Eye: Anatomy

Outer Layer- cornea/sclera

anterior part of sclera covered by mcous menbrane-the conjunctiva

Cornea:

• 3 layers-epithelium, substantia propria(stroma),Descemet's membrane with endothelium
• epithelium-stratified, basal cells lie on Bowman's membrane
• stroma-90% of corneal thickness - regularly arranged thin fibrils of collagen ensheathed by acid mucopolysaccharides set in a ground substance- form ribbon like bundles & give the stroma a laminated appearance- fibrils circular in CS-spaced equidistant- hexagonal lattice
• transparency related to regularity of stromal components-interfibrilar spacing less than a wavelength of light-tangential rows of fibres act as diffraction gratting resulting in destructive interference of scattered rays
• Descemet's membrane-thin elastic membrane-covered on posterior surface by endothelium
• stromal hydration maintained by endothelium- electrolytes removed & water flows passively
• endothelium examined by 500x specular microscope
• endothelial cells decrease in number with age-residual individual cells enlarge to compensate
• corneoscleral junction- limbus- cornea set into a sclera like a watch glass
• nerve supply-trigeminal
• no blood vessels- minute arcades @ limbus (1mm)
• corneal nourishment- diffusion of aqueous humour & peripheral vessels

Lining the Inner Aspect of Sclera-

1. Uveal Tract-highly vascular-for nutrition
2. Retina

Uveal Tract:

• Choroid
• Ciliary Body
• Iris- anteriormost

Anterior Chamber:

• aquous humour
• between cornea & iris 2.5 mm deep in centre
• peripheral recess- angle of anterior chamber
• canal of Schlemm- circular venous sinus in inner layer of sclera- often more than one lumen
• trabecular meshwork between canal of Schlemm & recess of anterior chamber

Iris:

• anterior surface-single layer endothelium-not continuous@crypts
• stroma contains branched connective tissue cells
• iris stroma usually pigmented-unpigmented in blue sclera
• blood vessels in iris-radial
• tissue spaces communicate directly with anterior chamber through crypts@ ciliary border
• thinnest@attachment to ciliary body
• posterior surface-2 pigmented epithelium-developmentally from retina-continuous@pupillary margin-anterior layer flattened cells-posterior layer cuboidal cells
• pupillary smooth muscles derived from from anterior epithelial cells
• sensory- trigeminal
• sphincter pupillae-occulomotor
• dilator pupillae-sympathetic (cervical chain)

Ciliary Body:

• like isosceles triangle- base forwards
• chief mass-ciliary muscle-unstriped-3 parts-circumferential-blends with scleral spur
• most muscles meridional in anteroposterior direction, 2nd portion v-shaped interdigitating concentrically@base of iris, 3rd portion -insertion@ root of iris-just anterior to pigmentary epithelium-closely related to dilator muscle
• anterior surface-corrugated-pars plicata-contains ciliary processes(tufts of blood vessels-like glomeruli) in between
• posterior part-smooth-pars plana
• covered on inner surface by 2 layers of epithelium-belongs to retina-only outer layer pigmented
• posterior extent-ora serrata-transition from ciliary body to choroid gradual
• ora serrata more anterior on nasal side than temporal
• sensory-trigeminal
• motor-occulomotor & sympathetic

Choroid:

• extremely vascular membrane in contact everywhere with sclera with a potential space-epichoroidal/suprachoroidal space in between
• inner side lamina vitrea/membrane of Bruch
• blood vessels increase in calibre from inside to outside-choriocapillaries(fenestrated vessels) immediately beneath membrane of Bruch
• sensory-trigeminal
• autonomic supply-for vasomotor

Retina:

• Outer layer epithelium-hexagonal single layer pigment epithelium
• Inner layer epithelium-suddenly changes@ora serrata into highly complex visual retina

Retina formed by 3 strata of cells & their synapses:

• visual cells-externally
• relay layer of bipolar cells-intermediate
• ganglion cells-internally

1. pigment epithelium-hexagonal cells-single layer-assist metabolism of retina- products of metabolism are freely exchanged between receptor cells & pigment epithelium-melanin granules prominent(absorbs light)-phagosomes present-
2. rods & cones-neural epithelium-discs renewed continuously-rod discs have limited life,eventually lost to pigment epithelium
3. external limiting membrane-perforated by rods & cones
4. outer nuclear layer-nuclei of rods & cones
5. outer plexiform layer-synaptic layer-transmissive region
6. inner nuclear layer-nuclei of bipolar cells-
7. inner plexiform layer-synaptic
8. ganglion cell layer
9. nerve fibre layer-axons of ganglion cells running centrally to optic nerve
10. internal limiting membrane-separates retina from vitreous

• Fibres of Muller-better developed vertical cells-supportive neuroglial cell- nutritive function

• Fovea Centralis- at posterior pole (3mm in the temporal side of the optic disc)-only cones present- other layers almost completely absent- most sensitive part of retina

• Macula lutea(yellow spot)-surrounds fovea centralis-nuclear layers get thinned out-plexiform layer present-ganglion cells heaped up into several layer(in stead of consisting of a single row of cells)-no blood vessels present-nourishment entirely by choroid)-more sensitive than other parts of retina,less than fovea

• Optic Disc-fibres of nerve fibre layer pass into the optic nerve-other layers of retina stop abruptly@ the edge of the aperture in the scleral canal- spanned by transverse layer of connective tissue containing much elastic tissue(lamina cribrosa)-through its meshes the optic nerve fibres pass-on posterior side the nerve fibres abruptly become surrounded by medullary sheaths

Lens:

Biconcave mass of peculiarly differentiated epithelium-developed from invagination of surface ectoderm of the fetus(compare with plantar corns)-original surface goes inside@centre-peripheral cells correspond to the basal cells of epidermis-inner old cells undergo sclerosis-changes analogous to that of stratum granulosum in epidermis-becomes massed together in the form of nucleus-Lens is devoid of nerve supply

1. Accommodation is one of the focusing mechanisms of the eye.
   Accommodation
2. In order to focus rays from objects at varying distances, the lens must change it's refractive power.
3. To change it's refractive power, the lens changes shape
4. The exact shape of the lens is determined by seventy or so suspensory ligaments.
5. The suspensory ligaments attached to the lens are called zonula.
6. The zonula are attached radially around the lens.
7. The zonula pull the edges of the lens towards the clilary body.
8. When the eye is accommodated for distant vision, both the circular and meridional fibres of the ciliary muscle are relaxed. (a)
9. When both the circular and meridional fibres of the ciliary muscle are relaxed, the zonula is stretched.
10. When the zonula is stretched it pulls the elastic lens into a flattened shape.
11. When the eye is accommodated for near vision, both the circular and meridional fibres of the ciliary muscle contract. (b)
12. When both the circular and meridional fibres of the ciliary muscle contract, the tension in the zonula is released.
13. When the ciliary muscle contracts, the ciliary process and choroid move forward toward the lens.
14. When the tension in the zonula is released, it allows the the elastic lens to bulge.
15. The lens of the eye is elastic.
16. Because the lens is elastic, it bulges, shortens, and thickens.
17. The ciliary process is the aqueous humor factory.
18. The aqueous humour is drained out of the scleral venous sinus.