This beautiful object is a corrosion cast of bronchi and trachea, c. 1880-1890, most likely from a rabbit, sheep, or dog. It's part of the new Body of Knowledge exhibition at the Harvard Museums of Science & Culture.
Corrosion casts have been part of anatomical teaching from the 17th century to the present, particularly for creating display specimens. A rapidly hardening substance, often metal or plastic, is injected into blood spaces or other cavities. Then the tissue is dissolved away by strong acids or bases. This cast was created using a mixture of bismuth, lead, tin, and cadmium. After injection, the tissue was dissolved in potassium hydroxide.