THE LACRIMAL APPARATUS AND PRECORNEAL TEAR FILM 

Introduction

Throughout the Middle Ages, tears were thought to be waste products of the brain. It was not until about 50 years ago that Eugene Wolff described in clinical terms the basic concepts governing tear film and structure.
The precorneal tear film is produced and drained by the secretory and excretory lacrimal system, respectively. Eyelids play an important role in the distribution of the tear fluid by regulating the amount of tear fluid in the eye and spreading it all over the ocular surface.
Human tear film is a complex layer of fluid that covers the surface of the cornea. Its main function is to keep the corneal epithelium moist, so that the surface epithelial cells have a medium in which to live, thus preserving the most important refractive mechanism of the eye. A secondary function is to lubricate the front of the eye so that it moves freely beneath the eyelids.
About 90% of the film is made of an aqueous component most commonly known as tears. But there are two other vital components (lipid and mucus), so we often refer to the tear fluid as the three-layered precorneal tear film.

The lacrimal apparatus

The lacrimal apparatus is a group of structures that secretes tear fluid onto the eye, distributes it over the surface of the eye, then drains excess fluid out of the eye. The structures within the lacrimal apparatus may be differentiated according to their
function: 

• The secretory system consists of glands that produce the various components of the tear film
• The distribution system comprises the eyelids which spread the tear film over the surface of the eye during the blinking process
• The excretory system is made up of ducts that must effectively drain a certain amount of tear fluid from the ocular surface into the nasal cavity. 

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