The Lyman's Ear, also called the Sentinel, is the 11th of the 19 genetically-engineered gene-seed organs that are implanted into a Space Marine Neophyte to produce a new Astartes. Not only does this implant's improved inner ear structure make a Space Marine immune to dizziness or motion sickness but it also allows Space Marines to consciously filter out and greatly enhance certain sounds over the capabilities of normal human hearing. The Lyman's Ear completely replaces one of a Space Marine's original ears. It is externally indistinguishable from a normal human ear in size and shape. Lyman's Ear is named after one of the scientists who worked in the Emperor's gene-laboratories on Terra in the 29th Millennium who aided in the project to produce the first Astartes using the Emperor's own genome as the starting point.
- Codex: Space Marines (5th Edition)