Actually, "filling the lung with fixative......" is infusion not
perfusion.  For our rodent specimens we cannulate the trachea and
infuse the fixative  at 25 to 30 cm of water.  Following infusion, the
trachea are tied with suture material (or dental floss for the
spendthrifts) below the cannulation site and then submersion fixed. 
The 25 to 30 cm figure is quite common among species, in my reading I
have found a picture of intact blue whale lungs that were infused at
that pressure.  The two men who did it were quite proud of their
accomplishment, standing next to it along with the 55 gallon drum of
fixative and fire hose.  Ian, are the "wee differences" you mention at
the light or ultrastructureal level (or both)?  Rob   At 10:45 PM
1/25/99 -0600, you wrote: >Embedding is the actual casting/ places the
tissue into a mold in the >correct position. Filling the lung with
fixative is >perfusion..........Bonnie G. Greer ,histo lab St. Jude
Childrens >Hospital/   Memphis,TN >  <italic> Robert S. Geske  Research
Associate  Center for Comparative Medicine and Department of Pediatrics
 Baylor College of Medicine  Houston, TX  713-798-3602 </italic>