Eye Anatomy PDF

The eye The eye is an almost spherical structure located in bony orbit that also contains extraoccular muscles, ligaments, adipose tissues, blood vessels, nerves and glands The eye consists of a wall that encloses several compartments containing refractive media The lacrimal appa...

The eye The eye is an almost spherical structure located in bony orbit that also contains extraoccular muscles, ligaments, adipose tissues, blood vessels, nerves and glands The eye consists of a wall that encloses several compartments containing refractive media The lacrimal apparatus, the  The eye wall consists of three tunics 1. The outer fibrous tunic (tunica fibrosa bulbi) which in turn is subdivided into a. The sclera- the white tough posterior portion of the eye bulb b. The cornea- the transparent portion of the fibrous tunic which bulges slightly in the center of the rostral pole of eye 2. The middle vascular tunic (tunica vasculosa bulbi) composed of a. The choroid b. The ciliary body c. The iris 3. The inner tunic of the eye (tunica interna bulbi or retina) d. An optic portion containing the sensory receptors e. A blind portion that is epithelial in nature and covers the ciliary body and posterior surface of iris  The eye is sub divided into compartments that contains refractive media A. The anterior compartment is filled with aqueous humour and is located between the cornea and the vitreous body. It is further subdivided into 1. The anterior chamber(camera anterior bulbi) located between the cornea and the iris 2. The posterior chamber (camera posterior bulbi) between the iris and the vitreous body B. The posterior compartment(camera vitrea bulbi)of the eye located between the lens and the retina and is filled with vitreous body.  Tunica fibrosa Sclera The sclera is white tough layer of dense irregular connective tissue that protects the eye and maintains its form(shape) Bundles of collagen fibers containing a few elastic fibers and elongated fibroblasts are arranged parallel to the surface of the eye bulb These bundles are intricately(complicated) interwoven and arranged predominantly in an equatorial direction near the junction between sclera and cornea called limbus and around optic nerve  In the layer of sclera adjacent to the choroid, elastic fibers predominate and pigment cells are present This layer is referred to as the lamina fusca sclerae The optic nerve leaves the eye through numerous perforations in a disc like area referred as area cribosa sclerae  Cornea The transparent cornea is a convex- concave lens thicker at periphery than at the center and with a smaller radius of curvature centrally than peripherally The normal cornea is completely devoid of blood vessels It receives nutrients through blood vessels located in the area of transition where sclera overlaps the cornea rostrally This region is called limbus  The cornea composed of five layers 1. The anterior epithelium 2. Subepithelial basement membrane 3. Substantia propria 4. Posterior limiting membrane 5. Posterior epithelium(corneal endothelium) Anterior epithelium: The corneal epithelium is Stratified squamous nonkeratinized between 4-12 layers of thickness The epithelial cells are tightly packed and interdigitate profusely and adhere through numerous desmosomes They contain abundant microfilaments , little rER and a small golgi complex The basal cell layers are rich in glycogen Subepithelial basement membrane: consists of a basal lamina and underlying layer of reticular fibers Substantia propria consists of varying no. of layers of collagen fibers Predominant cell type is fibroblast located mainly between collagen fibers These cells are elongated and branched with little cytoplasm Cells and fibers are embedded in the amorphous ground substance that stains metachromatically owing to the presence of sulfated glycosaminoglycans(chondroitin sulfate, keratan sulfate)  The ground substance plays an essential role in the transparency of the cornea by maintaining an optimal degree of hydration(swelling) Excessive water content cause opacification of the cornea Corneal wound that penetrate anterior chamber result in formation of a fibrin plug from the aqueous humor with subsequent invasion by the polymorphonuclear leukocytes and monocytes After this there is increased no. of fibroblasts of corneal and non corneal origin Posterior limiting lamina: is highly refractile thick basement membrane in H&E stained preparation It gives a PAS reaction and stains with dyes specific for elastic fibers It consists of collagen fibrils and arranged in hexagonal array In the vicinity of the corneal endothelium the lamina consists of basal lamina material Corneal endothelium: is a single layer of flat hexagonal cells covers the caudal surface of the cornea The epithelium functions in the maintenance of the transparency of the cornea Corneal scleral junction(limbus): At the junction between sclera and cornea there is slight rostral lapping of the sclera over cornea The corneal epithelium gradually changes into conjunctival epithelium which rests in loose connective tissue The collagen fibers of substantia propria assume a more irregular arrangement, become associated with elastic fibers and are continuous with the bundles of sclera The posterior limiting lamina splits and is continuous with the connective tissue trabeculae of the corneoscleral trabeculae  Tunica vasculosa The tunica vasculosa comprises three portions the choroid, the ciliary body and the mesodermal component of iris 1. The choroid - is a thick , highly vascularised layer that extends caudally from the ora serrata and continue rostrally with the ciliary body Peripherally it is connected with the sclera and centrally it is adjacent to the pigmented epithelium of retina  The choroid is subdivided into five layers i. Supra choroid layer- the most peripheral layer consisting of bundles of collagen and elastic fibers ii. Vessel layer- Numerous large arteries and veins iii. Tapetum lucidum- is a light reflecting layer increasing light perception under conditions of poor illumination to keep the animal in state of alertness Herbivores- it is fibrous consisting of collagen and elastic fibers Carnivores-flat polygonal cells, 10-15 layers of rod like structures arranged parralel ro retina. The rods are rich in zinc with ability to reflect light iv.Choriocapillary layer (lamina choroidocapillaries)- is a dense network of capillaries immediately adjacent to the pigmented epithelial layer of the retina V. Basal complex(complexus basalis) is also referred to as elasticmembrane, glassy membrane or bruch’s membrane 2. Ciliary body is the direct rostral continuation of choroid It begins caudally at the oraserrata, a sharply outlined dentate border that marks the transition between the optic part(pars optica retinae) and the blind or ciliary part( pars ciliary retinae) of the retina Rostrally it is continuous with the iris participates in formation of trabecular meshwork of iris angle  In longitudinal section through the eye bulb the ciliary body is a triangle with narrow base oriented toward the anterior chamber and the iris The caudal outer portion contains the ciliary muscle The innermost portion within the ciliary process is a highly vascularised connective tissue The vessel layer of ciliary body is a continuation of the same layer of the choroid The elastic membrane of the choroid continues into the ciliary body The ciliary body is covered by two layers of cuboidal epithelial cells of retinal origin  The pigmented epithelial layer is continuous with the pigmented epithelium of retina The inner nonpigmented epithelium layer consists of cuboidal or columnar cells with a basal lamina that separates it from the posterior chamber  Iris The iris: is located rostral to the lens and separates the anterior and posterior chambers which communicate through the central round or oval opening referred as pupil The iris consists of a stroma of pigmented highly vascularised loose connective tissue, the sphincter and dilator muscles and posterior pigmented epithelium Iris angle: The periphery of the anterior chamber at the level of the corneoscleral junction and the attachment of the ciliary body and the base of the iris Toward the anterior chamber, bundles of collagen fibers , fibroblasts and pigment cells surrounded by a basal lamina and flat cells( a continuation of the corneal epithelium) form the trabeculae of the ligamentum pectinatum  Retina Retina is composed of a sensory portion referred as pars optica retinae and nonsensory portion which begins at the ora serrata and covers the ciliary body and iris as a double epithelium Retina consists of 10 layers 1.Pigment epithelium 2. Layer of rods and cones 3. Outer limiting layer 4. Outer nuclear layer 5.Outer plexiform layer 6. Inner nuclear layer 7. Inner plexiform layer 8. Layer of ganglionic cells 9. Optic nerve fiber layer 10. Inner limiting layer The area centralis retinae is a round oval area of retina It is characterised by increased number of cones , a thickening of the inner plexiform layer, an increased no. of ganglion cells, absence of optic nerve fiber and large blood vessels  Lens Lens is a transparent structure situated between the iris and the vitreous body and is suspended by the zonular fibers the ciliary process It consists of lens capsule- lens is entirely surrounded by the lens capsule. It consists of several layers of collagen fibrils lens epithelium- Beneath the anterior lens capsule is the lens epithelium. It is a layer of simple cuboidal epithelium lens fibers- long prismatic fibers, hexagonal in cross section Zonular fibers: originate from the ciliary inner limiting membrane at level of ciliary processs and composed of collagen fibrils and are attached to the lens capsule  Vitreous body It occupies the posterior compartment , the space between the lens and retina It is a jelly like substance adheres intensely to the optic pupila, ora serrata and orbiculus ciliaris It is intimately attached to the lens capsule It contains a network of collagen fibrils, it contains hyaluronic acid and hygroscopic protein vitrein  Lacrimal apparatus Lacrimal gland is a compound tubuloacinar or tubulo alveolar gland, predominantly serous in ungulates except in pigs mucous cells predominate It is serous in the cat and seromucous in the dog The interstitial connective tissue is composed of reticular fibers and some collagen fibers and is rich in lymphocytes, plasma cells and histiocytes The intercalated and secretory ducts are lined with simple and stratified cuboidal epithelium The lacrimal ductules with stratified cuboidal epithelium  Excess tears accumulate In the lacrimal lake(lactus lacrimalis) The nasolacrimal duct is lined by a stratified columnar epithelium with goblet cells or by transitional epithelium(pig) Towards the nasal end of the duct simple branched tubuloacinar mucous(or seromucous in sheep and goat) glands are present  The eye lids are movable folds of skin that protect the eyes The third eyelid is a conjunctival fold fortified by hyaline(ruminats, dog) or elastic( horse, pig, cat) cartilage The conjunctiva is a pseudostratified columnar (horse and carnivores) or transitional(pig, ruminants)epitheliumwith goblet cells It is based on a propria of highly vascularised loose connective tissuerich in fibrocytes, histiocytes, mast cells and plasma cells  It also contains numerous lymphatic nodules and glands, subdivided into superficial and deep glands The superficial gland of the third eyelid is serous in the horse and cat and seromucous in all other domestic animals with predominantly mucous secretion in the pig The deep gland of the third eyelid is present only in the pig and secretes a fatty product  Ear The ear is composed of three divisions: The external ear The middle layer The inner layer The external ear comprises the auricle and the external auditory canal and is separated from the middle ear by the tympanic membrane The middle ear represents a cavity within the temporal bone, which opens into the pharynx through auditory tube  The middle ear cavity(tympanum or tympanic cavity) is separated from the external ear by the tympanic membrane and from the inner ear by the oval and round windows(fenestra vestibuli and fenestra cochleae) Three ossicles(small bones) lie within the middle ear cavity and through their articulations form a bridge between the tympanic membrane and the oval window  The inner ear lies within the petrous temporal bone and is represented by canals containing receptor organs for auditory and vestibular sensory systems External ear Auricle comprises a central plane of elastic cartilage covered with skin or both sides The skin covering the convex surface of auricle is rich in hair and associated with sebaceous and sweat glands  On the concave surface of the auricle , hair and sweat glands are sparse but sebaceous glands are large and abundant The extrinsic muscles of the ear attach to the cartilage through a dense irregular connective tissue containing many elastic fibers These muscles are skeletal and striated  External auditory canal The external auditory canal extends from the auricle to the tympanic membrane It is round and oval in cross sections and held open by the rigidity of its wall The skeleton of the walls of the external three fourths of the canal is cartilagenous auricle The inner one quarter of the canal has an osseous skeleton The canal is lined with the skin containing small hair follicles and sebaceous , sweat and ceruminous glands(modified large sweat glands) combine to form a dark waxy material known as cerumen(earwax)  Tympanic membrane The tympanic membrane closes inner extrimity of the external auditory canal and is round or oval shape The epithelial lining of the tympanic cavity is simple squamous or low cuboidal epithelium Intercalated between these two layers is a connective tissue stratum composed of outer radially and inner circularly arranged collagen fibers  Collagen fibers are sparse in the dorsal portion of the tympanic membrane and this region is referred to as the flaccid part(Shrapnell’s membrane) The manubrium of the malleus (one of the ossicles of the middle ear) is firmly attached to the inner surface of the tympanic membrane  Middle ear The middle ear cavity lies between the tympanic membrane laterally and the petrous temporal bone medially Ventrally the chamber is enlarged and consists of cells separated by bony spicules The cavity is lined with simple squamous or low cuboidal epithelium  ossicles The three ossicles of middle ear cavity are the malleus, incus and stapes The malleus is shaped like a hammer and its handle (manubrium) is firmly attached to the tympanic membrane The head of the malleus articulates through a synovial articulation with incus and incus inturn articulates with the stapes The stapes is stirrup shaped with its foot plate in the oval window held by fibrous ligament  The tensor tympani muscle attaches to the malleus and the stadius muscle attaches to the stapes These are skeletal muscles arising from fosse within the dorsal wall of tympanum  Auditory tube The auditory tube is a connection between tympanic cavity and the pharynx It is surrounded by a V shaped hyline and elastic cartilage The lining epithelium is pseudostratified columnar with cilia The propria submucosa consists of loose connective tissue containing lymphocytes , lymphatic nodules Near the pharyngeal extremity of tube is a tonsil known as tubal tonsil In horse the auditory tube has a large diverticulum from its ventral surface the guttaral pouch  Inner ear Inner ear has a system of canals and cavities in the petrous temporal bone within which are a second set of membranous canals and cavities The osseous system is referred as osseous labyrinth and the contained membranous system is the membranous labyrinth The membranous labyrinth is filled with fluid known as endolymph Fluid called perilymph fills the spaces between the osseous and membranous labyrinth  Osseous labyrinth is divided into three compartments the vestibule lies adjecent to medial wall of tympanic cavity Semicircular canal Spiral cochlea The membranous labyrinth lies within the osseous labyrinth and is in part separated from it by the perilymphatic space  Vestibule contains round oval windows, the oval window is closed with the foot plate of stapes and round window by secondary tympanic membrane Semicircular canals – Three semicircular canals open into the vestibule, Two of these canals are vertically oriented and at right angles to each other The third canal is horizontal and at right angle with two vertical canals The canals open into vestibule The three canals are named according their orientation rostral vertical, caudal vertical and horizontal semicircular canals  The oseous cochlea resembles a snail shell with two and half to three coils The central core of of cochlear spiral is cone shaped and is called the modiolus The apex of the coil is referred as the helicotrema An oseous shelf projects into the lumen of the cochlear tube and this shelf is the spiral lamina of osseous cochlea  Membranous labyrinth – lies within the osseous labyrinth and is in part separated from it by perilymphatic space The perilymphatic space communicates with the subarachnoid space and perilymph is continuous with cerebrospinal fluid through the perylymphatic duct Within the vestibule of the osseous labyrinth are two membranous sacs connected by small communicating duct  The dorsocaudal sac is utriculus and the ventrorostral sac is the sacculus The endolymphatic duct is a membranous duct branching off the communicating duct terminates in endolymphatic sac a blind sac The membranous labyrinth is lined with a simple squamous epithelium  Cristae ampularis The cristae ampullares of semicircular canals project transversely into the lumina of membranous ampullae and partially occlude them The epithelium consists of sensory hair cells and supporting cells There are two types of hair cells type I and type II, type I are flask shaped and type II are cylindric  Utricle The macula of the uitricle lies in the lateral wall of the chamber and represents a rounded area of sensory epithelium The surface of macula is covered with gelatinous mass which contains concretions of calcium carbonate and protein -otoliths The macula of the utricle is an important receptor for the detection of orientation of the head in the field of gravity and of linear acceleration(equilibrium)  Saccule The macula of the saccule lies on the medial wall of the saccule and histologically similar to utricle These organs are commonly referred as otolithic organs The macula of the saccule is most sensitive to low frequency vibrations and plays no role in postural adjustments  Cochlea The membranous labyrinth extends into the spiral bony cochlea from the saccule by a small duct and ends as a blind tube at helocotrema or apex of cochlea The triangular shaped tube the cochlear duct divides the osseous cochlea into two compartments above and below the duct Dorsal compartment –scala vestibuli Ventral portion- scala tympani Both of these compartments contain perilymph where as the cochlear duct(scala media) is filled with endolymph  Cochlear duct The lateral wall of the cochlear duct (scala media) is lined by an epithelium with two cell types marginal cells and basal cells Marginal cells form a superficial layer of cuboidal or columnar cells and basal cells are located in deeper layer composed of cuboidal cells The ventral wall of the cochlear duct is covered by a complex epithelium that forms the spiral organ (organ of corti) This represents the sensory epithelium of the auditory system and comprises both sustentcular and sensory hair cells  Spiral organ of corti The outermost cells of the spiral organ are the external limiting cells(of Claudius) and external supporting cells(of Hensen) They form several rows Outer phalangeal cells Outer sensory hair cells Outer pillar cells Outer and inner sensory hair cells Inner pillar cells Inner tunnel of spiral organ Inner phalangeal cells Basilar membrane- zona arcuata, zona pectinata Tectorial membrane