Accommodation. Binocular vision.

Accommodation

Accommodation - ability of a human eye to enlarge the refracting force while translating a look from distant subjects on near that is to see good both afar, and close. A point of a visual axis on the minimum distance from which the eye can is still distinct distinguish any subject at the maximum strain of accommodation, it is accepted to name a near point of clear sight (punctum proximum). Hence, accommodation is an ability of an eye accurately to distinguish the subjects which are settling down between the further and nearest points of clear vision. It is possible to tell, that accommodation is provided a sharp image, that is clear definition of the subjects located more close of the further point of clear sight.

Accommodation mechanisms: during the moment translating to a look from distant subjects on the near there is a reduction of a ciliary muscle owing to what its diameter decreases, are relaxed Zinn ligaments, and the lens becomes more convex, that its refractivities are enlarged.

Eye refraction in a state of work of the accommodative apparatus name a dynamic clinical refraction.

At a difference distance from an eye of the further point of clear sight and the nearest point of clear sight it is possible to define in linear measures range, or length of accommodation for each eye. The accommodation volume (width, force) is characterised by a difference in refracting force of optical system of an eye. Position of the nearest point of clear sight corresponds to the maximum strain of accommodation. To define distance of this point from an eye it is possible, if until when there is appreciable its illegibility.

The accommodation volume in diopters is defined under the formula And = t/P - t/p = P-R, where p and r - refraction sizes in the diopters, corresponding to the nearest and further points of clear sight. The accommodation volume is peer to that increase to a refraction of an eye, which resulting the maximum strain of the accommodative apparatus of an eye, that is a difference between maximum dynamic (P) and static R refraction.

The accommodation defined for one eye, is called as absolute. If sight is carried out by two eyes, binocularly accommodation process is necessarily accompanied by convergence, the data of visual axes of eyes on fixed a subject. Such accommodation is characterised as relative. Accommodation and convergence at the human having an emmetropia, are usually made in parallel and in coordination.

That the human could work freely and longly at a short distance, it is necessary, that, except a spent strain of accommodation (a negative part of relative accommodation), remained in a store (a positive part) not less than half spent. If the accommodation store is small, in an operating time quickly there is a visual fatigue. With the years accommodative ability of an eye weakens.

So, in 20-30 years the nearest point of view is on distance approximately 10 see D = 1/F = 100 cm/10 cm = 10 diopters.

Thus, surveying subjects about 10 sm we enhance the refraction on 10 the human see Usually reads from distance in 25 sm: D = 1/F = 100 sm / of 25 sm = 4 diopters.

Gradually reduction of accommodative possibilities of an eye can be caused change of physical and chemical structure of a lens, pauperisation by its water, inspissation in connection with kernel formation (from 20 summer age), elasticity loss, myopia hypercorrection. Thereof the nearest point of clear sight gradually leaves from an eye. After 40 years this point is already on big enough distance and consequently for examining of fine subjects they should to be approached, and to remove from an eye all further and further. There is a so-called presbyopy, that is, senile (from Greek - presbys - old) a hyperopia.

So, in 50 years the nearest point of view is peer to 1 metre. Correction inventory:

Thus, selection of lenses can be carried out according to the formula:

Db = Dd + A-30/10, where Db - force of an orbicular lens for close (diopters), Dd - force of a lens, corrective sight afar (diopters), And - age of the patient in years.

With the years sight and a refraction, and ability to accommodation variates not, and illusion is only framed, that mope-eyed humans by an old age see better: depending on size of short-sightedness they put on points for work later or do not use them absolutely.

Binocular vision

Binocular vision is a sight two eyes provided that the image dropping on macular range in a cortex of a brain merges in a uniform cortical mode. Thanks to a binocular vision we define distance from a subject to a subject, volume, a relative positioning of subjects.

Newborns do not have associated locomotions of eyes, they appear only in 2-3 weeks, however the binocular vision still is not present. Binocular vision consider generated to 3-4 years, it is definitively established by 6-7 years. Thus, the preschool age on is most dangerous to development of disturbance of a binocular vision (squint formation).

Conditions for formation of a normal binocular vision:

• good optical apparatus (the transparent medium, rays of light should collect on a retina).
• good photoreceptive apparatus
• good muscular apparatus

It is possible to measure by one eye only approximate distance. At a binocular vision following mechanisms are used:

• that life experience - knowledge of sizes of subjects
• linear prospect - the further a subject, the it is less
• air prospect - the further the subject, the is more air layer - indistinct contours
• angle rate - a monocular parallax - for example, at driving in the car nearby subjects are carried by quickly, distant - slowly.
• the nearest subjects shield slowly
• light and shade allocation - convex parts more light
• while translating a view the cortex "calculates" distance

At a view afar there is a divergence (delution of visual axes), and at a view close - convergence (data of visual axes). The brain cortex depresses physiological doubling while translating a view on near subjects and on the contrary.

Any disorder of a binocular vision conducts to strabismus concomitans. Educes at children's age is more often, locomotion of eyes remains in full. A problem of oculists.

The paralytic squint - educes owing to a lesion of outside muscles of an eye, or their innervation at different levels: restriction towards the amazed muscle is always observed.

The method of treatment of a squint - pleontoortoptosurgical, includes:

1. Correction carrying out
2. Carrying out of special exercises for rising of visual acuity of a mowing eye
3. Surgical treatment to put axes it is correct
4. Training and binocular vision development