We all know dementia by now: the organ of the brain breaking down in substance and function much as a heart or liver does. By the time a person dies from complications of Alzheimer’s disease, his or her brain is significantly smaller than its normal size. There are several major variants of this process, and the disorder’s progress takes many forms: insidious, incremental, dramatic, fast, and slow. The biology of loss is complicated and not entirely predictable; but in every case, memory, language, and motor control eventually slip away until a person finally sinks into silence and immobility. One…