It is the point where the fibers of the retina leave the eye and become part of the optic nerve. It is the only part of the retina that is insensitive to light. At its center the porus opticus marks the point of entrance of the central artery of the retina.
By Andrew Alden. The lithosphere is the brittle outer layer of the solid Earth. The plates of plate tectonics are segments of the lithosphere. The lithosphere is basically the layer of the Earth that is strong enough to flex rather than flow. Its top is easy to seeit's at the Earth's surfacebut the base of the lithosphere is a transition rather than a sharp boundary.
Report Abuse. 1  Tectonic plates are able to move because of the relative density of oceanic lithosphere and the relative weakness of the asthenosphere. Dissipation of heat from the mantle is acknowledged to be the original source of energy driving plate tectonics. 2  They float on a layer of rock called the Asthenosphere.