Housed in the vast, timbered roof space of the 17th-century St Thomas' church, which once abutted the hospital's female wards, the former herb- drying garret/dispensary and women's operating room were forgotten for almost a century, until a resourceful historian with a ladder rediscovered the old entrance in the Fifties.
Today, the restored Herb Garret Museum houses an alarming collection of early surgical instruments that look as if they belong in B&Q's DIY joinery section; a replica dispensary where you are invited to make your own pillules with the aid of a mahogany "pill machine", some withered leaves and a spot of glycerine; and an atmospherically aromatic thicket of dusty dried botanicals with arcane cultural, and astonishing medical, tales to tell.
With its past links with astrology and assorted magical and esoteric arts, herbalism is a lecturer's dream ticket when combined with the imagined flash of steel and bone. In the replicated standings of the Old Operating Theatre, where medical students from old St Thomas' and Guy's (including the delicate student John Keats) used to cram to witness gory procedures, the public now sits on Sunday afternoons to listen to a splendid oration by the museum curator, Karen Howell, or a specialist guest speaker.
Among the planned lecture delights later this year, for example, is "Shakespeare and the Medicines of the Apothecaries", on 6 December. There are always school-holiday workshops on the horrific, but popular, subject of "Victorian Surgery Before Anaesthesia". The marketing manager, Stewart Caine, says they never have any problems with attention span from the GCSE students who come to the museum to do their history of medicine module. "We just point to the operating table and ask for a blindfolded volunteer," he explains.
The lecture I attended was on the intriguing subject of "The Herbal Surgeon", which put forward the premiss that the battle fields of history not only provided medical personnel with practical experience of the "hand craft" of surgery, but also introduced a raft of sure-fire herbal remedies and stupefactives.
Yarrow, generic name Achillea, is named after Achilles, who is said to have used it to heal his soldiers' battle wounds. During the First World War it was still being used as a field dressing, as was moss. Comfrey, or bruisewort, and boneset - since found to contain allantoin, which stimulates the growth of connective tissue and mends cartilage and bone - were also must-have herbs for the battlefield. The astrologer and physician Nicholas Culpeper said of it in his 1640 Materia Medica that, spread upon a leather poultice, it "tends to heal gangrenes".
The rosy pink infused oil of St John's wort, once known as "Jesus Oil", was carried into battle by the Greeks, Romans and Crusaders for its efficacy in stemming bleeding.
Dioscorides, who was doctor to the Roman legions in the first century AD and also personal physician to Antony and Cleopatra, made a study of the painkilling and soporific qualities of plants. He prescribed willow bark, which contains salicin, a precursor of aspirin, to take the edge off pain.
Until the 17th century, a decoction of anaesthetic hedgerow narcotics such as henbane, mandrake, opium poppy, ground ivy, sea holly and wild lettuce was wafted under the noses of those about to undergo major surgery. Patients were brought round by a whiff of onions and vinegar.
In 1797, at the Battle of Cape St Vincent, Horatio Nelson had his wounded arm hurriedly cut off on the deck of his ship with only some opium and rum to dull the horror. In his book Operations That Made History (Greenwich Medical Services), Harold Ellis describes Nelson's stoic response when asked whether he had any complaints about the operation. He replied that the knife had been too cold, and ordered that all his ships carry portable primus stoves in future, against such ghastly eventualities.
The Old Operating Theatre, Museum and Herb Garret is at 9a, St Thomas Street, London SE1 9RY (0171-955 4791). Open 10am-4pm daily. Admission pounds 2.90 adults, concessions pounds 2, children pounds 1.50, family pounds 7.25