The Ossmodula, also called the Ironheart, is the 2nd of the 19 gene-seed organs implanted into a Space Marine Neophyte during the induction process to create a new Astartes. This implant, surgically placed alongside the Neophyte's pituitary gland at the base of the brain, thus becoming a part of the Space Marine's endocrine system, secretes a specially engineered form of human growth hormone. When the effects of this hormone are combined with a diet laced with microscopic ceramic-based minerals, they act to catalyse the rapid growth of an Astartes' skeletomuscular system which results in an Astartes' superhuman strength and massive size compared to a baseline human male. Two years after the surgery to implant the Ossmodula is completed, the Neophyte Space Marine's skeleton will be larger and exponentially stronger than a normal man's with growth having topped out at around 7-7.5 feet in height with an equivalent amount of skeleto-muscular mass. During this time the rib cage will fuse into a solid mass of bullet-proof, interlaced bone plates. The resulting structure protects the Space Marine's organs from damage in a way the normal human skeleton never could, though at the price of producing greater difficulties for the Chapter's Apothecaries when they must perform surgeries on a Space Marine's body cavity.
- Codex: Black Templars (4th Edition), pg. 10
- Codex: Space Marines (5th Edition), "The Making of a Space Marine," pg. 11
- Index Astartes I, "Rites of Initiation - The Creation of a Space Marine" by Rick Priestley & Gav Thorpe, pg. 3
- Warhammer 40,000: Compendium (2nd Edition), "The Origins of the Legiones Astartes" by Rick Priestley, pg. 7
- White Dwarf 98 (UK) (1988) "Chapter Approved: The Origins of the Legiones Astartes" by Rick Priestley, pg. 13