In hospitals and laboratories, interstitial spaces are spaces designed adjacent to regular-use floors to contain mechanical infrastructure. Designing these as separate spaces allows for the possibility for rearrangement of regular-use floors (e.g. turning patient rooms into operating theaters) and allows for infrastructural maintenance to be completed without interruption of patient care. But hospital interstices, I argue, are found beyond those formally demarcated by architects and hospital engineers. The interstices, as depicted in van Gogh's 1889 painting, are sites of intense engagement between patients, practitioners, and families. They are the hallways and waiting rooms and storage closets. While the "cell" of the patient room is often imagined as the center of care, the hospital corridors are often where the most important consultation and communication takes place. These are spaces of fluidity: where specialty and hierarchy bleed, where patients--waiting for their formal admittance into a cell--dwell, often unaccounted.