Instead of a longer article, I went for one article per structure.
Here are the others:
It is actually not pure white. It can lead to confusion because the internal wall is a starker white than the white line. It is located between the internal wall and the sole and its role is to maintain them both together. It also serves as a barrier from external intruders (bacterial or stone like) and protects the corium and coffin bone.
It is however a softer material compared to sole or wall material, and, when unhealthy, becomes an easier entrance for unwanted trespassers. This can lead to more serious infection problems and lameness.