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...
Our Earth is divided into three different spheres; the geosphere, the hydrosphere and the atmosphere. The geosphere is the most solid part of the Earth and is divided into three separate layers; the core, the mantle and the crust. The core is the EarthÂ´s center. It is made up of metals and its temperature is extremely high.