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:Hearts were excised and perfused retrograde via the aorta with modified Krebs-Henseleit (K-H) buffer oxygenated with 95% O2-5% CO2 as previously described (12).
Category: