At one point in last week's party to celebrate the V&A's new Medieval and Renaissance Galleries, I slipped the roaring champagne-chugging throng and visited the (refurbished) loo for relief and reflection, each of a profound nature. Lavatories are always revealing of any civilisation's achievements. Here I found some of ours. There was a Duravit urinal with a trompe l'oeil fly in the target area and a Dyson hand-drier. I'm not one to repudiate the modern world, but compared to upstairs, these made me a bit sad.  For the first time, the museum's astonishing treasures from these defining moments of European civilisation are rationally and beautifully displayed. And the effect is exalting, transcendental. It is an entirely new museum-within-a-museum. If these galleries were a standalone in any other country, it would immediately become one of the world's great museums. Whole institutions have been built around less than Leonardo's Codex Forster, but this is only one of nearly 2,000 superlative objects on display. That these new galleries are only a portion of the whole V&A is bewilderingly wonderful. Here is a resource of incalculable value and meaning.  Architecturally, bright new space has been created within the dark, eclectic chaos of the old museum. What we think of as the V&A is, in fact, mostly an Edwardian facade by Sir Aston Webb, disguising confused layers of different buildings behind it. The unifying effect is illusory: it has always been difficult for even the keenest visitors to make sense of the V&A's collections. Until now.  The new galleries have been designed by McInnes Usher McKnight Architects (Muma) who won the commission in 2003, just three years after the practice was founded. It is their first substantial work. Essentially, Muma ingeniously recovered dark, neglected space in the old museum and fused it with existing cavernous galleries to create three levels bound by a "central orientation hub".  The recovered space is roofed by heroic structural glass, making natural top-lighting a feature of what had hitherto been Stygian gloom. The new volumes are so accommodating that one exhibit is the entire front elevation of Sir Paul Pindar's house. Pindar was the ambassador to the Ottoman court and consul at Aleppo. His house, once on the site of what is now Liverpool Street station was one of the few survivors of the Great Fire. It has not looked so good since 1666.  To describe the individual treasures, mesmerising as a list may be, does nothing to indicate the whole magic of what is now available. Still, it would be negligent not to record that you can see the Devonshire Hunting Tapestries and the Thomas à Becket enamelled casket from Limoges, a superlative object which once contained relics of the martyr. There is the chapel of Santa Chiara, built in Florence in 1494, which the V&A claims to be the only Italian renaissance building outside Italy. Indeed, the museum's collection of Italian sculpture is rivalled only by Florence and Rome. Now it is properly accessible.  Indeed, accessibility is our new best friend. When I was at the V&A in the 80s, the great Giambologna sculpture, Samson and a Philistine (an audacious design since its vast substance is anchored at only five slender points) was in the gloom and once carelessly thwacked by a contractor's scaffolding pole. A Michelangelo drawing had been lost and the hapless director posed for photographers holding a postcard of it. Now, Giambologna is available in all his dramatic swagger in a courtyard garden with tinkling fountain while Leonardo's great Codex is interactively digitised. The modern display cases have fabulously sophisticated mitred glass by Hahn of Frankfurt allowing perfect visual access, works of art in their own right.  On any basis this would all be marvellous, but I sensed something special in the air on the opening night. There was quite extraordinary passion in the crowd. Why was this? Because the exhibits offer rare access to the sense of wonder. They satisfy appetites for physical quality and moral substance which have, by and large, been ignored in the exploitative lightweight crapola served up by, shall we say, the Turner prize. Of course, no one bright enough to walk around unaided needed to be persuaded that Donatello is an artist of the very highest rank, but to have the manipulated enterprise and care that is the Chellini Madonna presented with such immediacy is an epiphany. And not one available in Italy.  In these troubled times, there was a mood of near-religious enthusiasm among the guests. Not that the hedge-funded crowds were dressed in Primark, but the sight of Opus anglicanum needlework does remove you from the comings and goings of shopping. It was extraordinary to see great art enhancing moods: for those who find the Duravit fly and the Dyson Airblade fail fully to address the enigmas of existence, there are object lessons in the objects on display here.  So it is melancholy to note that among the crowds on the opening night, I did not see any leading representatives of the architecture or design professions. So much for Renaissance man. Cue Dark Ages?
V&A Museum, LondonThe V&A has unveiled a cavernous and brilliant new space. But it's the art within that is truly spectacular, says Stephen Bayley