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.
These boundaries mark the collision between two of the planet's tectonic plates. The plates are pieces of crust that slowly move across the planet's surface over millions of years. Where two tectonic plates meet at a subduction zone, one bends and slides underneath the other, curving down into the mantle.(The mantle is the hotter layer under the crust.). Tectonic plates can transport both continental crust and oceanic crust, or they may be made of only one kind of crust.Oceanic crust is denser than continental crust. At a subduction zone, the oceanic crust usually sinks into the mantle beneath lighter continental crust.he plates are pieces of crust that slowly move across the planet's surface over millions of years. Where two tectonic plates meet at a subduction zone, one bends and slides underneath the other, curving down into the mantle.