The retina is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which translates that image into electrical neural impulses to the brain to create visual perception, the retina serving a function analogous to that of the film or image sensor in a camera; the neural retina consists of several layers of neurons interconnected by synapses, is supported by an outer layer of pigmented epithelial cells. The primary light-sensing cells in the retina are the photoreceptor cells, which are of two types: rods and cones. Rods function in dim light and provide black-and-white vision. Cones function in well-lit conditions and are responsible for the perception of colour, as well as high-acuity vision used for tasks such as reading. A third type of light-sensing cell, the photosensitive ganglion cell, is important for entrainment of circadian rhythms and reflexive responses such as the pupillary light reflex.
Light striking the retina initiates a cascade of chemical and electrical events that trigger nerve impulses that are sent to various visual centres of the brain through the fibres of the optic nerve. Neural signals from the rods and cones undergo processing by other neurons, whose output takes the form of action potentials in retinal ganglion cells whose axons form the optic nerve. Several important features of visual perception can be traced to the retinal encoding and processing of light. In vertebrate embryonic development, the retina and the optic nerve originate as outgrowths of the developing brain the embryonic diencephalon, it is the only part of the CNS. The vertebrate retina is inverted in the sense that the light sensing cells are in back of the retina, so that light has to pass through layers of neurons and capillaries before it reaches the rods and cones; the ganglion cells, whose axons form the optic nerve, are at the front of the retina. In this region there are no photoreceptors.
In contrast, in the cephalopod retina the photoreceptors are in front, with processing neurons and capillaries behind them. Because of this, cephalopods do not have a blind spot. Although the overlying neural tissue is transparent, the accompanying glial cells have been shown to act as fibre-optic channels to transport photons directly to the photoreceptors, light scattering does occur; some vertebrates, including humans, have an area of the central retina adapted for high-acuity vision. This area, termed the fovea centralis, is avascular, has minimal neural tissue in front of the photoreceptors, thereby minimizing light scattering; the cephalopods have a non-inverted retina, comparable in resolving power to the eyes of many vertebrates. Squid eyes do not have an analog of the vertebrate retinal pigment epithelium. Although their photoreceptors contain a protein, that recycles retinal and replicates one of the functions of the vertebrate RPE, one could argue that cephalopod photoreceptors are not maintained as well as in vertebrates and that, as a result, the useful lifetime of photoreceptors in invertebrates is much shorter than in vertebrates.
Having replaced stalk-eyes or retinae occurs. The cephalopod retina does not originate as an outgrowth of the brain, it is arguable that this difference shows that vertebrate and cephalopod eyes are not homologous but have evolved separately. From an evolutionary perspective, a more complex structure such as the inverted retina can come about as a consequence of two alternate processes: an advantageous "good" compromise between competing functional limitations, or as a historical maladaptive relic of the convoluted path of organ evolution and transformation. Vision is an important adaptation in higher vertebrates. A third view of the "inverted" vertebrate eye is that it combines two benefits: the maintenance of the photoreceptors mentioned above, the reduction in light intensity necessary to avoid blinding the photoreceptors, which are based on the sensitive eyes of the ancestors of modern hagfishes; the vertebrate retina has ten distinct layers. From closest to farthest from the vitreous body: Inner limiting membrane – basement membrane elaborated by Müller cells.
Nerve fibre layer – axons of the ganglion cell bodies. Ganglion cell layer – contains nuclei of ganglion cells, the axons of which become the optic nerve fibres, some displaced amacrine cells. Inner plexiform layer – contains the synapse between the bipolar cell axons and the dendrites of the ganglion and amacrine cells. Inner nuclear layer – contains the nuclei and surrounding cell bodies of the amacrine cells, bipolar cells, horizontal cells. Outer plexiform layer – projections of rods and cones ending in the rod spherule and cone pedicle, respectively; these make synapses with dendrites of horizontal cells. In the macular region, this is known as the Fiber layer of Henle. Outer nuclear layer – cell bodies of rods and cones. External limiting membrane – layer that separates the inner segment portions of the photoreceptors from their cell nuclei. Inner segment / outer segment layer – inner segments and outer segments of rods and cones; the outer segments contain a specialized light-sensing appar
Wallace Warren Cross was a medical doctor and long serving politician from Alberta, Canada. Cross was elected in the 1935 Alberta general election for the Alberta Social Credit Party for the Hand Hills district, he served 6 terms in office retiring from provincial politics in 1959. Cross became minister of health under Premier William Aberhart. During his time as minister, he opened a series of cancer clinics across the province, focused the Department of Healths resources on cancer treatment, he died in Edmonton in 1973. The Cross Cancer Institute, which opened in 1968, is named in his honor. Cross Cancer Institute History
John Noonan is a conservative national security commentator and analyst. He was the national security policy advisor to Jeb Bush during his presidential campaign, foreign policy advisor and speechwriter for Mitt Romney in 2012, he was a principal defense writer for The Weekly Standard and a policy director of the Foreign Policy Initiative. Before moving to the FPI, the Captain Noonan served in the United States Air Force's Global Strike Command as a Minuteman III launch officer. While still on active duty, he started a defense blog for Military.com called OPFOR. OPFOR was one of the first military blogs drawn on by Michael Goldfarb of the Weekly Standard to refute the war reporting of Scott Beauchamp in 2007; that year, Noonan joined the Standard as a defense stringer and blogger. He is the second military blogger, after the blog's Andrew Exum joined the Center for a New American Security, to assimilate into Washington's influential defense policy community. Noonan has drawn fire from liberal and progressive groups for his foreign policy positions on US nuclear weapon issues.
In June 2010, the position of Noonan and others on the Obama administration's Nuclear Posture Review was criticized by the Institute for Policy Studies. He assumed a hawkish posture on the 2008 South Ossetia War, calling the refusal of European nations to admit Georgia into NATO the result of "Chamberlain-esque conflict aversion."His practical experience working with nuclear weapons placed him in a point position for conservative opposition to President Obama's nuclear disarmament initiatives, as has his ability to project the outcome of certain geopolitical events such as the recent wars in Lebanon and Georgia. On the topic of nuclear deterrence, in 2010 he listed reasons why the Obama administration should modernize the US nuclear arsenal. Noonan was involved in the movement to recruit retired Marine Corps General James Mattis to be the 2016 Republican presidential nominee. In August 2016, Noonan wrote in an op-ed for the Los Angeles Times that Republican presidential candidate Donald Trump "cannot be trusted with weapons that can kill millions.
He cannot be handed the nuclear'football' – a briefcase containing the war plans and codes for our nuclear forces—and be made responsible for its contents." His editorial followed a series of posts on Twitter expressing the same sentiments. Noonan is one of several dozen Republican security officials who signed an open letter pledging not to support Trump for president. Stop Trump movement John Noonan on Twitter