Magic Circuit

Magic Circuits (魔術回路, Majutsu Kairo) are a pseudo-nervous system that spreads itself through the human body and what qualifies a person to be a magus. Their normal functions are to act as paths that convert Life Force into magical energy, but humans learned how to control it and use it to perform mysteries. They are that which connects the material world to the flow of karma. If magical energy is the force that actualize the rules of a Thaumaturgical Foundation’s system, then the Magic Circuits are the pipelines that converts the flow of karma and transmits that energy from the magus to the system.

Mechanisms
Magic Circuits reside within the magus’s soul, and what is found on the body is just a physical expression of them that stretches itself throughout the body in a fashion similar to the nervous system.

They are composed of core components, which are the actual Circuits so to say, and bypasses that connect those components to the brain. Magic Circuits do not change and once damaged they cannot be repaired. Because organic activity is necessary for the operation of Circuits, it is a common mistake to think that they are literally a part of the magus body. There are rare occasions in which the Circuits continue working even after the practitioner’s body has ceased all vital functions. In these cases, destruction of the brain is necessary to stop the Circuits. Their Quality and Quantity are ranked through letters like Magecraft, and their Composition often varies between magi.

Like any other organ, the number of Magic Circuits that one possesses is determined at birth and cannot decrease or increase naturally, meaning that those from older lineages will be more powerful, but it is possible to change that number artificially through a number of methods. One of those methods is through the establishment of a mystic pass between two magi from which the Circuits can transferred. As the Circuits are located in the soul, a fusion of body and mind is required during the transplant. However, the process carries the same risks and complications as a transplant of organs. Furthermore, as the Magic Circuits become unstable if tampered while in use, there is a possibility of mutual destruction if a contract similar to that between a magus and his familiar has not been established beforehand.

For that and other reasons, lineages of magi seek not to tamper with the Circuits of their members while they are alive, but rather try to produce descendants with the greatest "extra" number of Magic Circuits possible. Note that number is not everything, and quality and precision of the Circuits are also an important factor that differs from lineage to lineage. The Aozaki bloodline is known for producing magi with a low quantity of Magic Circuits, but the great quality of their Circuits is more than enough to compensate for that. Meanwhile, the Barthomeloi possess a powerful brand of Magic Circuits known as Blue Blood Noble Magic Circuits (貴い魔術回路（ブルーブラッド）, Burū Buraddo Tōtoi Majutsu Kairo).

A normal person rarely acquires Magic Circuits but in these cases they are considered as sudden mutations. While Kirei Kotomine was born with sudden Magic Circuits not of his lineage, he is not of this type. Kirei's Circuits were a gift from the divine sacrament, rewarding his father, Risei Kotomine, for years of pious worship. Kirei was born with "the right to recreate the miracles in the divine sacrament". This "right", in other words, was the Magic Circuits.[6]

Still, the Circuits are ultimately something unnatural for the human body and their activity will cause pain and numbness to the magus. Their very presence in the human body is something uncommon and an ordinary person will have little to none. Also, the use of Magic Circuits increases the magus bodily temperature, which is what allowed Kiritsugu Emiya to discover enemy Masters on the battlefield through the employment of a thermal sensor during the 4th Grail War.

Magic Circuits are “opened” when magical energy is run through them for the first time. From that point on, the user can activate them at will through the use of a mental trigger. The image that is related to that trigger will differ from a magus to another: Shirou’s image involves firing the hammer of a gun, while Rin Tohsaka’s is that of a knife stabbing her heart, and Luvia Edelfelt's is imagining her heart as a jewel shattering. There are also those that can only use that trigger under predetermined conditions, such as after performing self-injury or when sexually excited.

Prior to the Fifth Grail War, Shirou Emiya believed that he had to “make” a new Circuit every time he wanted to perform Magecraft. In each attempt to do so, he temporarily (and inadvertently) converted parts of his nerves into makeshift Magic Circuits. A very dangerous procedure, as failure would result in his body breaking down from the inside out and even with success the resulting pain would be comparable to inserting a hot iron rod into the spine. Furthermore, when his natural born Circuits were finally opened, his nervous system went haywire due the sudden intake of energy. At first, neither of his teachers of Magecraft (Kiritsugu and Rin) noticed that habit of his.

Differing from the Magic Circuits of regular humans, Saber and Saber of Black when he was alive have a Magic Core (魔術炉心, Majutsu Roshin) granted by their Dragon blood. Its capacity is far greater than any human can achieve. While a magus' body can be considered a machine which creates magical energy, Saber's Core acts as a factory which creates magical energy. Something like Prelati's Spellbook has a Magical Energy Reactor (魔力炉), supplying essentially limitless power to its wielder.

Magic Circuits By Characters
 * Average magus: 20 Magic Circuits.
 * Touko Aozaki: 20 Magic Circuits. She is the first in the history of her family with so many Magic Circuits. Her parents had none whatsoever and it has been implied that her sister does not have much more than them.
 * Souren Araya: 30 Magic Circuits.
 * Cornelius Alba: Unknown. Implied to have more than both Touko and Araya.
 * Shirou Emiya: 27 Magic Circuits. Each Circuit can barely handle 10 units of magical energy while being damaged in the process. Considering the amount that he has, such low quality of Magic Circuit is inconsistent.
 * Rin Tohsaka: 40 inborn Magic Circuits and subs that are 30 each.Maximum output is of 1000 units of magical energy.
 * Sakura Matou: Unknown. Stated to have the same maximum output as Rin with her natural circuits. However, whatever circuits she had were corroded and assimilated by the Holy Grail Shards implanted into her.
 * Luviagelita Edelfelt: 100 Magic Circuits. It is unknown how many of these are natural and how many are from her family Crests.