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Two Boys Blowing Up a Bladder - Joseph Wright of Derby (1769-70)
(upload.wikimedia.org)
Wander the gallery. Look at the art. Be polite. If you feel able please post some great art :)
Lungs are like a sponge inside (maximum surface area) so more dense than an empty bladder, not as much light would go through. We (the English) used these bladders as the first rugby and footballs in the 1800's.