1. Fascia penis 2. Corpus cavernosum 3. Coronal sulcus 4. Corona of glans 5. Foreskin 6. Glans penis 7. Meatus of the urethra 8. Navicular fossa of male urethra 9. Tunica albuginea of penis 10. Corpus spongiosum 11. Urethra

The foreskin can generally be retracted over and past the glans, and may automatically retract during an erection.

In comparison, the glans of felids is short and spiny, while that of viverrids is smooth and long.

Ejaculation example

Ejaculation

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Discharge of semen from the male reproductory tract as a result of an orgasm.

Discharge of semen from the male reproductory tract as a result of an orgasm.

Ejaculation example
Diagram of the male pelvic and reproductive organs

A usual precursor to ejaculation is the sexual arousal of the male, leading to the erection of the penis, though not every arousal nor erection leads to ejaculation.

A flaccid penis, with surrounding pubic hair removed to show anatomical detail

Human penis

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External male intromittent organ that additionally serves as the urinal duct.

External male intromittent organ that additionally serves as the urinal duct.

A flaccid penis, with surrounding pubic hair removed to show anatomical detail
Lateral cross section of the penis
Anatomical diagram of a human penis
Various sized penises
Stages in the development of the male external genitalia
A urinating puer mingens by Annibale Carracci, 1600, Palazzo Farnese
The development of a penile erection, also showing the foreskin gradually retracting over the glans. 
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See also: Commons image gallery''
A ventral view of a penis flaccid (left) and erect (middle); a dorsal view of a penis erect (right).
Hypospadias
Kanamara Matsuri festival in Japan
Papuan man wearing traditional penis sheath
A labelled dorsal view of a circumcised penis: (1) Shaft, (2) Circumcision scar, (3) Corona, (4) Glans, (5) Meatus.
Dissection showing the fascia of the penis as well as several surrounding structures
Image showing innervation and blood-supply of the human male external genitalia

The main parts are the root (radix); the body (corpus); and the epithelium of the penis including the shaft skin and the foreskin (prepuce) covering the glans penis.

An erection is the stiffening expansion and orthogonal reorientation of the penis, which occurs during sexual arousal.

A stallion

Stallion

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Male horse that has not been gelded .

Male horse that has not been gelded .

A stallion
Mustang stallion (right) with part of his band of mares and foals
Stallion exhibiting the flehmen response
Genitourinary system of a stallion
A stallion's secondary characteristics include heavier muscling for a given breed than is seen in mares or geldings, often with considerable development along the crest of the neck, as shown in this image.
Even well-trained stallions require firm and consistent handling by experienced individuals.
Aggressive and even violent behavior between stallions not habitually living together or in the presence of mares adds to the challenges in stallion management.
Provided with sufficient space and food with no distractions from mares in estrus, even stallions previously used for breeding may coexist peacefully. Not all individuals are suited for this kind of arrangement, however.
Stallions are capable of achieving a high level of discipline and training.

the penis, within the "penile sheath". Stallions have a vascular penis. When non-erect, it is quite flaccid and contained within the sheath. The retractor penis muscle is relatively underdeveloped. Erection and protrusion take place gradually, by the increasing tumescence of the erectile vascular tissue in the corpus cavernosum penis. When not erect, the penis is housed within the prepuce, 50 cm long and 2.5 to 6 cm in diameter with the distal end 15 to 20 cm. The retractor muscle contracts to retract the penis into the sheath and relaxes to allow the penis to extend from the sheath. When erect, the penis doubles in length and thickness and the glans increases by 3 to 4 times. The urethra opens within the urethral fossa, a small pouch at the distal end of the glans. A structure called the urethral process projects beyond the glans.

Transverse section of the penis.

Corpus cavernosum penis

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Transverse section of the penis.
Structure of the penis
The deeper branches of the internal pudendal artery.
The penis in transverse section, showing the bloodvessels.
Male pelvic organs seen from right side.
Diagram of the arteries of the penis.
Cross section of penis.
Medical ultrasonography of a normal penis.

A corpus cavernosum penis (singular) (literally "cave-like body" of the penis, plural corpora cavernosa) is one of a pair of sponge-like regions of erectile tissue, which contain most of the blood in the penis during an erection.

Martin van Maële's print Francion 15

Sexual arousal

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Sexual arousal (also known as sexual excitement) describes the physiological and psychological responses in preparation for sexual intercourse or when exposed to sexual stimuli.

Sexual arousal (also known as sexual excitement) describes the physiological and psychological responses in preparation for sexual intercourse or when exposed to sexual stimuli.

Martin van Maële's print Francion 15
Two birds that appear to be exhibiting affection

Male arousal will lead to an erection, and in female arousal the body's response is engorged sexual tissues such as nipples, vulva, clitoris, vaginal walls, and vaginal lubrication.

The ring surrounds the penis, but does not constrict or cause discomfort.

Baculum of a dog's penis; the arrow shows the urethral sulcus.

Baculum

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Baculum of a dog's penis; the arrow shows the urethral sulcus.
A raccoon baculum
Walrus baculum, around 22 in long

The baculum (also penis bone, penile bone, or os penis, os genitale or os priapi ) is a bone found in the penis of many placental mammals.

In humans, the rigidity of the erection is provided entirely through blood pressure in the corpora cavernosa.