Eye

Anatomy of the Eye

The eyeball is located in the orbital cavity, which contains and protects it. It is a pyramid-shaped bone structure with posterior apex and anterior base.
The wall of the bulb is made up of three concentric tunics which, from the outside towards the inside, are:

1. External (fibrous) tunic: formed by the sclera and cornea
2. Medium (vascular) tunic also called uvea: formed by the choroid, the ciliary body and the lens.
3. Inner (nerve) cassock: the retina.

The external tunic acts as an attachment for the extrinsic muscles of the eyeball, ie those which allow its rotation downwards and upwards, to the right and left and obliquely, towards the inside and the outside.
In its five posterior sixths it is formed by the sclera, which is a membrane resistant and opaque to light rays, and in its anterior sixth by the cornea, which is a transparent structure devoid of blood vessels, and which is therefore nourished by those of the sclera. The cornea is made up of five superimposed layers, of which the outermost one is made up of epithelial cells arranged in several superimposed layers (multilayered epithelium); the underlying three layers are made up of connective tissue and the last, the fifth, again from epithelial cells but in a single layer, called the endothelium.
The media or uvea is a membrane of connective tissue (collagen) rich in vessels and pigment and is interposed between the sclera and retina. It supports and nourishes the layers of the retina that are in contact with it. It is divided, from "forward to" backward, into iris, ciliary body and choroid.

The iris is the structure that typically carries the color of our eyes. It is in direct contact with the lens and has a central hole, the pupil, through which the light rays pass.
The ciliary body is posterior to the iris and is lined internally by a portion of the retina called "blind" because it does not contain any photoreceptor and therefore does not participate in vision.
The choroid is a support for the retina and is very vascularized, precisely to nourish the retinal epithelium. It is rust-brown in color, due to the presence of a pigment that absorbs the light rays, preventing their reflection on the sclera.
The inner tunic is formed by the retina. It extends from the point of emergence of the optic nerve to the pupillary margin of the iris. It is a thin transparent film made up of ten layers of nerve cells (full-fledged neurons), including, in its non-blind portion - called the optic retina. - the cones and rods, which are the photoreceptors responsible for the visual function.
There are more rods than cones (about 75 million) and contain a single type of pigment. This is why they are deputed to twilight vision, that is, they see only in black and white.
The cones are fewer in number (about 3 million) and are used for the distinct vision of colors, containing three different types of pigment. Almost all of them are concentrated in the central fovea, which is an ellipse-shaped area that coincides with the posterior end of the optic axis (the line that passes through the center of the eyeball). It represents the seat of distinct vision.
The nerve extensions of the cones and rods all join together in another very important portion of the retina, which is the optic disc. It is defined as the point of emergence of the optic nerve (which carries visual information to the cerebral cortex, which in turn re-elaborates it and allows us to see the images), but also of the artery and central vein of the retina. The papilla is not covered by retina, it is blind.

Physiology of optics

Light is a form of radiant energy that allows the vision of the objects around us.
In a transparent medium, the light has a straight path; by convention (for established) it is said that it travels in the form of rays.
A beam of rays can consist of converging, diverging or parallel rays. The rays coming from infinity, which in optics are considered starting from a distance of 6 meters, are called parallel. The point where the converging or diverging rays meet is called fire.
When a beam of light meets an object there are two possibilities:

1. It will suffer the phenomenon of refraction, typical of transparent objects. The rays pass through the object undergoing a deviation that will depend on the refractive index of the object in question (which in turn depends on the density of the matter of which the same object is formed) and on the angle of incidence (angle formed by the direction of the light beam with the perpendicular to the surface of the object).
2. It will suffer the phenomenon of reflection, typical of opaque bodies: the rays do not cross the object but are reflected.

Spherical lenses are transparent means delimited by spherical surfaces, which can be concave or convex and which represent spherical caps. The ideal center of the sphere of which the surfaces are part is called the center of curvature, the radius of the sphere is called the radius of curvature, the ideal line that joins the two centers of curvature of the lens surfaces is called the optical axis.
The spherical surfaces of the lens can be convex or concave; they have the ability to measure the direction of the light rays (vergence) that pass through them.
In a convergent system, parallel rays, that is, coming from a luminous point placed at infinity, will be refracted posteriorly on the optical axis at a distance from the vertex of the lens correlated to the radius of curvature and to the refractive index of the same lens. bright point from infinity towards the lens (distance less than 6 meters), the rays will reach it no longer parallel but divergent. The rear focus tends to move away in proportion to the increase in the angle of incidence. As you progress in the approach of the light point to the lens, you will reach a position in which, by increasing the angle of incidence, the rays will emerge parallel. For further approaches of the luminous point, the rays will emerge divergent and their focus will be virtual, being on the extensions of the same rays.

Convex lenses induce a vergence positive, that is, they make the light rays that cross them converge towards a point called focus, enlarging the image. This is why they are called positive spherical lenses. The focus of these rays is real.
Concave lenses induce a vergence negative, that is, they make the light rays that cross them diverge, decreasing the size of the observed image. This is why they are called negative spherical lenses. The focus of these rays is virtual and can be identified by extending the rays emerging from the lens backwards.

The power of the lenses, that is the amount of convergence or divergence induced by a given diopter (the lens), is called dioptric power and its unit of measurement is the diopter. It corresponds to the inverse of the focal distance expressed in meters. , According to the law

d = 1 / f

where d is the diopter and f is the focus. Therefore one diopter is one meter.

For example, if the focus is 10 centimeters, the diopter is 10; if the focus is one meter, the diopter will be one. The smaller the focus, the greater the dioptric power, that is, the smaller the distance, the more convergence increases.

The fundamental property of the eye is the ability to modify its characteristics according to the observed object, so that its image always falls on the retina. For this reason the eye is considered as a compound diopter, made up of several surfaces. The first separation surface is the cornea, the second is the lens. They form a converging lens system.
The cornea has a very high dioptric power, equal to about 40 diopters. This value is explained by the fact that the difference between its refractive index and that of air is very high. Under water, on the other hand, we do not see each other because the refractive index of cornea and water are very similar, so the focus is not on the retina but far beyond it.
The pupillary foramen has a diameter of about 4 millimeters, it widens when the brightness of the environment decreases and narrows when it increases. The average length of the eyeball is 24 millimeters, and it is the length that allows the parallel rays that cross the lens to be focused on the retina, which suggests that a greater or lesser length of the bulb causes visual defects.
That said, we can say that in a normal eye (emmetrope) the rays coming from infinity (from 6 meters onwards) fall exactly on the retina. In order to have emmetropia, therefore, there must be a correct relationship between the ocular dioptric power and the length of the bulb. When this does not happen, the eye is said ametrope and we have the vices of refraction that cause the most common sight defects.