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...
What percent of the Earth is the mantle? The mantle comprises some 85% of the earth. It is a thick layer between the crust and the core, and the depth to which it goes (by definition) is such that it takes up most â¦ of the earth's volume. Use the link below for more information.9 people found this useful.n Plate Tectonics. The mantle comprises some 85% of the earth. It is a thick layer between the crust and the core, and the depth to which it goes (by definition) is such that it takes up most â¦ of the earth's volume.