the lens exterior to the inner cells through many layers of cells. some vertebrates need to see well both above and below water at times. one example is diving birds which have the ability to change focus by 50 to 80 dioptres. compared with animals adapted for only one environment diving birds have a somewhat altered lens and cornea structure with focus mechanisms to allow for both environments. even among terrestrial animals the lens of primates such as humans is unusually flat going some way to explain why our vision, unlike diving birds, is particularly blurry under water. function focusing an image that is partially in focus, but mostly out of focus in varying degrees. eye and detailed ray path including one intraocular lens layer in humans the widely quoted helmholtz mechanism of focusing, also called accommodation, is often referred to as a "model". direct experimental proof of any lens model is necessarily difficult as the vertebrate lens is transparent and only functions well in the living animals. when considering all vertebrates aspects of all models may play varying roles in lens focus. external forces the model of a shape changing lens of humans was proposed by young in a lecture on the 27th nov 1800. others such as helmholtz and huxley refined the model in the mid 1800's explaining how the ciliary muscle contracts rounding the lens to focus near and this model was popularized by helmholtz in 1909. the model may be summarized like this. normally the lens is held under tension by its suspending ligaments being pulled tight by the pressure of the eyeball. at short focal distance the ciliary muscle contracts relieving some of the tension on the ligaments, allowing the lens to elastically round up a bit, increasing refractive power. changing focus to an object at a greater distance requires a thinner less curved lens. this is achieved by relaxing some of the sphincter like ciliary muscles. while not referenced this presumably allows the pressure in the eyeball to again expand it outwards, pulling harder on the lens making it less curved and thinner, so increasing the focal distance. there is a problem with the helmholtz model in that despite mathematical models being tried none has come close enough to working using only the helmholtz mechanisms. schachar model of lens focusschachar has proposed a model for land based vertebrates that was not well received. the theory allows mathematical modeling to more accurately reflect the way the lens focuses while also taking into account the complexities in the suspensory ligaments and the presence of radial as well as circular muscles in the ciliary body. in this model the ligaments may pull to varying degrees on the lens at the equator using the radial muscles while the ligaments offset from the equator to the front and back are relaxed to varying degrees by contracting the circular muscles. these multiple actions operating on the elastic lens allows it to change lens shape at the front more subtly. not only changing focus, but also correcting for lens aberrations that might otherwise result from the changing shape while better fitting mathematical modeling. the "catenary" model of lens focus proposed by coleman demands less tension on the ligaments suspending the lens. rather than the lens as a whole being stretched thinner for distance vision and allowed to relax for near focus, contraction of the circular ciliary muscles results in the lens having less hydrostatic pressure against its front. the lens front can then reform its shape between the suspensory ligaments in a similar way to a slack chain hanging between two poles might change it's curve when the poles are moved closer together. this model requires fluid movement of the lens front only rather than trying to change the shape of the lens as a whole. internal forces tracing of scheimpflug photographs of 20 year old human lens being thicker focusing near and thinner when focusing far. internal layering of the lens is also significantwrinkled lens fibers in picture below compared to straight fibers abovewhen thomas young proposed the changing of the human lens's shape as the mechanism for focal accommodation in 1801 he thought the lens may be a muscle capable of contraction. this type of model is termed intracapsular accommodation as it relies on activity within the lens. in a 1911 nobel lecture allvar gullstrand spoke on "how i found the intracapsular mechanism of accommodation" and this aspect of lens focusing continues to be investigated. young spent time searching for the nerves that could stimulate the lens to contract without success. since that time it has become clear the lens is not a simple muscle stimulated by a nerve so the 1909 helmholtz model took precedence. pre-twentieth century investigators did not have the benefit of many later discoveries and techniques. membrane proteins such as aquaporins which allow water to flow into and out of cells are the most abundant membrane protein in the lens. connexins which allow electrical coupling of cells are also prevalent. electron microscopy and immunofluorescent microscopy show fiber cells to be highly variable in structure and composition. magnetic resonance imaging confirms a layering in the lens that may allow for different refractive plans within it. the refractive index of human lens varies from approximately 1.406 in the central layers down to 1.386 in less dense layers of the lens. this index gradient enhances the optical power of the lens. as more is learned about mammalian lens structure from in situ scheimpflug photography, mri and physiological investigations it is becoming apparent the lens itself is not responding entirely passively to the surrounding ciliary muscle but may be able to change its overall refractive index through mechanisms involving water dynamics in the lens still to be clarified. the accompanying micrograph shows wrinkled fibers from a relaxed sheep lens after it is removed from the animal indicating shortening of the lens fibers during near focus accommodation. the age related changes in the human lens may also be related to changes in the water dynamics in the lens. lenses of birds, reptiles, amphibians, fish and others diving bird (cormorant) lens focusing can be up to 80 dioptres for clearer underwater vision.bony fish eye. note the spherical lens and muscle to pull the lens backwardin reptiles and birds, the ciliary body which supports the lens via suspensory ligaments also touches the lens with a number of pads on its inner surface. these pads compress and release the lens to modify its shape while focusing on objects at different distances; the suspensory ligaments usually perform this function in mammals. with vision in fish and amphibians, the lens is fixed in shape, and focusing is instead achieved by moving the lens forwards or backwards within the eye using a muscle called the retractor lentus. in cartilaginous fish, the suspensory ligaments are replaced by a membrane, including a small muscle at the underside of the lens. this muscle pulls the lens forward from its relaxed position when focusing on nearby objects. in teleosts, by contrast, a muscle projects from a vascular structure in the floor of the eye, called the falciform process, and serves to pull the lens backwards from the relaxed position to focus on distant objects. while amphibians move the lens forward, as do cartilaginous fish, the muscles involved are not similar in either type of animal. in frogs, there are two muscles, one above and one below the lens, while other amphibians have only the lower muscle. in the simplest vertebrates, the lampreys and hagfish, the lens is not attached to the outer surface of the eyeball at all. there is no aqueous humor in these fish, and the vitreous body simply presses the lens against the surface of the cornea. to focus its eyes, a lamprey flattens the cornea using muscles outside of the eye and pushes the lens backwards. while not vertebrate, brief mention is made here of the convergent evolution of vertebrate and molluscan eyes. the most complex molluscan eye is the cephalopod eye which is superficially similar structure and function to a vertebrate eye, including accommodation, while differing in basic ways such as having a two part lens and no cornea. the fundamental requirements of optics must be filled by all eyes with lenses using the tissues at their disposal so superficially eyes all tend to look similar. it is the way optical requirements are met using different cell types and structural mechanisms that varies among animals. crystallins and transparency graph showing optical density (od) of the human crystalline lens 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 lens. 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 lens fibers, thus increasing the index of refraction of the lens while maintaining its transparency. β and γ crystallins are found primarily in the lens, 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 lens proteins, which must last a human for their entire lifetime. another important factor in maintaining the transparency of the lens is the absence of light-scattering organelles such as the nucleus, endoplasmic reticulum, and mitochondria within the mature lens fibers. lens fibers also have a very extensive cytoskeleton that maintains the precise shape and packing of the lens fibers; disruptions/mutations in certain cytoskeletal elements can lead to the loss of transparency. the lens 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 lens epithelium. high intensity ultraviolet light can harm the retina, and artificial intraocular lenses are therefore manufactured to also block ultraviolet light. people lacking a lens (a condition known as aphakia) perceive ultraviolet light as whitish blue or whitish-violet. nourishment the lens is metabolically active and requires nourishment in order to maintain its growth and transparency. compared to other tissues in the eye, however, the lens has considerably lower energy demands. by nine weeks into human development, the lens 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. channels regulate lens transport.after regression of the hyaloid artery, the lens 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 lens 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 lens epithelium. the interaction of these pumps with water channels into cells called aquaporins, molecules less than 100 daltons in size among cells via gap junctions, and calcium using transporters/regulators (trpv channels) results in a flow of nutrients throughout the lens. glucose is the primary energy source for the lens. as mature lens 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 lens consumes very little oxygen. clinical significance cataracts are opacities of the lens. while some are small and do not require any treatment, others may be large enough to block light and obstruct vision. cataracts usually develop as the aging lens becomes more and more opaque, but cataracts can also form congenitally or after injury to the lens. nuclear sclerosis is a type of age-related cataract. diabetes is another risk factor for cataract. cataract surgery involves the removal of the lens and insertion of an artificial intraocular lens. presbyopia is the age-related loss of accommodation, which is marked by the inability