REFRACTION AND ACCOMMODATION OF THE EYE

The anomalies of eye refraction are extremely widespread. Espe­cially important problem of ophthalmology is nearsightedness (myo­pia). In different areas of our country the frequency of this disease among the schoolboys composes 3.5-33%. A high nearsightedness — the most often reason of physical inability owing to eye diseases. All this puts the struggle with myopia on a level of the state problem and indicates necessity of realization of active and broad measures of pre­vention of this disease and its complications, that should be realized not only by oculists, but also by doctors of a general profile.

The refraction is a refractive ability of any optic system (refractivity). A refraction of the eye is its refractivity in the condi­tion of accommodation, i.e. refractive force of the eye owing to its anatomic constitution.

OPTIC SYSTEM OF THE EYE

The refractive apparatus of the eye consists of the cornea, aque­ous humor (liquid of the anterior chamber), lens and vitreous. The refractive apparatus of the eye consists of a few refractive surfaces and environments and represents a difficult compound optic system.

The refraction force of any optic system is measured in diopters. The unit of the refractive force is the force of the lens with a focal length of 1 m. The less a focal length, the more the force of lens.

For example, the force of lens with a focal length of 50 cm amounts to two diopters; the force of lens with a focal length of 25 cm amounts to four diopters etc.

So for the aim to calculate refractive force of the eye optic sys­tem, it is necessary to know a focal length of this system. The fo­cal length is calculated on the basis of optic values.

The optic values consist of: 1) curvature radiuses of the cornea and lens surfaces; 2) factors of environments refraction; 3) distances between refractive surfaces, i.e. between the cornea and anterior surface of the lens between anterior and posterior surfaces of the lens. Knowing these values, it is possible to calculate a position of cardinal points, i.e. two main points, two nodular and two main focuses, and then focal length (these concepts are known for you from a course of physics).

The optic values are determined with the help of very delicate and sometimes very difficult methods of investigation. As these va­lues have personal oscillations in this connection the mean values are obtained as a result of measurements of series of eyes. Such an eye with mean optic indices is named a schematic eye.

The schematic eye of Gullstrand is most widespread, it consists of six refractive surfaces: the anterior and posterior surfaces of the cornea, anterior surface of the lens, anterior and posterior surfac­es of the lens nucleus, posterior surface of the lens. They divide seven environments: air, the cornea, fluid, anterior and posterior cortical layers of the lens and its nucleus and vitreous. The refrac­tive force of a schematic Gullstrand's eye is 58.64 D. 43.05 D be­long to the cornea and 19.11 D — to lens in accommodation's rest. The schematic eye is used for the solution of many problems of physiologic optics but in the whole number of cases the calcula­tion of the date which is necessary for the clinical purposes is pos­sible with the help of the most simplified scheme of the optic eye model. It is named as the reduced eye in which there is one aver­age index of refraction, one refractive surface and one main plane. The reduced eye of Verbitski is the perfect model. The constants of this eye include the index of refraction — 1.4; the radius of re­fractive surface curvature — 6.8 mm; radius of retina surface — 10.2 mm; length of eye axis — 23.4 mm.

Last years the simplified schemes of optic indices calculation gain large practical value for definition of focal point of the optic eye system at opticoreconstractive operations.

Date: 2015-01-11; view: 1267