Biological Leylines

Internal Channeling is a concept that explains how humans and other similar beings are able to cast magic without the use of a focusing device or magic circle.

Placement
Internal ley lines exist as part of the human body, and remain undetected by human science as of yet. When these ley lines are in groups, such as a human body or an area of nerves that form a suitable casting device, they are referred to as magic circuits.The leylines, or circuits, are mainly comprised of barrier-like tissue, acting to contain the magic energies inside. They are usually spread throughout the body; all nerves contain at least one ley line.

Composition
The ley lines are essentially channels through which magic energies can be channeled and shaped before being finalized and cast. Magic circuits are not limited to nerves, however, and may expand farther in some cases.

Use
The circuits are like muscles. When used often, they will grow, and when ignored, they will atrophy. Due to this, archmages and experienced casters will have significantly less trouble casting, and can use much more complex magic. And, like muscles, magic circuits and magic tendencies will be passed down genetically somewhat (concept of evolution, etc). This allows the cloning of those with high magic capacity, creating what essentially acts as a magic clone army (computer chips with secret orders to kill their master sold separately).

Depending on the configuration of energy run through the circuits, it is possible to create much more complex magic circles outside the body, which can then be activated. In this way internal channeling can create external channeling. This skill is complicated, but essential for any mage worth their salt.