Dancer to the music of time: Portraits of Jane Avril

If ever there was an exhibition just waiting to be researched and displayed, it was the Courtauld Gallery's show of Henri de Toulouse-Lautrec's portrayals of the dancer Jane Avril.

Most shows of work by this wonderful artist – and it has been some 20 years since a solo exhibition was held in Britain – present him primarily as the celebrant of the Belle Epoque and its world of cafés, nightclubs and dance halls. He was that, of course. But he was so much more, as this exhibition shows in moving detail.

Jane Avril was almost anorexically thin. Given to jerky movements and sudden contortions (she was nicknamed "La Mélinite", after an explosive), she was fond of fashion and a favourite in intellectual circles. She also became the subject of a succession of portraits, prints, posters and studies by Toulouse-Lautrec.

By any standards, they made an odd pair. He was the scion of one of the noblest families in the south-west of France, who had both legs shortened by riding accidents in his youth. She was the daughter of a courtesan who had run away from home at 13, been treated for what was then called St Vitus' Dance in a mental asylum, and then carved out a career with an English name and a style of ecstatic dance that was all her own.

They do not appear to have been lovers and it would also be wrong to describe Avril as Toulouse-Lautrec's "muse". Their relationship was more equal and professional than that. It was a mutual sense of being different from others, made singular by their defects, that drew them together.

Toulouse-Lautrec was fascinated by the world of performance and entertainment, from the brothels in which he spent time sketching to the dance halls where he got to know and promote the chief dancers. He has been accused of chauvinism and worse for doing so but although he may have used the services of some of the prostitutes, it was not that which drew him to the women of this world.

What he had – because of his own physical oddity, or because it was in his nature anyway – was a deep interest and profound understanding of the whole business of the way people, male and female, present themselves to the world. There is no artist who so completely understands that dress, make-up and gesture are part of an armoury which a woman dons, in order to give herself a front with which to face the world.

Avril was a perfect model for this, but she was also in her way a fellow spirit. Her life had been a harsh one – she came from an abusive home and spent a couple of years in a hospital for the mentally ill before being taken up as the mistress of an Englishman, who gave her the name Jane Avril. She made a determined pursuit of stardom through cabaret. She felt, as Toulouse-Lautrec did, that she was an outsider, needing to air an extravagant personality and to cut a dash in the world.

The Courtauld exhibition is built around the gallery's own Toulouse-Lautrec picture, Jane Avril at the entrance of the Moulin Rouge. It is a painting, in dark colours, of a performer arriving to work, elegantly dressed, with a great flowered hat and a yellow bag, her eyes concentrated inwards and her pale face taut with the job at hand. It is almost impossible to think that this is a girl in her early twenties, let alone someone earning her money by so flamboyant a task as cabaret dancing.

But then, Avril wasn't an extrovert or a naturally voluptuous dancer. She was a creature of her own creation. Time and again Lautrec captures, with infinite care in the brushwork around the eyes, the look of a woman who has lived, a woman who knows the world and who is determined to keep herself a space above it.

The curator of the show, Nancy Ireson, has been extremely fortunate – and persuasive – in being able to borrow a dozen paintings and prints from museums in the US and France that were clearly intrigued by the project. The Courtauld's Avril portrait is placed at one end of a wall that has at its other end Leaving the Moulin Rouge, from Hartford in Connecticut and from the same period. Alongside it is a truly extraordinary picture of Avril from Williamstown, this time face on, her cape filled in sketchily, her hat again on the extravagant side but her face almost regal, as her eyes glance aside, to evade intrusion.

Over by the famous lithograph of Jane Avril at the Jardin de Paris, which helped make her reputation, there is a dazzling gouache study for the poster – a brilliant portrayal of movement and purpose that is now in private hands – and an oil sketch of Avril dancing that is done in rapid-fire blue strokes which illustrate exactly the peculiar, twisted actions that made up her dance.

The centrepiece of the wall of portraits, however, is the large-scale At the Moulin Rouge. With Avril seated with her back to the viewer, Toulouse-Lautrec appears in the background, with his giraffe-like cousin; to the right in the foreground is Avril's great friend, the English dancer May Milton, who may or may not have been her lover.

As a composition, it is devastating. The wooden balcony in the left hand corner forces a diagonal which throws the party of Avril and others into counterpoint, only for the eye to be pulled down by the figure of Milton. Avril is (again) separate but apart from the table group, looking down while they look up at each other in their conversation. The red of Avril's hair leads, via her white skin and the white of the table, straight to the heavily made-up face of the performer La Macaron and then on to the dancer La Goulue (the "Glutton"), who is adjusting her hair, until finally the eye is fixed by the harshly-lit face of Milton. All life, the picture seems to say, is a matter of presentation, of artifice and performance. This is where Toulouse-Lautrec is most modern – not in style, but in understanding.

Toulouse-Lautrec died in 1901, at the age of 36. Avril lived on until 1943, her marriage blighted by the discovery that her husband was a transvestite, and stayed in the performing world. The Courtauld has gone to great efforts to track down information on her, and in a room adjoining the main gallery there is assembled a plethora of facts and photos, including an Edvard Munch-designed playbill for the first French performance of Ibsen's Peer Gynt, in which she is named in the cast list, and a series of playful pen-and-ink drawings, by Picasso, of her dancing in her later years.

Avril was not the only woman to catch Toulouse-Lautrec's interest, of course. Go to the Musée Toulouse-Lautrec in Albi and see his extraordinary study of Yvette Guilbert, and his sketches of the prostitutes at Rue des Moulins, or view in this exhibition the portrait La Goulue at the Moulin Rouge to see how performers of all sorts intrigued him. But Avril did bring out a special sympathy and an enduring fascination in the artist. Note the different spirits as well as guises that he captures in the various portraits.

By concentrating on this particular relationship, the Courtauld exhibition brings out exactly the combination of brilliance and human understanding which made Toulouse-Lautrec such a great portrayer of the performer and the mask we all present to the world. Along with the Courtauld's exhibition of Cezanne's Card Players last winter and the current show of Egon Schiele's women at the Richard Nagy Gallery in Old Bond Street, this is one of the best small exhibitions of the year.

Toulouse-Lautrec and Jane Avril: Beyond the Moulin Rouge, The Courtauld Gallery, London (020 7872 0220; www.courtauld.ac.uk) to 18 September