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.
Crustal Tectonic Plates and their movement. How the Earth's crust is split into large sections called tectonic plates is described. Their movement and effects at plate boundaries are explained e.g. earthquakes, volcanoes, mountain building, ocean ridges/trenches, subduction (part of the rock cycle).