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...
Earth's mantle is the thickest part of the planet and consists of very hot and plastic like solid rock. The mantle is divided into the lower and upper mantle, with the upper mantle making up 15.3 percent of the mantle crust mass and 10.3 percent of Earth's total mass. Learn more about Layers of the Earth