12.1.0 SENSORY ORGANS: EYE: INTRODUCTION
The eyeball is embedded in orbital fat. It is encapsulated by three layers:
1. Fibrous layer.
2. Vascular layer.
3. Neural layer
The fibrous layer (1) consists of the transparent anterior part (cornea) and an opaque posterior part (sclera). The fibrous layer (tunic) is resistant to the intraocular pressure and provides a suitable attachment for the extraocular muscles. It is essential for the shape and dimensions of the eye ball.
12.1.2 - images: Sclera and Cornea
The diameter of the cornea , a translucent disc-shaped area, is about 11.5 mm, thickness about 1.2 mm at the periphery and about 0.5 mm in the center.
It consists of five layers: Epithelium, Bowman's membrane, stroma, Descemet's membrane and endothelium.
See: 12.1.2 POJA-L2536 et al; 12.1.2 POJA-L4396+2959; 12.1.2 POJA-L4393 et al.
The sclera is a dense layer (a fibrocollagenous coat of the globe) and maintains the shape of the eyeball. It is thin (0.4 mm) at the corneoscleral junction and thick (1 mm) near the entry of the optic nerve posteriorly. It is composed of three layers: The episclera (external layer of loose fibrocollagenous tissue), the stroma (middle layer of thick bundles of collagen), and lamina fusca (inner part of the sclera) with small numbers of elastic fibres.
See: 12.1.2 POJA-L4391 et al; 12.1.4 POJA-L4425 et al.
12.1.3 - images: Iris and Ciliary body
The ciliary body is involved in the suspension of the lens and with accommodation due to intrinsic muscle fibres. It also contributes to the production of aqueous fluid for the anterior eye chamber as well as for glycosaminoglycans of the vitreous body. The ciliary body has an anterior part, the pars plica (corona ciliaris with ciliary processes) surrounding the base of the iris and posterior to this is the pars plana (ciliary ring or orbicularis ciliaris). The peripheral ring of the pars plana is the ora serrata at which the optic part of the retina becomes non-optic due to reduction into two cell layers that covers the whole ciliary body. Its stroma is continuous with that of the retina at the ora serrata. The ciliary muscle controls the tension of the suspensory fibres.
See: 12.1.3 POJA-L4414 et al; 12.1.3 POJA-L2556 et al.
The iris is a circular diaphragm behind the cornea, presenting a central aperture i.e. the pupil (diameter from 1-8 mm) to control the amounts of entering light. The concentration of melanocytes and the distribution of the pigment determines the shade and hue of the iris. The sphincter pupillae is a flat annulus (ca 0.15 mm thick and 0.75mm wide) of myocytes while the dilatator pupillae is composed of myoepithelial cells.
See: 12.1.3 POJA-L2534 et al; 12.1.3 POJA-L2553+4410; 12.1.3 POJA-L2548 et al.
12.1.4 - images: Retina + Choroid + Optic nerve
The vascular layer (2) formed a continuous thin structure and is pigmented. It comprises of choroid, ciliary body and iris, collectively termed the uveal tract (Gray’s Anatomy). The ciliary body continues from the anterior edge of the choroid and extends into the iris.
The choroid is a thin, brown and highly vascular tissue, and lines the majority of the internal surface of the sclera. Its posterior site is thicker, firmly attached to the sclera and it is pierced behind by the optic nerve. The external surface is continued to the sclera by the lamina fusca (suprachoroid lamina) and internally it is firmly attached to the pigmented layer of the retina (neural layer).
See: 12.1.2 POJA-L4391 et al; 12.1.4 POJA-L4416 et al; 12.1.4 POJA-L2568 et al.
The neural layer (3) or the retina is the sensory part of the eye ball. It is derived from the invaginated optic vesicle (see Embryology). The outer layer becomes the layer of pigmented cells and the inner layer develops into a multilayered complex. The retina is at its thickest (ca 0.55 mm) near the optic disc and decreasing to ca 0.1 mm toward the ora serrata (where the non-optic region starts with a two-layer lining).
See: 12.1.4 POJA-L4418 et al; 12.1.4 POJA-L3571 et al; 12.1.4 POJA-L4422+4423.
12.1.5 - images: Lens
The ocular refractive media includes the cornea, aqueous humor, vitreous body and the lens.
Aqueous fluid (humor) is formed by active transport and diffusion from the capillaries in the ciliary processes. It enters the posterior and the anterior chambers and exits at the iridocorneal angle (clinical ‘filtration‘ angle). It is a nutritious fluid containing a.o. ascorbic acid, glucose, amino acids for avascular structures as cornea and lens. It also maintains and regulates the intraocular pressure and the form of the eye ball.
See: 12.1.3 POJA-L4411 et al.
The vitreous body is a colourless transparent gel-like mass, it contains water, hyaluronate, salts, fine collagenous fibrils. There are no blood vessels, remnants of a hyaloid artery might be found, a varying amount of collagen fibrils as well as circulating rounded hyalocytes are present. The periphery of the gel is condensed into a vitreous membrane, it is attached to the ciliary epithelium and the optic disc. Nutrition of the vitreous body is maintained by diffusion from the ciliary and retinal capillaries.
See: 12.1.2 POJA-L4391.
The transparent, avascular adult lens is biconvex and lies between the iris and vitreous body, it is encircled by the ciliary processes. A transparent capsule of thick basement membrane material encloses the lens. The lens consists of a cortical substance and a firm nucleus. The anterior surface of the lens is covered by a transparent layer of nucleated cuboidal epithelium. At the equator mitotic activity is maximal and these cells elongate and gradual transform into fibres. They retain their nuclei for a while but the oldest and hence the deepest fibres lose the nuclei. The terminations of the extremely elongated, hexagonal lens cells (fibres) form so-called lens sutures.
See 12.1.5 POJA-L3562 et al; 12.1.5 POJA-L2590 et al; 12.1.5 POJA-L4405 et al.
12.1.6 - images: Eye lid/Glands
The accessory visual apparatus comprises the eyelids, conjunctiva, lacrimal glands and also the extraocular muscles, fasciae and eyebrows.
The eyelids are folds of skin and contain a dense fibrous connective tissue support (tarsal plate), skeletal muscle fibres of the orbicularis oculi muscle, smooth tarsal muscle cells above the tarsal plate and modified sebaceous glands (tarsal glands of Meibom). Eyelashes (cilia) are associated with sebaceous glands of Zeiss and modified sweat glands (ciliary glands of Moll).
See: 12.1.6 POJA-L4400 et al; 12.1.6 POJA-L4402 et al.
The conjunctiva is a transparent mucous membrane that covers the internal palpebral surfaces. The palpebral conjunctiva is very vascular, has numerous connective tissue papillae and closely adhered to the tarsi. Over the sclera the thin transparent ocular (or bulbar) conjunctiva is loosely connected to the eyeball.
See: 12.1.6 POJA-L4398 et al; 12.1.6 POJA-L3576 et al.
The lacrimal gland secretes tears via excretory ducts to the surface of the eye, lacrimal canaliculi, lacrimal sac and nasolacrimal duct and into the nasal cavity. Superolateral in the orbit the human lacrimal gland has a large, upper orbital and lower small palpebral part (lateral part of the palpebra). Many small accessory lacrimal glands occur more in the upper lid than in the lower one and near the fornix.
See: 12.1.6 POJA-L3578 et al; 12.1.6 POJA-L3580 et al.
12.1.7 - images: Pathology
Cortical cataract: See: 12.1.7 POJA-L3379+3378+3380
Retinoblastoma: See: 12.1.7 POJA-L3850+3851+3852
COPYRIGHTS
All rights reserves worldwide for the POJA collection are hold by L.G. Poels and P.H.K. Jap and Radboud University Medical Center (Radboud UMC Nijmegen). No one may modify, copy, distribute, transmit, display, or publish any materials contained in the POJA collection without prior written permission of the authors Poels and Jap or the UMC St Radboud. Any commercial use of the POJA collection is forbidden. POJA images are partly deposited in the Health Education Assets Library (HEAL database) as well as in the MedEdPortal/AAMC database).
All rights reserves worldwide for the POJA collection are hold by L.G. Poels and P.H.K. Jap and Radboud University Medical Center (Radboud UMC Nijmegen). No one may modify, copy, distribute, transmit, display, or publish any materials contained in the POJA collection without prior written permission of the authors Poels and Jap or the UMC St Radboud. Any commercial use of the POJA collection is forbidden. POJA images are partly deposited in the Health Education Assets Library (HEAL database) as well as in the MedEdPortal/AAMC database).