“…In recent years the perfusion method has prevailed more and more (Buss et al 1973, Buss and Hollweg 1977, Buss et al 1976, DeBruijn and VanMourik 1975, Edanaga 1974, Elemer et al 1976, Fishman et al 1975, Fonkalsrud et al 1976, Frenzel et al 1976, Hess and Staubli 1963, Shimamoto 1974, Svendsen and Jorgensen 1978, Webster et al 1974). Unfortunately there are also additional methodological variations and possibilities of error involved in this more complicated technique, making comparisons more difficult.…”