Anatomy and physiology of the eye

The orbit is a bony cavity or depression in which the eyeball is located. It consists of seven small bones that protect the eye from physical damage.

Adjacent organs — The tissues surrounding the eye, including the eyelashes and eyelids, are called the adjacent organs.

Eyelashes consist of several rows of short vellus hairs located along the edge of the eyelids and perform the following functions:

• by trapping air dust particles, eyelashes prevent eye clogging;
• promote rapid closure of eyelids when there is a threat of damage.

Eashlashes are characterized by natural growth and loss. New eyelashes grow quickly.

Each eyelid consists of a movable sheet of fibrous tissue that covers and opens the front of the eyeball. The outside of the eyelids is covered with skin, and the inside is covered with skin. mucous membrane. By distributing the tear fluid secreted by the lacrimal glands, the eyelids protect the eye from any foreign substances that can damage the cornea. There are also lacrimal openings on the eyelids through which tears are drained.

Both eyelids are covered by a sensitive membrane called the conjunctiva. The palpebral conjunctiva lines the inner surface of the eyelids and becomes continuous with the bulbar conjunctiva. The bulbar conjunctiva is located at the front of the eyeball (the visible white part of the eye) and is in contact with the surface of the palpebral conjunctiva.

Since the conjunctiva — continuous membrane, contact lenses cannot be behind it, that is, behind the eye. The conjunctiva is in constant direct interaction with the lenses, so successful contact lens use requires a healthy conjunctiva. When the eye is irritated, the blood vessels of the conjunctiva dilate and the eye turns red. The conjunctiva contains glands that promote the formation of the tear film to moisturize the surface of the eye.

The layer of water and nutrients on the cornea is called the tear film. This film is constantly produced and removed from the surface of the eye. The lacrimal glands of the eyelids participate in its formation, the outflow occurs through the lacrimal openings into the nasolacrimal duct and further into the nose. The tear film provides nutrition to the cornea in the form of oxygen, glucose, salts and minerals. It moisturizes the cornea and eyelids, preventing them from drying out, and also acts as a lubricant, facilitating the sliding of the eyelids along the cornea. Foreign particles dissolve in the tear film and are removed from the surface of the eye. Dangerous microorganisms are exposed to antibacterial enzymes (lysozymes) in tears. The tear film consists of three layers: an outer lipid (fatty) layer, a central aqueous layer and an inner mucin layer, which facilitates the connection of the film with the cornea.

Lipid layer — a fatty film that prevents the tear film from drying out. The aqueous layer is a slightly alkaline solution (pH 7.35), consisting of water (98%), minerals and nutrients, enzymes, (ions), dissolved salts and proteins. This layer, which makes up up to 90% of the thickness of the tear film, provides the cornea with oxygen and nutrients. The mucin (mucous) layer connects the tear film and the corneal epithelium.

An important factor in adaptation to contact lenses is the chemical structure of the tear film, since the lenses rest on it. Typically the film thickness is 7 microns. The average volume of tear fluid in the eye — 6 µl. 10-20 seconds would be enough for all the tear film to evaporate, but we involuntarily blink every 5-10 seconds and restore the film.

A patient with disruption of any layer of the tear film is unsuitable for wearing contact lenses. The condition of the tear film can be assessed in two ways: the Schirmer test uses strips of paper to indirectly determine the amount of aqueous layer of the film; the disintegration time test (TBT) shows the time during which the tear film evaporates from the surface of the cornea (normally - 10-15 seconds) .

Cornea — a transparent, avascular, highly sensitive, multilayered, dome-shaped membrane that borders the anterior chamber of the eye. Convex in the center, the cornea flattens at the junction with the sclera, which is called the limbus. The changing curvature of the cornea makes measuring and fitting contact lenses difficult. Therefore, contact lenses usually have different curvatures: central peripheral.