Noun. 1. optic disc-the point where the optic nerve enters the retina; not sensitive to light. optic disk, blind spot. retina-the innermost light-sensitive membrane covering the back wall of the eyeball; it is continuous with the optic nerve.
The mantle layer is the largest of the class. About half of our planetâs mass. The mantle is composed of very hot dense rocks, That move and flow, always on the go, they never lock, Extract from http://www.educationalrap.com/song/layer...
A: The lithosphere comprises the crust and the upper section of the mantle. It forms the rigid exterior of the Earth, which is composed of the crust, mantle and core.