May refer to Grub Street, which was in Moorfields, but more probably to Bethlehem Hospital, the madhouse there. Cf. Gay's epistle to Mr. Thomas Snow, where he seems to have had this passage in his mind —
'Through famed Moorfields extends a famous seat,
Where mortals of exalted wit retreat;
Where, wrapt in contemplation and in straw,
The wiser few from the mad world withdraw.'