Possible mechanisms of peritoneal tissue-oxygen tension changes during CO2-pneumoperitoneum: the role of design, methodology and animal models: Editorial commentary on the articles 'Peritoneal tissue-oxygen tension during a carbon dioxide pneumoperitoneum in a mouse laparoscopic model with controlled respiratory support' by Bourdel et al. (2007) and 'Effects of supplemental perioperative oxygen on post-operative abdominal wound adhesions in a mouse laparotomy model with controlled respiratory support' by Matsuzaki et al. (2007)