This is a dormant organ that connects to both Astartes hearts. In times of extreme stress, or should the warrior's body undergo violent, damaging trauma, it expels great blurts of self-synthesized chemicals -- a hyper-cocktail that simulates the biological action of combat stimms while also aiding in the rapid regrowth of tissue, bone and muscle. The gland then falls dormant again, and takes some time to metabolically build itself up once more for the next usage.
- Codex: Adeptus Astartes - Space Marines (8th Edition), "The Making of a Space Marine", pp. 10-11