Inner Secrets of the Lung

This incredible 19th century lung cast was one of the medical curiosities on display in a 2014 Harvard Museums exhibition. It shows how finely detailed models of inner lung structures were made by filling the airways with a hardening substance and dissolving the outer tissues. While this technique loses features such as muscles and blood vessels, it nevertheless gives some sense of the complexity of the airway tree.

Even this amazing detail doesn’t come close to capturing the full lung structure. In reality the airway tree branches more than 18 times, each branch becoming finer until they are too small for the casting process to work, and finally reaching hundreds of millions of tiny alveoli, the air sacks which fill the lung and enable your body to take in oxygen and get rid of carbon dioxide. And there’s much more to the lung than airways - whole other trees of arteries and veins which can be separately cast in similar ways.

This technique dates from at least the 17th century. What may surprise you is that, despite the incredible developments in imaging technology over the last 50 years, our best computer simulations of the lung still use measurements made in the 1960s, based on casts similar to this one, with length, radius and angles measured painstakingly by hand using precise instruments. The casting process helps preserve the shape of the lung, which would otherwise collapse without the normal pressure from the air and blood flow and the tension-reducing surfactant in the living lung. Nowadays we can see inside the body using imaging techniques like CT, but their resolution is still nowhere near fine enough to see the intimate inner structures of the lung. These casts remind us that in science, even antiquated methods may yield knowledge that serves us for decades to come.