Accommodation An image that is partially in focus, but mostly out of focus in varying degrees. The<unk> is flexible and its curvature is controlled by ciliary muscles through the zonules. By changing the curvature of the<unk>  one can focus the eye on objects at different distances from it. This process is called accommodation. At short focal distance the ciliary muscle contracts, zonule fibers loosen, and the<unk> thickens, resulting in a rounder shape and thus higher refractive power. Changing focus to an object at a greater distance requires the relaxation of the<unk> and thus increasing the focal distance. The refractive index of human<unk> varies from approximately 1.406 in the central layers down to 1.386 in<unk> dense layers of the<unk>  This index gradient enhances the optical power of the<unk>  Aquatic animals must rely entirely on their<unk> for both focusing and to provide almost the entire refractive power of the eye as the water-cornea interface does not have a large enough difference in indices of refraction to provide significant refractive power. As such,<unk> s in aquatic eyes tend to be much rounder and harder. Crystallins and transparency optical density (OD) of the human crystalline<unk> for newborn, 30-year-old, and 65-year-old from wavelengths 300-1400 nm.Crystallins are water-soluble proteins that compose over 90% of the protein within the<unk>  The three main crystallin types found in the human eye are α-, β-, and γ-crystallins. Crystallins tend to form soluble, high-molecular weight aggregates that pack tightly in<unk> fibers, thus increasing the index of refraction of the<unk> while maintaining its transparency. β and γ crystallins are found primarily in the<unk>  while subunits of α -crystallin have been isolated from other parts of the eye and the body. α-crystallin proteins belong to a larger superfamily of molecular chaperone proteins, and so it is believed that the crystallin proteins were evolutionarily recruited from chaperone proteins for optical purposes. The chaperone functions of α-crystallin may also help maintain the<unk> proteins, which must last a human for their entire lifetime. Another important factor in maintaining the transparency of the<unk> is the absence of light-scattering organelles such as the nucleus, endoplasmic reticulum, and mitochondria within the mature<unk> fibers.<unk> fibers also have a very extensive cytoskeleton that maintains the precise shape and packing of the<unk> fibers; disruptions/mutations in certain cytoskeletal elements can lead to the loss of transparency. The<unk> blocks most ultraviolet light in the wavelength range of 300–400 nm; shorter wavelengths are blocked by the cornea. The pigment responsible for blocking the light is 3-hydroxykynurenine glucoside, a product of tryptophan catabolism in the<unk> epithelium. High intensity ultraviolet light can harm the retina, and artificial intraocular<unk> s are therefore manufactured to also block ultraviolet light. People lacking a<unk> (a condition known as aphakia) perceive ultraviolet light as whitish blue or whitish-violet. Nourishment The<unk> is metabolically active and requires nourishment in order to maintain its growth and transparency. Compared to other tissues in the eye, however, the<unk> has considerably lower energy demands. By nine weeks into human development, the<unk> is surrounded and nourished by a net of vessels, the tunica vasculosa lentis, which is derived from the hyaloid artery. Beginning in the fourth month of development, the hyaloid artery and its related vasculature begin to atrophy and completely disappear by birth. In the postnatal eye, Cloquet's canal marks the former location of the hyaloid artery. After regression of the hyaloid artery, the<unk> receives all its nourishment from the aqueous humor. Nutrients diffuse in and waste diffuses out through a constant flow of fluid from the anterior/posterior poles of the<unk> and out of the equatorial regions, a dynamic that is maintained by the Na+/K+-ATPase pumps located in the equatorially positioned cells of the<unk> epithelium. Glucose is the primary energy source for the<unk>  As mature<unk> fibers do not have mitochondria, approximately 80% of the glucose is metabolized via anaerobic metabolism. The remaining fraction of glucose is shunted primarily down the pentose phosphate pathway. The lack of aerobic respiration means that the<unk> consumes very little oxygen as well.
Lens 
Eye
Globe 
Optician
DermAtlas
Pituitary gland
Heart
Optic stalk