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.
conÂ·verÂ·gent plate boundary. A tectonic boundary where two plates are moving toward each other. If the two plates are of equal density, they usually push up against each other to form a mountain chain.If they are of unequal density, one plate usually sinks (subducts) beneath the other. Also called collision zone.See more at tectonic boundary.onÂ·verÂ·gent plate boundary. A tectonic boundary where two plates are moving toward each other. If the two plates are of equal density, they usually push up against each other to form a mountain chain.