Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:Intact, beating hearts from young ( 4 wk, n 37) and old ( 52 wk, n 37) guinea pigs were prepared by the Langendorff method and perfused with Krebs-Ringer (KR) solution as previously described by us in detail (1, 2, 9, 10, 35, 37, 38).
Category: