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...
The surface of the Earth is called the crust which is geological  terms is the outermost layer of planet or satellite. In the case of  the Earth, the crust is composed of â¦ igneous, metamorphic and  sedimentary rocks and is about 20miles to 50miles thick on the  continent and about 3 to 6miles thick in the ocean.