The visual system includes the eyes. They provide animals eyesight, the capacity to receive and interpret visual information, and the capacity to perform many photo response functions that are not dependent on vision. Light is detected by the eyes and converted into electrochemical impulses in the neurons. The eye is a complex optical system in higher organisms that collects light from the environment, regulates its intensity through a diaphragm, focuses it through an adjustable assembly of lenses to form an image, converts this image into a set of electrical signals, and transmits these signals to the brain via complex neural pathways that connect the eye to the brain via the optic nerve. There are 10 essentially diverse types of eyes with resolving capacity, and 96 percent of animal species have a complex optical system. Molluscs, chordates, and arthropods all have image-resolving eyes.
Pit eyes are the most basic eyes. They are eye-spots that are placed in a pit to restrict the angles of light that enter and effect the eye-spot, allowing the organism to determine the angle of incoming light. Retinal photosensitive ganglion cells provide signals to the suprachiasmatic nuclei to modify the circadian clock and to the pretectal region to govern the pupillary light reflex from more sophisticated eyes through the retinohypothalamic tract.
The ciliary body has a triangle horizontal portion and is covered by the ciliary epithelium, a double layer. The inner layer is translucent and extends from the retina’s neural tissue to cover the vitreous body. The cells of the dilator muscle are made up of the outer layer, which is highly pigmented and continuous with the retinal pigment epithelium.
The vitreous is a clear, colorless gelatinous material that fills the gap between the eye’s lens and the retina, which lines the back of the eye. It’s made by a subset of retinal cells. It has a similar composition to the cornea, but it contains very few cells (mostly phagocytes that remove unwanted cellular debris in the visual field, as well as the hyalocytes of Balazs of the vitreous surface, which reprocess hyaluronic acid), no blood vessels, and 98–99 percent of its volume is water (as opposed to 75 percent in the cornea), salts, sugars, and vitrosin (a type of vitreous protein With so little solid content, it manages to keep the eye tight.
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