Magic Circuits [Others]
The pseudo-nerves existing in the body of a magus.
They are the paths that convert life force into magical energy, as well as the paths which connect to the foundations underlying Grand Rituals.
The number of Magic Circuits is determined at birth. Mage lineages will resort to any means to alter themselves in hope of giving their descendants just one more Circuit.
This is why magi from older lineages are more powerful.
In terms of the number of Magic Circuits, and the quality of the lineage, Cornelius Alba is undoubtedly a first rate magus.
In fact, neither Aozaki Touko nor Araya Souren have many Magic Circuits. Touko has about 20, Araya about 30.
Still, the two are considered geniuses: Touko has talents that are independent of her lineage. Araya's overwhelming power is probably a result of the anguish that he ceaselessly accumulated through the ages.

Kara no Kyoukai Special Pamphlet - Encyclopedia: Magic Circuits [Others], p.039