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...
Which type of rock makes up most of the Earth's crust and why? The type of rock that makes up most of the Earth's crust is igneous. It is the most common rock type because it has been forming billions of years ago, along with most for â¦ mations on Earth. Is the oceanic crust dense?