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.
At this type of boundary, the only effects are earthquakes. The sliding process is not smooth and moves in fits and starts. The pressure for the plate to move builds over time, and the rocks compress and strain. At some point, the stored energy is released as the plates move, which can be several feet at one time.