LONDON — The classical facade of the Wellcome Collection here makes it seem as if this museum, which opened last June, were going to treat medical history the way the nearby British Museum treats Greek and Near Eastern civilizations, with an ordered, carefully annotated display of marvels and antiquities. But it doesn’t take long before that notion is thoroughly overturned. Perhaps it happens when you watch the famous 1929 Surrealist film (“Un Chien Andalou” ) made by Salvador Dalí and Luis Buñuel, in which an eyeball is slit by a razor. Or perhaps it is when you come upon a fragment of skin from the dissected body of the 19th-century philosopher Jeremy Bentham.