A mechanism within the body semi-analogous to the nervous system, which serves to replenish the organism’s Od when it is depleted. Most organisms are not born with Magic Circuits and thus possess a fixed amount of Od which, once depleted, cannot be restored. The possession of Magic Circuits and the ability to control them is what differentiates a normal person from a true Mage. An “average” Mage possesses roughly 20 Magic Circuits.
It is possible for a person devoid of Magic Circuits to perform acts of Magecraft, however this is very rare and takes a great deal of study and focus. Most people who lack Magic Circuits turn to the use of a magical focus of some sort imbued with Prana, such as a talisman, dagger, jewelled scepter, etc.