The draconis fundamentum, often called the "Fundamentum", was the Common name for a large blood vessel found only in true Dragons, forming a vital part of the biological system that enabled their breath weapons. While early anatomists believed the vessel itself produced the elemental power of a dragon's breath, later study revealed that the true source lay within the dragon's heart. There, magical energy was continuously generated and carried throughout the body by the blood. The draconis fundamentum served as the primary conduit for this energy, directing it from the heart to a specialized chamber within the gizzard where it accumulated until released.
This reservoir allowed dragons to gather and contain tremendous quantities of magical power before expelling it in a devastating blast. Although the form of the breath varied between species, including elemental weapons, breath of Transmutation, and many stranger manifestations, the underlying process remained the same. The fundamentum did not determine the nature of the breath itself, but instead transported the magical essence that made such attacks possible. Once discharged, the chamber gradually replenished as energy continued to flow from the heart through the vessel.
The existence of the draconis fundamentum demonstrated that a dragon's breath was neither a product of its lungs nor simply a reaction of Alchemy. It was the physical expression of the creature's innate magical nature, supported by specialized organs found nowhere else in the natural world. This anatomy also explained why dragons could maintain internal temperatures capable of sustaining certain elemental breaths and why their blood was often regarded as possessing potent magical properties. Though hidden deep within the body and seldom seen outside the dissection of a fallen dragon, the draconis fundamentum remained one of the defining features that separated true dragons from all lesser Draconic creatures.