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...
Earthâs interior is divided into three concentric layers;Crust, mantle and core. Crust is the thinnest and outermost layer, mantle middleone whereas core is the innermost and the most dense layer of the earth. Thematerial of the crust is composed of rocks. Rock is composed of one or moreminerals.