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:Cardiopulmonary anatomy of sheep is mainly similar to the humans (Bianco et al. 2004) but MBs are more frequently found in sheep hearts (Ozbag and Kervancioglu 2004) than human hearts.
Category: