"off to the British Museum looking up some authorities on ancient medicine"
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The British Museum Reading Room
Creative Commons Attribution Share AlikeThe British Museum Reading Room - Credit: Eneas

The British Museum was founded in 1753 as the first national public museum.  Today, with a permanent collection of around 8 million items, it is one of the world's greatest museums of human history and culture.  It is located on Great Russell St, London.

The museum is best known for treasures such as the Rosetta Stone and the Elgin Marbles, but it does also hold various ancient medical texts:

The London Medical Papyrus

Babylonian Medical Tablets

Tibetan Medical Tantra

Medical Tablets from the Library of Ashurbanipal

However it is more likely that Van Helsing was headed for the British Library, which was at the time a department of the British Museum.  The Library, now at St Pancras, contains a multitude of medical texts, such as:

Medieval Medical Manuscripts in the Harleian Collection

Leyden Medical Dissertations, 1593-1746