Anatomy of the Human Eye

Eng.Mohammed ShurrabBSc. Medical Engineering

Overview

Additional Structures Related to the Eye

Inner Structures1. Fibrous Tunic2. Vascular Tunic3. Nervous Tunic4. Ocular Refractive Medium5. Interior of the Eye Ball

Muscle Innervations of the eye.

Eye lids or Palpebrae are thin and moveable folds protecting the eyes from injury by rapid closure. These are skin tissues consisting of subcutaneous connective tissue, Orbicularis Oculi muscle, tarsal plate and tarsal glands (Meibomian gand) secreting oily substance for lubrication and innermost mucosal conjunctiva tissue.

Related Structures

The tarsal plate is thin and elongated made up of firm and dense fibrous tissue which determines the eye lid form. The tarsal glands are yellow and arranged in a single row of about 25 in the upper lid and fewer in the lower lid. These glands are modified sebaceous glands, secreting oily secretions that spread over the margins of the eyelids, reducing evaporation and contributing to tear film stability and inhibiting the overflowing of tears on the face.

Related Structures

Eye lids

Blinking: is to spread the tear film over the cornea.

12 Blinks/min Blink lasts in average 0.3 of second

Related Structures

Palpebral Fissure

30 mm wide

15 mm high

Eye lashes: Hair follicles growing at the end of the eye lids, protecting the eyeballs from foreign bodies, perspiration and direct sun rays. Pairs of ciliary sebaceous glands and ciliary sweat glands open into the eyelash follicles.

Eye brows: located above the eyes and play protective role similarly to the eye lashes.

Related Structures

Conjunctiva: A transparent and mucosal membrane that extends from the junction of the cornea and the Sclera to the inner portion of the eyelid. There are two parts: Bulbar conjunctiva that covers the Sclera loosely connected to the eyeball and the Palpebral conjunctiva that lines the inner surface of the upper and lower lids. The Bulbar conjunctiva is thin and transparent and slightly vascular. Palpebral conjunctiva is very vascular and dense subepithelial layer of capillaries, and contains mucosa-associated lymphoid tissue, it is continuous with the skin, lining epithelium of the ducts of the tarsal glands, and with the lacrimal canaliculi, sac and hence the nasolacrimal duct and nasal mucosa.

It helps lubricate the eye by producing mucus and tears, although a smaller volume of tears than the lacrimal gland.[1] It also contributes to immune surveillance and helps to prevent the entrance of microbes into the eye.

Related Structures

Related Structures

Palpebral

Bulbar

The lacrimal apparatus is the physiologic system containing the orbital structures for tear production and drainageIt consists of:

(a) the lacrimal gland, which secretes the tears, and its excretory ducts, which convey the fluid to the surface of the eye;

(b) the lacrimal canaliculi, the lacrimal sac, and the nasolacrimal duct, by which the fluid is conveyed into the cavity of the nose, emptying anterioinferiorly to the inferior nasal conchae at the nasolacrimal duct.

(c) the nerve supply of lacrimal apparatus done by carotid plexuse of nerves along artery internal and external sympathetically but parasympathetic from lacrimal nucleus of the facial nerve

Related Structures

Name Containers Secretors Function

Lipid Layer oils

Mebomian glands (or tarsal glands)

coats the aqueous layer; provides a hydrophobic barrier that evaporates and prevents tears from spilling onto the cheek. These glands are found among the tarsal plates.

Thus, the tear fluid deposits between the eye proper and oil barriers of

the lids.

Aqueous layer

water and other substances such as proteins (e.g.,

tear lipocalin, lactoferrin,

lysozyme and lacritin)

Lacrimal Gland

promotes spreading of the tear film; promotes the control of infectious

agents; promotes osmotic regulation

Mucous layer mucinconjunctiv

al goblet cells

coats the cornea;provides a hydrophilic layer;allows

for even distribution of the tear film; covers the

cornea

1

2

3

OUTER

Inner

Inner Structures: The Eyeball of diameter about 1 inch is divided

into three parts: Fibrous Tunic, Vascular Tunic and Nervous Tunic (Retina).

Fibrous Tunic: Outer covering of the eye made up of anterior Cornea and posterior Sclera.

Vascular Tunic: Middle layer of the eye, made up of 3 portions: Choroid, Ciliary Body and Iris

Nervous Tunic (Retina) Ocular Refractive Medium (Lens)

Inner Structures

Fibrous Tunic

The cornea is the outer most layer of the Fibrous tunic in the eyeball. It is a transparent, dome shaped, non-vascularized layer, made up of highly organized group of cells and collagen fibers. The cornea must remain transparent to refract light properly; in fact the cornea refracts almost 75% of the light entering the eye due to its convex structure. Consequently for better vision, the layers of the cornea must be free of any cloudy or opaque area. The Cornea’s dome shaped structure is what makes it thicker at the edges (1mm) and thinner at the center (0.5 mm), the cornea is 78% water be weight.

The corneal tissue is arranged in five basic layers, each having an important function. These five layers are:

Fibrous Tunic

1

2

3

4

5

The epithelium is the cornea's outermost region, comprising about 10 %of the tissue's thickness. It is comprised of 5-7 layers of epithelial cells. The outermost cells are squamous cells followed by wing cells, basal cells and bordered by the basal membrane that serves as a foundation for the epithelial cells. The corneal epithelium replaces itself about once a week by the constant multiplication of the basal cells forming new wing cells that migrate upwards becoming squamous cells and ultimately lose their attachments and slough off into the tear film.

Functions: A. to block the passage of foreign material, such as dust, water,

and bacteria, into the eye and other layers of the cornea. B. to provide a smooth surface that absorbs oxygen and cell

nutrients from tears, then distributes these nutrients to the rest of the cornea.

C. to prevent the flow of the fluid and electrolytes from the tears to the stroma, keeping the stroma dehydrated.

Fibrous Tunic

Bowman’s Membrane Lying directly below the basement membrane of the epithelium is a transparent acellular sheet of tissue known as Bowman's layer. It is a condensation of the superficial stroma, composed of strong layered protein fibers called collagen. Once injured, Bowman's layer cannot be replaced and forms a scar as it heals. If these scars are large and centrally located, some vision loss can occur. It has some tiny perforations that permit the passage of corneal nerves to the epithelium.

Fibrous Tunic

Beneath Bowman's layer is the stroma, which comprises about 90% of the cornea's thickness. It consists primarily of water (78%) and collagen (16 %), keratocytes and does not contain any blood vessels. Collagen gives the cornea its strength, elasticity, and form. The collagen's unique shape, arrangement, and spacing are essential in producing the cornea's light-conducting transparency. The space not occupied by the fibrils and cells is filled with ground substance made up of keratin sulfate and chondroitin sulfate. In swollen cornea, the volume of ground substances increases. Other cells that might be present in the cornea in some abnormal conditions are the lymphocytes, histiocytes and occasionally the polymorphonuclear leukocytes.

Fibrous Tunic

Under the stroma is Descemet's membrane, a thin but strong sheet of tissue that serves as a protective barrier against infection and injuries. The anterior portion of the Descemet's membrane adherent to the stroma is fibrous and composed of collagen fibers (different from those of the stroma) while the posterior end, adjacent to the endothelial cells and is made by the endothelial cells that lie below it. Descemet's membrane is regenerated readily after injury. 

Fibrous Tunic

The endothelium is the extremely thin, innermost layer of the cornea. The endothelial cells are flat hexagonal and in direct contact with the humor. This layer has limited if any reproductive capacity. Endothelial cells are essential in keeping the cornea clear. Normally, fluid leaks slowly from inside the eye into the middle corneal layer (stroma). The endothelium's primary task is to pump this excess fluid out of the stroma and to allow the diffusion of nutrients to the cornea. Without this pumping action, the stroma would swell with water, become hazy, and ultimately opaque. In a healthy eye, a perfect balance is maintained between the fluid moving into the cornea and fluid being pumped out of the cornea. With aging, the endothelial cells are lost but the remaining cells enlarge, reorganize and migrate to maintain the intact monolayer so that Descemet’s membrane is maintained but once endothelium cells are destroyed by disease or trauma, they are lost forever. If too many endothelial cells are destroyed, corneal edema and blindness ensue, with corneal transplantation the only available therapy.

Fibrous Tunic

Function of the Cornea:

When light strikes the cornea, it bends, or refracts, the incoming light onto the lens. The lens further refocuses that light onto the retina.

The cornea also serves as a filter, screening out some of the most damaging ultraviolet (UV) wavelengths in sunlight. Without this protection, the lens and the retina would be highly susceptible to injury from UV radiation.

Fibrous Tunic

Sclera ; The white of the eye, spreading around the eyeball except for the cornea. It is made up of dense collagenous tissue mixed with elastic fibers and interspaced with flat fibroblasts and few blood vessels. It is a viscoelastic structure that provides rigidness and specific shape for the eye. The anterior part is covered by conjunctiva reflected on to it from the inner layer of the eyelid. The internal surface is attached to the choroid. Anteriorly it is continuous with the cornea and attached to the ciliary body, posteriorly, it is pierced by the optical nerve. The canal of Schlemm is an endothelial canal located near the internal surface of the Sclera and at the cornea-sclera junction. Through this canal the aqueous humor filters from the anterior chamber in to the ciliary veins.

Fibrous Tunic

Vascular Tunic

Choroid: The layer located underneath the Sclera, adhering tightly to the latter and loosely to the Retina. It is dark brown tissue that is highly vascularized, providing nutrients and Oxygen to the Retina. The choroid paper lies internal to the suprachoroidal lamina and is subdivided into an external vascular lamina of small arteries and veins and loose connective tissue with some pigment cells, an intermediate capillary lamina and a thin apparently structureless basal lamina. The choroidal pigment cells prevent the passage of light across the sclera into the retina.

Vascular Tunic

Ciliary Body: thickest portion of the Vascular Tunic, directly continuous with the choroid behind and the iris in front, extending from the Ora Serrata of the Retina to the Sclero-corneal junction. It is a highly vascularized region, involved in secretory and muscular activities. It is made up of:

Ciliary processes: that secrete the aqueous Humor.

Ciliary muscles

Vascular Tunic Ciliary processes: that secrete the aqueous Humor. The Humor is the fluid secreted by the ciliary processes

and is drained into the posterior chamber and then through the pupil into the anterior chamber of the eye. It nourishes and oxygenates the cornea and lens which are non-vascularized. The Humor is then drained into the Sclera through the canal of Schlemm and then passes into the blood. It is responsible for exerting the intraocular pressure in the eyeball. It is this fluid and the pressure it exerts what keeps the retina smoothly applied to the choroid and maintains a rigid shape of the eyeball. If the Intraocular pressure increases to exceed the normal pressure of 16 mmHg, glaucoma results, which degenerates the retina and causes blindness. The Humor is completely replaced every 90 minutes to be able to supply the required nutrients to the deprived areas. It constitutes of glucose, amino acids and respiratory gases, and also many other minor constituents.

Ciliary muscles: smooth muscles that affect the shape of the lens (dilation or contraction). Three types of muscles have been recognized, meridional, oblique and circular or sphincteric. Stimulated by the parasympathetic nerves, the ciliary muscles contract affecting the shape of the lens during accommodation.

The ciliary body has three functions: • accommodation, essentially means that when the ciliary

muscle contracts, the lens becomes more convex, generally improving the focus for closer objects. When it relaxes,it flattens the lens, generally improving the focus for farther objects.

• aqueous humor production and the production and maintenance of the lens zonules. which is responsible for providing most of the nutrients for the lens and the cornea and involved in waste management of these areas.

• It also anchors the lens in place.

Vascular Tunic

Iris: the colored slightly bulged portion of the eyeball, positioned between the cornea and the lens, made up of circular and radial smooth muscle fibers that regulate the amount of light passing through the Pupil into the posterior cavity, preserving useful vision under highly variable conditions. The color of the iris is due to iridial connective tissue and the pigment cells that selectively absorb and reflect different frequencies of light. The iris partially divides the anterior segment into anterior chamber and posterior chamber.The muscles of the iris are the Sphincter papillae and the Dilatator papillae.

The Sphincter papillae: flat smooth muscles around which connective tissue lie in front and behind along with high innervation. They spread circumferentially around the pupil. In bright light and in response to stimulation by parasympathetic nerve fibers, these muscles contract reducing the pupil size.Dilatator papillae: myoepithelial cells in which myofilaments appear in their basal muscular processes, when these radial muscles dilate upon stimulation by the Sympathetic nerve fibers during dim light, the pupil dilates.

Vascular Tunic

Nervous Tunic

The Retina is a vascularized portion of the eyeball that clearly shows blood vessels by means of ophthalmoscope. External to the Retina is the choroid, internal to it is the hyaloid membrane of the vitreous body. Conventionally, 10 layers of the retina are distinguished beginning at the choroidal edge and passing to the vitreous body:

1. Retinal pigment epithelium2. The lamina of rods and cones3. The external limiting lamina4. Outer nuclear layer5. Outer plexiform layer6. Inner nuclear layer7. Inner plexiform layer8. Ganglion cell layer9. Lamina of nerve fibers(Stratum

Opticum)10. Internal limiting lamina

Retinal Pigment Epithelium: sheet of melanin-containing epithelial cells lying between the Choroid and the neural portion. These cells are cuboidal cells which form a single continuous layer extending from the periphery of the optic disc to the ora serrata, then continuing from there into the ciliary epithelium. They bear microvilli which contact or project between the outer ends of rod and cone processes. Functions of pigment cells:

1. They are involved in the turnover of rod and cone photoreceptor cells as the terminals of the latter cells are constantly shed and phagocytosed by the pigment cells.

2. This layer acts as an anti-reflection device as the cytoplasm of the pigment cells contains melanin, the role of which is to absorb the light rays, avoiding reflection and scattering of light within the eyeball. The absence of melanin as in the case of Albinism is what causes vision disorders for albinos in bright light. Very intense light damages the pigments cells and cause epithelial breakdown.

3. The epithelium forms an important blood- retinal barrier between the retina and the vascular system of the choroid. If damaged, the epithelial cells can be limitedly replaced by mitosis.

Nervous Tunic

The neural portion of the Retina is comprised of the retinal neurons ranging from the layer of rods and cones to the internal limiting lamina. This portion is considered an outgrowth from the diencephalon of the brain dedicated to the detection and early analysis of visual information before transmitting nervous impulses to the Thalamus. It is soft, translucent and purple in the fresh unbleached state (due to the presence of Rhodopsin) and becomes opaque and bleached when exposed to light.

Nervous Tunic

The retinal neurons are classified into different categories through which light passes in the following sense:

A. The ganglion cell layer: The final common pathway for the nervous impulse as their dendrites are synaptically connected to the Bipolar cells and their axons reaching the CNS. Ganglion cells are highly concentrated in the Macular area but diminishing again in the fovea, where they are almost absent.  

B. The bipolar cell layer: radially oriented neurons, each with one or more dendrites synapsing with cones or rods and horizontal cells and interplexiform cells. Their somas are located in the inner nuclear layer. They have short axons. There are two main types of bipolar cells:

Cone bipolar cells that communicate with cone cells, these are of three major subtypes: midget, blue cone and diffuse cone bipolar.

Rod bipolar cells receiving photoreceptive inputs from rods exclusively C. Photoreceptor layer that transduces light rays into receptor potentials due to the

presence of 120x 106 rod neurons and 6x106 cone neurons. Both rods and cones are made up of outer segments and inner segments together forming a cone process or rod process. The outer and inner segments are connected by a short cilium. The inner segment is what determines the type of the photoreceptor. Furthermore, these cells contain external fiber, cell body, inner fiber and synapses (rod spherule and cone pedicle).

The rods are responsible for the discrimination between white and black, identification of shapes and movement, dominating the peripheral retina.

The cones are responsible for color vision and high visual acuity in bright light. Cones are highly concentrated at the fovea centralis in the center of the macula lutea which makes it the area of sharpest vision. Rods are excluded from the fovea.

Nervous Tunic

Macula and Fovea The Macula Lutea is a yellowish oval area, it is referred to as the central retina and is the most sensitive area of vision, having a central depression, the fovea centralis, where the retina is exceedingly thin making the choroid layer visible underneath, fovea is the thinnest area of the retina. The fovea has no rods, only a group of specialized cone cells that provide maximum visual discrimination.

Nervous Tunic

The optic disc or optic nerve head is the location where ganglion cell axons exit the eye to form the optic nerve. There are no light sensitive rods or cones to respond to a light stimulus at this point. This causes a break in the visual field called "the blind spot" or the "physiological blind spot". The Optic Disc represents the beginning of the optic nerve (second cranial nerve) and is the point where the axons of retinal ganglion cells come together. The Optic Disk is also the entry point for the major blood vessels that supply the retina.[1] The optic nerve head in a normal human eye carries from 1 to 1.2 million neurons from the eye towards the brain.

Nervous Tunic

The lens: A transparent encapsulated biconvex body placed

between the iris and vitreous body. Its diameter in adults is about 9 to 10 mm. It is surrounded by the ciliary processes and attached to them by means of thin filaments known as Zonular fibers. The lens is divided into capsule, cortex and nucleus. The nucleus is present in adults 30 years and older as it is the result of regenerating new lens fibers in the cortex and pushing the old fibers into the center in a concentric fashion, progressively darkening the color of the center. In adults, the lens is non-vascular, colorless and transparent, but with age the transparency is gradually lost and the nucleus becomes harder and darker.

Ocular Refraction Medium

The lens consists of stiff, very elongated, prismatic cells known as lens fibers, very tightly packed together. Fibers near the surface are nucleated and very closely packed by means of gap junctions and desmosomes. The fibers are rich with cytoskeleton actin filaments that maintain the shape of cells and provide elasticity. These fibers are also rich with proteins known as crystallins which are responsible for transparency and refractile properties and elasticity. Anterior lens cells constitute a transparent layer of simple cuboidal epithelium over the anterior surface of the lens, they duplicate at the surface and migrate backwards to the equator to form lens fibers. When older cells occupy older positions in the lens, they lose their nuclei and are packed down by new recruits to the lens fiber population.

The lens capsule is dense thick basement membrane that covers the outer surface of the lens. It resembles the Descemet’s membrane in its structure. It is elastic and allows the stretching of the lens without tearing.

The value of the lens is defined by its ability to vary its dioptric power depending on the capacity to change its shape, however this power decreases with age.

Ocular Refraction Medium

Capsule

Cortex

Nucleous

Interior of the Eyeball: Ocular refractive medium, large space divided by the lens, made up of:

Anterior cavity located in front of the lens filled with aqueous Humor that nourishes the cornea and the lens. The anterior chamber is further divided into:

anterior chamber located between the cornea and the Iris posterior chamber located between the Iris and the lens

Vitreous chamber located behind the lens and contains the vitreous body. The vitreous body is formed once during the embryonic life and not replaced afterwards. The vitreous body is colorless, constituting mainly of 99% water, along with organized cellular and fibrous content is present in a gel like consistency due to presence of sodium hyaluronate and collagen

Interior Eye Ball

The globe is kept in position and moved by a set of six muscles. Five of these (as well as

the levator palpebrae) originate in the back of the orbit forming what is known as themuscle cone. They include: the superior rectus, the inferior rectus, the lateral rectus, themedial rectus, and the superior oblique. The sixth muscle, the inferior oblique originateson the floor of the orbit. All six muscles connect to the globe at various points,depending on the movement they control.

Muscle Innervations

Medial Rectus (MR): moves the eye toward the nose External Rectus (ER): moves the eye away from the

nose Superior Rectus (SR): primarily moves the eye upward

and secondarily rotates the top of the eye toward the nose

Inferior Rectus (IR): primarily moves the eye downward and secondarily rotates the top of the eye away from the nose

Superior Oblique (SO): primarily rotates the top of the eye toward the nose and secondarily moves the eye downward

Inferior Oblique (10): primarily rotates the top of the eye away from the nose and secondarily moves the eye upward

Muscle Innervations

Medial Rectus (MR): Cranial Nerve III External Rectus (ER): Cranial Nerve VI Superior Rectus (SR): Cranial Nerve III Inferior Rectus (IR): Cranial Nerve III Superior Oblique (SO): Cranial Nerve IV Inferior Oblique (10): Cranial Nerve III

Muscle Innervations

ER6(SO4)3

Conclusion

Thank You

END of The Presentation