Gustave Courbet's celebrated and enigmatic 1866 L'origine du Monde (Origin of the World) is a key work in the artistic depiction of sexuality and has inspired many contemporary references and appropriations. These range from the purely imitative to political comment on gender (below).
Now housed in the Musée D'Orsay, Paris, the exact origin of L'origine itself is still subject to speculation, although it was probably commissioned by Khalil-Bey, a wealthy Turkish diplomat.
Vik Muniz, from Pictures of Soil, 1999
Pipilotti Rist, Pikelporno, 1992
John Currin, After Courbet, 2008
Orlan, L'Origine de la Guerre (The Origin of War), 1989
Bourgeois rarely depicts the sexual act itself, preferring instead to address themes of sexual identity in works that blur distinctions between male and female gender.
From the 1960s to 1980s in particular, the artist created a large number of sculptural pieces conveying a notion of sexuality as neither purely male or female, but a far more fluid combination of the two.
The latex sculpture Fillette (1968) clearly represents a phallus, although exaggerated to the point where it seems almost monstrous, grotesque. Yet it also incorporates subtle elements of the female, with the overly rounded testicles appearing almost breast-like, and vaginal folds of skin at its tip.
Above all, its name - 'little girl' in French - confounds its apparently masculine status. Bourgeois has never considered herself a feminist artist as such, but here she actively diminishes the patriarchal by assigning it a female identity, one which very consciously employs the French dimunitive '-ette'.
With this act, the work's threatening sexuality is tamed or even neutered: in a celebrated photograph by Robert Mapplethorpe she holds the piece casually tucked under her arm as if it were simply a toy or small pet.
A later work, Avanza, (left) extends the idea of hermaphroditic sexuality through rounded forms that immediately suggest the feminine - buttocks, breasts, pregnant stomachs - but also the tips of a penis.
Janus Flueris, ( bottom left) which exists in many versions, even more clearly represents female and male genitalia as separate though conjoined entities.
The strategy of placing two penile forms at each side of a vaginal centre allows them to vaguely approximate buttocks or breasts, as well as providing specific reference to the mythological 'Janus' of the title, whose two faces gazed into the past and the future.
In this way, time also becomes a subject of this work, with Bourgeois appearing to assert that sexuality always has been, always will be, a conjunction rather than a division.
Mapplethorpe's male nudes focus almost entirely on genitalia. His subjects' faces are never shown, and much of the body is likely to be cropped from view.
Occasionally, the images are titled after the models involved, but the names seem almost meaningless except as a kind of sexual reference system, a reminder of who owns what.
These photographs serve not only as studies of the penis itself, but as documents detailing a highly fetishistic view of the male member.
Within such a scenario, associations equating the phallus with power, domination and virility are inextricably connected to the erotic, and given such a correlation, the larger the penis, the more sexually charged it becomes.
In addition, the genitals become such intense objects of desire that they assume a sexual identity of their own and can be almost entirely disassociated from the bodies to which they are attached.
Mapplethorpe's pictures of the penis unapologetically acknowledge and, indeed, celebrate this fact, yet they cannot be seen solely as records of a gay male sexuality.
To begin with, Mapplethorpe's erotica was not aimed at specific viewers, and the artist was keenly aware that both gender and sexual orientation would drastically alter its perception.
To this end, sensuality for Mapplethorpe is never dependent on subject alone, but something he strives to achieve through technique, composition and lighting.
Moreover, any depiction of the male or female genitalia holds significance as one of the very few universal subjects in art.
In this important respect, Mapplethorpe's studies are no more specifically gay than Courbet's Origin of the World is rigidly heterosexual; and in fact, there are striking formal parallels between Courbet's work and Mapplethorpe's.
Yet tellingly, while even the most sedate of Mapplethorpe's male nudes have provoked (generally male) disapproval, his extensive female nude studies - almost all of which, like Breasts (left), similarly objectify the body - have largely escaped controversy.
This, in itself, appears to provide a fascinating comment on male sexuality.
Mapplethorpe's erotic view of the male body revels in deep-seated masculine stereotypes - the alpha male, the stud. And for many male viewers, the suggestion that they might not measure up - a parallel to the 'ideal' body image which has confronted women for years - may just prove a little too uncomfortable.
While sex is just one of various themes in the work of art duo Tim Noble and Sue Webster, their approach to the subject is always completely matter-of-fact.
Unsurprisingly, it is also mediated by their own relationship, both as a couple and artistic collaborators.
A 2002 neon sculpture proclaims I Love Sex, and as if to underline the point, in 2005 the artists meticulously depicted their own sex life in a spoof on the '70s 'self-help' classic, 'the Joy of Sex' (left).
Noble and Webster's partnership has long provided a specific visual motif for their work, and although the Joy of Sex flouts conventions of privacy, its intimacy is very different in tone and concept to Jeff Koons' similarly explicit portrayal of his own short-lived marriage.
Although a focus on sexual relationships is perhaps more characteristic of our previous sections, another work by the couple provides a fascinating and highly specific take on the penis and its symbolic, as well as physical, presence.
Black Narcissus was commissioned for installation in the Sigmund Freud Museum, London, and cleverly fuses the sexual element of Freudian psychology with the couples' existing artistic practice.
Taking their acclaimed shadow-picture technique as a starting point, the piece combines casts of Noble's penis and Webster's fingers.
The phallus, of course, is a primary Freudian motif, and when light is projected onto the sculpture from a particular angle, its shadow portrays a conjoined silhouette of the two artists - neatly illustrating both physical union and the notion of the psycho-sexual in a work that no doubt would have delighted Freud himself.