In Membranes, Laura Otis examines how the image of the biological cell became one of the reigning metaphors of the nineteenth century. At the heart of her story is the rise of a fundamental assumption about human identity: the idea that selfhood requires boundaries showing where the individual ends and the rest of the world begins. Otis focuses on the scientific and creative writing of four physician-authors: American neurologist S. Weir Mitchell; Spanish neurobiologist Santiago Ramón y Cajal, who won the Nobel prize in 1906 for proving that neurons were intact, independent cells; British author Arthur Conan Doyle; and Austrian writer Arthur Schnitzler, a contemporary of Sigmund Freud in fin-de-siècle Vienna. Membranes also compares the scientific and political thinking of German scientists Rudolf Virchow, the founder of cellular pathology and an active liberal politician, and Robert Koch, who discovered the bacteria that cause cholera and tuberculosis and whose studies of foreign bacteria provided a scientific veneer for German colonialism. Finally, the book presents a unique reading of Thomas Mann's Death in Venice.

Otis argues that belief in impermeable personal and national borders is increasingly dangerous. Defying the traditional boundary between science and the humanities, she concludes by proposing a notion of identity based on relations and connections.