\bibitem{Sapoval02}   B.Sapoval, M.Filoche, E.R.Weibel  \hskip 2mm
                      {\it Smaller is better -- but not too small: A physical scale for 
                      the design of the mammalian pulmonary acinus},
                      PNAS {\bf 99} (16), 10411-10416, 2002.