En inglés

What is the eye like, and how do we see

Anatomy and Physiology of the Human eye

by Juan Cuesta
Traslated by J.G. Provencio

	In order to explain what the more usual visual problems consist of, it is essential a general knowledge of the anatomy and physiology of the human eye and the general visual system, too.
	To outline, we could say that visual apparatus consists of: EYEBALL, OPTICAL PATHWAYS AND ACCESORY STRUCTURES
	THE EYEBALL has three layers of tissue and three chambers.
	The layers are: FIBROUS TUNIC, VASCULAR TUNIC and RETINA.
	Chambers are: anterior and posterior chambers and vitreous cavity
	The fibrous tunic is the outermost layer and it is formed by the SCLERA and the CORNEA.
	The SCLERA is the fibrous portion that is the "white portion" of the eye, and its function is protect. The outer surface is covered with a cleat epithelium called bulbar conjunctiva, whose irritation leads to usual and well-known pink -eye (conjunctivitis). The also well-known "nails" problems of the conjunctiva.
	The CORNEA is the clear portion of the outer layer; it is the "optic-window" , and its function is, of course, an optic one.
	The VASCULAR TUNIC is the middle-layer and consists of three portions: The CHOROID, that lays on the posterior wall and has both functions: nutritive and acting as a tanned screen (to avoid light coming into the eye using "bad" pathways.) The CILIARY BODY, in the middle zone, is formed by the CILIARY PROCESS (which is in charge of secreting the liquid that fulfils the anterior chamber. This liquid is known as AQUEUOUS HUMOR and the CILIARY MUSCLE; the last one changes the shape of the lens. This allows to focus at different distances. Finally, the third portion is the IRIS, laying on the anterior area (this is the coloured part of the eye), has the function of regulating the amount of light entering the eye. This is accomplished by changing the size depending on the light intensity.
	Just after the iris, and joined to the ciliary muscles we find the LENS. It is oval shaped and can change its shape, and thus the dioptre power, through the action of ciliary muscles. This is what allows to focus at different distances from the eye.
	The RETINA is the most sensitive layer of visual apparatus. It is here where images should be formed in order to be able to see them properly. Its anterior layer is blind, and sensitivity grows when going away the anterior layer. The highest sensitive point is a small depression called FOVEA CENTRALIS, where the higher amount of sensitive cells is concentrated. These cells are the CONES RODS. On the posterior layer there is a blind area where optic nerve connects ; this part is called the OPTIC DISK(or BLIND SPOT).
	As far as the chambers are concerned, we can say they are three: ANTERIOR, POSTERIOR and VITREOUS.
	ANTERIOR chamber is the area between the cornea and the iris. It is plenty of AQUEOUS HUMOR, a clear liquid produced by the ciliary processes. This liquid empties between the angle formed by the iris and the cornea. Problems with the emptying process lead to a increase of intraocular pressure and produce the terrible glaucoma.
	The POSTERIOR chamber contains aqueous humor and it is the layer that extends between the iris and the lens. It is here where the ciliary processes are.
	VITREOUS cavity is the layer between the lens and the retina; it is plenty of a clear gel and not-vascular called VITROUS HUMOR .
	OPTIC PATHWAYS allow the nervous impulse transmission from the retina to cerebral cortex. This is held trough the optic nerve. Receptor-cells are the already named cones and rods which transmit the received images into nervous impulses; These are carried to the cerebral following the named nerve.
	The ACCESORIES of the visual apparatus are: THE OCULO-MOTOR SYSTEM, consisting of six external muscles which make the eyeball mobility; THE PROTECTION SYSTEM consisting of THE EYE SOCKET, EYELIDS, CONJUNCTIVE, TEARS, LACRIMAL DUCTS and LACRIMAL GLANDS.
	THE ACT OF SEEING consists of 4 stages: 1.- Formation of the image on the retina using the optic system (cornea, aqueous humor, lens and vitreous humor) 2.- Origin of the nervous stream that leads to: 3.- The relaying of the nervous impulse trough the optic nerve. 4.- Interpretation of the nervous impulse on the cerebral cortex.
	But formation of the image on the retina isn’t a simple process nor a static one either.
	A normal eye, focused at infinite (from 5 metros more or less) is in a resting state. Apart from the possible iris contraction(to regulate the amount of light—like the diaphragm in a photographic camera), the other dynamic part of the optic system, i.e. the lens, is resting too. That is, the human eye doesn’t need any additional effort to have a proper far sight.
	Of course, a focused at infinite eye, if anything changing in its optic system, won’t see clearly at a near distance. The same happens if we focus a camera at a distance but we take the photo of a nearest one. The photo won’t be clear.
	And the thing that changes is the lens thickness. When focusing at a near distance is needed, ciliary muscles come to action and cause the lens to be thicker, boosting the lens power (in the end it is just a biconvex lens). This gets the correct focus. This mechanism is called accommodation and it is the failure which produces the presbiocya, or "AGING SIGHT". We’ll deal with this topic in other page of this web.