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 Earth consists of three layers: the crust, the mantle and the core. The core, the innermost layer, is rich in iron and very dense. It can be further subdivided into the inner and outer core. The mantle is the intermediate layer of the Earth and can be subdivided into the inner and outer mantle.Most of the mantle is a thick fluid that moves in currents, but the very outermost portion of the outer mantle is solid. This portion and the solid crust make up the lithosphere.hile it is the thinnest layer of the Earth at only 60 to 70 kilometers thick, it makes up the majority of the lithosphere and is the portion of the Earth that supports life. The crust surface is shaped by characteristics of the lithosphere that cause formations like mountains and fault lines.