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...
There are four major layers of the Earth: the crust, the mantle, the liquid outer core, and the solid inner core. The crust is the thinnest outside layer of the Earth where the continents reside. The next layer is the mantle which is the...