The loss of pigmentation in the substantia nigra of the midbrain at the left in a patient with Parkinson disease is contrasted with a normal midbrain at the right. Parkinson disease is marked clinically by a "pill-rolling" tremor at rest, mask-like facies, and cogwheel rigidity of limbs, among other findings. Some cases occur in association with diffuse Lewy body disease, a form of dementia similar to Alzheimer disease.

---