Art in Uzbekistan Plan

Accomodations 
Plan: 
1. Accommodation 
2. Land based animals and the shape changing lens 
Accommodation is the process by which the vertebrate eye changes optical 
power to maintain a clear image or focus on an object as its distance varies. In this, 
distances vary for individuals from the far point—the maximum distance from the 
eye for which a clear image of an object can be seen, to the near point—the 
minimum distance for a clear image. Accommodation usually acts like a reflex, 
including part of the accommodation-vergence reflex, but it can also be 
consciously controlled. The main ways animals may change focus are: 
-Changing the shape of the lens. 
-Changing the position of the lens relative to the retina. 
-Changing the axial length of the eyeball. 
-Changing the shape of the cornea. 
Focusing the light scattered by objects in a three dimensional environment into a 
two dimensional collection of individual bright points of light requires the light to 
be bent. To get a good image of these points of light on a defined area requires a 
precise systematic bending of light called refraction. The real image formed from 
millions of these points of light is what animals see using their retinas. Very even 
systematic curvature of parts of the cornea and lens produces this systematic 
bending of light onto the retina. 
Due to the nature of optics the focused image on the retina is always inverted 
relative to the object. Different animals live in different environments having 
different refractive indexes involving water, air and often both. The eyes are 
therefor required to bend light different amounts leading to different mechanisms 
of focus being used in different environments. The air/cornea interface involves a 
larger difference in refractive index than hydrated structures within the eye. As a 
result, animals living in air have most of the bending of light achieved at the 
air/cornea interface with the lens being involved in finer focus of the image. 
Generally mammals, birds and reptiles living in air vary their eyes' optical power 
by subtly and precisely changing the shape of the elastic lens using the ciliary 
body. 
The small difference in refractive index between water and the hydrated cornea 
means fish and amphibians need to bend the light more using the internal structures 
of the eye. Therefore, eyes evolved in water have a mechanism involving changing 
the distance between a rigid rounder more refractive lens and the retina using less 
uniform muscles rather than subtly changing the shape of the lens itself using 
circularly arranged muscles.
[1]
 
Land based animals and the shape changing lens[edit] 
Varying forms of direct experimental proof outlined in this article show that most 
non-aquatic vertebrates achieve focus, at least in part, by changing the shapes of 
their lenses. 

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What is less well understood is how the subtle, precise and very quick changes in 
lens shape are made. 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 vertebrates, aspects of all models may play varying 
roles in lens focus. The models can be broadly divided into two camps. Those 
models that stress the importance of external forces acting on a more passively 
elastic lens and other models that include forces that may be generated by the lens 
internally. 
External forces[edit] 
The model of a shape changing lens of humans was proposed by Young in a 
lecture on the 27th Nov 1800.
[2]
 Others such as Helmholtz and Huxley refined the 
model in the mid-1800s explaining how the ciliary muscle contracts rounding the 
lens to focus near
[3]
 and this model was popularized by Helmholtz in 1909.
[4][5]
 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, stretching the ciliary body and relieving 
some of the tension on the suspensory 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 allowing the ciliarly body to spring back, 
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 has proposed a model for land based vertebrates that was not well 
received.
[7]
 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.
[8][9]
 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
[10]
 are relaxed to varying degrees by contracting the 
circular muscles.
[11]
 These multiple actions
[12]
 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.
[6]
 
The "catenary" model of lens focus proposed by Coleman
[13]
 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 precise fluid movement of the lens front only rather than trying to change 
the shape of the lens as a whole.

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