This road was once used as the main highway.
He finally returned to the right path.
internal medicine (traditional Chinese medicine)
Specialized term in the context of traditional Chinese medicine, referring to internal medicine as opposed to surgery or other branches.
See also: 内科 (ないか)
In Kampo medicine, 本道 refers to internal medicine.
本道 is the main road, directly contrasting with 横道, the smaller branching side street.
From 本 (main, true) + 道 (road, way). The medical sense derives from the idea of the 'main path' of medicine in traditional Chinese practice.