The eye (sensory apparatus) is a paired sensory organ that consists of the eyeball and the accessory organs. At the border of the brain and facial part of the head, the eye is embedded in the orbit: a bone cavity that serves to protect the eye from injuries.
It has the shape of an irregular sphere composed of several parts with various curvature radii. Two poles are differentiated on the surface: the anterior, which corresponds to the corneal top, and the posterior, which is situated near the exit of the optic nerve. Their connecting line – the geometric eye axis – is 24 mm in length. The eyeball wall is divided into three layers: fibrous exterior (surface), vascular central and photosensitive inner layer. The eyeball content represents transparent and clear structures that transmit light beams and refract them so that they fall onto the retina.
Cornea (from Latin)
The cornea is the most convexed part of the eye, resembling the glass of a watch, and is completely transparent. It occupies approximately 20% of the eyeball surface. Its thickness varies. It gets thinner from the edges (around 0.9 – 1.2 mm) to the center (around 0.4 – 0.7 mm). Due to its very delicate curvature, the cornea concentrates light beams that focus in the center of the eye. It takes around +40 dioptres out of the total optic capacity (+60dioptres). This number is constant throughout life. A number of nerve endings permeate the cornea. Therefore, it is very sensitive to the touch. It does not contain any vessels as it receives a majority of its oxygen supply from the surrounding air. The space behind the cornea is filled with other nutrients from the aqueous humor.
Sclera (from Latin)
The sclera is a continuation of the retina. However, this crossover is not continuous because both parts have different curves. The sclera is a firm, fibrous, milky-white membrane occupying around 80% of the eyeball surface. Its thickness is between 0.3 – 2 mm. It is composed of thick tissue that intertwines and forms a firm eye sheath (one of the firmest structures in the human body). Oculomotor muscles are attached to the sclera and it is permeated by the optic nerve at the back.
Choroid (lat. choroidea)
The choroid resembles a very thin red-brown membrane (around 0.2 – 0.4 mm) which is richly permeated by blood vessels. Its primary function is mainly to supply the retina. It consists also of a great number of pigment cells (melanin) which absorb redundant light falling into the eye and ensure darkness inside. Darkness is necessary for clear image formation on the retina. The choroid attaches to the sclera from the outer side and to the retina from the inner side. The frontal edge verges into the ciliary body.
Ciliary body (lat. corpus ciliare)
The ciliary body has a shape of muscular ring of tiny hairs attached to the frontal part of the sclera. It thickens towards the back and connects with the iris. Numerous projections protrude from the ciliac surface. These projections hold the lens by means of fibers. Muscle contractions enable the lens to change its shape and optic capacity. The projections also release aqueous humor into the posterior eye chamber. The humor supplies non-vascular eye parts, maintains its shape, and is involved in light refraction.
Iris (from Latin)
The iris, which recedes backwards into the ciliary body, has the shape of a circular target with an opening in the center – pupil (lat. pupila). In contrast to the choroid and ciliary body, it is not attached to the sclera but forms a sort of diaphragm between the posterior retinal wall and frontal lens wall. Thus, this space is divided into anterior and posterior chambers, both filled with aqueous humor.
The iris contains a layer of pigment cells (melanin) which permit light beams to enter the eye only through the retina. The amount and depth of pigment determine the color of the eye. Brown eyes contain more pigment, which is stored closer to the surface of the iris, blue eyes contain less pigment, stored deeper. Albinos, for example, have red eyes because their pigment is missing completely and only the iris and choroid blood vessels shine through. Eye color and design are different and unique in each person.
The iris is also composed of muscle cells organized in two directions – ray-like extending from the iris center, or ring-like widening or narrowing the pupil. Its diameter depends on the light intensity of the beams entering the eye. It can constrict up to 1 mm in bright light and dilate up to 8 mm in dim light. Such activity happens automatically and can be induced by very strong emotions as well. To sum up, the pupil behaves like a camera shutter and regulates the amount of light entering the eye.
Retina (from Latin)
The retina is a very delicate membrane with a thickness of around 0.2 – 0.4 mm. It lines the whole cavity of the eyeball, extending to the very edge of the iris – it attaches externally to the choroid and internally to the vitreous body. It consists of several cell layers – from a pigment layer to a sensory (photosensitive) layer, then to a bipolar layer which transmits impulses and finally to a nerve layer. The main function of the retina is to scan and pre-prepare light signals penetrating the lens.
The sensory cells absorb radiation and occur in the form of rods and cones. Rods, which number in the retina around 130 million, have an oblong shape. They are sensitive to light, and therefore are active in the semi-dark or dim light. However, they can differentiate only gray tones. On the contrary, cones, which number around 7 million, are receptors able to differentiate three basic colors – red, green, and blue. Combining these three colors, other colors and their details are generated. However, to do this more light is needed. Therefore, they are active mainly during the day. Photosensitive cells are not situated equally on the retina. The biggest cone concentration, around 100,000, is in the yellow spot (macula), which is the place of the sharpest vision. The picture that is observed by the eyes is projected onto the yellow spot. Starting from the yellow spot and moving outwards, the number of cones decreases and therefore at the edges of the retina we find almost exclusively rods. Such distribution allows subtle peripheral vision, but of course no detailed depiction. Another very important place is the blind spot, which does not contain any rods or cones (no image is formed here). All the nerve retinal fibers converge at the blind spot and exit the eyeball as one optic nerve transmitting impulses into the sensory center in the brain. Cones and rods are placed in the retina to achieve the best vision combinations possible in the daytime or at night.
Lens (lat. lens cristallina)
The lens is a very elastic transparent bioconvex organ (with a more curved posterior part) about 4 mm thin and 10 mm in diameter. It lies against the vitreous body at its back. Its frontal part is turned to the iris and is involved in posterior chamber demarcation.
The lens is principally a transparent, jelly-like substance situated in a thin, fibrous, elastic sheath. It hangs on several ciliac fibers through which the effects of the ciliary muscle are transmitted to the lens. The eye curvature is changed by its constrictions and thus the eye accommodates – adjusts to distances. Its main function is to adjust the focus point so that the eye sharpens the image of objects at various distances. While observing an object at a short distance the lens is domed – it is thicker and refracts the light more. On the contrary, while looking into the distance, around more than 6 m, the lens flattens (extends at the sides) and the refraction index decreases. The lens helps to refract light beams so that they convex on the retina and as a result much more detailed vision is provided. The optic capacity of the human lens is around 15 dioptres, which is approximately one quarter of the overall eye capacity.
Vitreous body (lat. corpus vitreum)
The vitreous body is a transparent, clear, colorless, jelly-like, and fibrous body that fills the inner eyeball cavity. Its frontal part is turned against the back of the lens and forms a socket-like space for the lens. The rest of the vitreous body leans towards the inner retina, forming almost a spherical shape. The function of the vitreous body is to keep the eyeball under pressure in order to maintain its spherical shape (similar to air in a ball). The vitreous body is formed only in the embryonic period and lacks any self-regenerative abilities. In case of injury, it is replaced by aqueous humor.
Eye chambers (lat. camerae bulbi)
There are two eye chambers inside of the eyeball. The back part of the retina together with the frontal iris part and the lens part in the area of pupil enclose the frontal eye chamber. The posterior chamber is a narrow, slit-like space that is restricted by the back iris, the frontal part of the lens and by the ciliary body. Both chambers inter-communicate via the pupil and they are filled by chamber liquid (aqueous humor), which is a thick, clear body liquid produced also by the ciliary body. The aqueous humor consists 99% of water, some ions, a small amount of carbohydrates, proteins and other substances. The chambers’ function is mainly to regulate pressure in the eye, nourish the surrounding tissues, drain harmful substances, and transport vitamin C into the cornea.
The accessory organs are the eyelids, conjunctiva, tear apparatus, oculomotor muscles, nerves, and vessels.
Eyelids (lat. palpebrae)
The eyelid is a skin prominence that is divided into an upper and lower part. The eyelid’s function is to protect the eye against injury, to polish the eye surface and to regulate beams of light approaching the retina. The outer eyelid part is covered by soft and sensitive skin. Beneath the skin an oculomotor muscle is situated which draws the eyelids nearer and closes the eye. The eyelids are able to move spontaneously or consciously at one’s own will. Their inner surface is covered by conjunctiva. Both eyelids join at the outer and inner corner, which consists of the tear canal where tears are released. The eyelid edges end in eyelashes, which have a similar function as the eyebrows, that is, they protect the eye from any foreign objects. The cerumen glands also lead to the eyelid edges and, if they become clogged as a result of inflammation, a sty appears.
Conjunctiva (lat. tunica conjunctiva)
The conjunctiva is a very thin, moist, and transparent mucosal membrane of dark pink color connecting the eyeball and the eyelid. It begins at the outer cornea edge covering the visible part of the outer sclera and fills the inner eyelid surface up to its edges. It is nourished by very tiny vessels that are almost invisible and contain many nerve endings. Thus, it is very sensitive to irritation. The narrow, slit-like space between the eye and the eyelid that covers the conjunctiva is called the conjunctiva sack. This is where the tear gland endings lead and transport the tears. The conjunctiva, due to its elasticity, enables smooth eye movement and produces mucosa, an important part of tear film.
The tear apparatus functions predominantly as a moisturiser of the frontal eye wall, as protection against infection, to maintain constant corneal temperature and to wash out the harmful substances. It functions as both a producer and transporter of tears. The place of tear production is composed of a tear gland situated above the external eye corner and of accessory tear glands that are placed in the upper and lower eyelid and in the conjunctiva. Tears produced here flow through a tear transporting passage (draining tear passages) into the tear sack and finally into the nasal cavity. Blinking helps tears to flow away. Increased tear production is triggered by foreign bodies in the eye but also by emotions (joy, sadness).
Oculomotor muscles (lat. musculi bulbi)
The oculomotor apparatus is composed of six outer oculomotor muscles (four linear and two transversal) situated in the orbit and attached to the sclera. They move the eyeball vertically, horizontally, linearly and transversally. Their functioning is regulated by the oculomotor centers situated in the medulla oblongata and in the central part of the brain. The muscles move the eyeball so that the image of observed objects project on the yellow spot in the retina. When gazing, both eyes look the same direction. In case of muscle coordination failure, squint (strabismus) appears.
Optic nerve (nervus opticus)
The optic nerve is a paired head nerve that transmits sensory information from the retina to the sensory centers in the brain. It exits the eyeball at the blind spot, continues further back and through a bone tunnel (optic canal) enters the skull cavity just beneath the brain’s pituitary region. There it also joins to the second optic nerve. Along the optic nerve line runs the central retinal artery, which after entering the eye divides into the tiny capillaries nourishing the retina. Next to the retinal artery lies a corresponding vein that carries deoxygenated blood from the retina.