Until 1965, there was a metropolitan borough of Holborn. Oddly, Holborn Viaduct wasn’t in it, and nor was Staple Inn, the impressive Tudor building that’s the trademark of Old Holborn tobacco. Even Holborn Circus only had a fingertip in the borough.
The reason for this goes back into the mists of time. Originally, Hole Bourn was another name for the River Fleet, or at any rate, its upper reaches. The river rose at Hampstead ponds, and ran down (now under) King’s Cross Road, Farringdon Road and New Bridge Street (as you’ll see if you head down them, they run along bottom of a valley, with streets leading uphill on both sides). It met the Thames at Blackfriars. The eastern end of the old Roman Road from London to Silchester, now called “Holborn” and “High Holborn”, was originally Holborn Street, meaning the street leading down to the River Fleet (just like Fleet Street).
Gradually the neighbourhood around the street took on the name of Holborn too. All of it was outside the walled City of London. But then the medieval City expanded its jurisdiction, from the old walls to what were called the City bars, which were toll gates on the roads leading into it (not to be confused with places where bankers drink). Temple Bar is the most famous of the City bars, but there was also a Holborn Bar, located in front of Staple Inn where two City boundary dragons now stand. The area to the south and east of this, though outside the old walls, were thus annexed to the City (“below the bars”) and were therefore excluded from what became the borough of Holborn (which was “above the bars”).
The borough did however extend westward, to take in St Giles and most of Bloomsbury, and all the way east to the River Fleet, to take in Hatton Garden and Saffron Hill, which was already London’s Little Italy back in the nineteenth century. More recently, a bunch of PR wonks decided to rebrand Holborn as “Midtown”, but strangely nobody other than the wonkers in question ever called it that.
In 1965, the metropolitan borough of Holborn was merged with Hampstead and St Pancras to form the London Borough of Camden, and the area called Holborn remains split between Camden and the City to this day. The metropolitan borough’s old town hall still stands, just where High Holborn meets New Oxford Street.