Aqueous humour

 Aqueous humour:

• It is a clear fluid.
• It fills the space between cornea in front and lens behind - anterior segment.
• This space is divided by iris into 

1. Anterior chamber

2. Posterior chamber

• Both compartments freely communicate with each other through pupil.

Aqueous humour

• It is secreted into the posterior chamber from the capillaries in the ciliary process
• It passes into the anterior chamber through the pupil
• From the anterior chamber, it is drained into the anterior cilia veins through 

1. the spaces of iridocorneal angle or angle of anterior chamber (Located between the fibres of ligament pectinatum.)

2. Canal of Schlemm.

• Interference with the drainage of aqueous humour into canal of Schlemm results in an increase in an increase in intraocular pressure, results in glaucoma 
• This produces cupping of optic disc and pressure atrophy of retina causing blindness.
• The intraocular pressure is chiefly due to aqueous humour.
• It maintains the constancy of the optical dimensions of the eyeball
• The aqueous is rich in ascorbic acid, glucose and amino acids.
• It nourishes the avascular tissues of cornea and lens

Clinical Anatomy:

• Over production of aqueous humour or lack of drainage or combination of both raise the intraocular pressure 
• It must be treated urgently.

Watch lectures on YouTube :

The Refractive Media of the Eye | Cornea | Aqueous Humour | Lens | Vitreous Humour

Inner coat / Retina

 Retina:

• This is thin, delicate inner layer of the eyeball.
• It is continuous posteriorly with the optic nerve.
• The outer surface of the retina formed by pigment cells is attached to the choroid.
• The inner surface is in contact with vitreous.
• Opposite the entrance of the entrance of the optic nerve, inferomedial to posterior pole, there is a circular area known as optic disc.
• It is 1.5mm in diameter

The retina:

• It diminishes in thickness from behind forwards.
• It is divided into optic, ciliary and iridium parts.

The optic part:

• It contains nervous tissue and is sensitive to light.
• It extends from the optic disc to the posterior end of ciliary body.
• The anterior margin of optic part of the retina forms a wavy line called Ora serrata

Ciliary & Iridial parts

• Beyond the ora serrata, the retina  is continued forwards as thin, non-nervous insensitive layer, which covers the ciliary body and iris. 
• This forms the ciliary and iridial parts of the retina.
• These parts are made up of two layers of epithelial cells

• The depressed area of the optic disc is called the physiological cup.
• It contains no rods or cones, therefore insensitive to light - physiological blind spot.

Macula:

• At the posterior pole of the eye, 3mm lateral to the optic disc, there is another depression of similar size called the macula.
• It is avascular and yellow in colour.
• The centre of the macula is further depressed to form fovea centralis.
• The fovea is the thinnest part of the retina.
• The fovea contains cones only and is the site of maximum acuity of vision

The rods: 

• These are light receptors of the eye.
• Rods contain a pigment called visual purple.
• They respond to dim light - scotopic vision
• The periphery of the retina contains only rods
• Fovea has no rods.

The cones:

• They respond to bright light - photopic vision
• They are sensitive to colours
• The fovea centralis contains only cones.
• Their number diminishes towards the periphery of the retina

The retina is composed of ten layers

1. Outer pigmented layer
2. Layer of rods and cones
3. External limiting membrane
4. Outer nuclear layer
5. Outer plexiform layer
6. Inner nuclear layer - bipolar cells
7. Inner plexiform layer
8. Ganglion cell layer 
9. Nerve fibre layer
10. Internal limiting membrane

• The retina is supplied by Central artery 
• This is an end artery
• In the optic disc, it divides into upper & lower branches.
• Each gives of nasal and temporal branches
• The artery supplies the deeper part of retina up to the bipolar cells.
• The rods and cones are supplied by diffusion from the capillaries of the choroid.
• The retinal veins run with arteries.

Clinical anatomy

• Retinal detachment occurs between outer single pigmented layer an inner nine nervous layers.
• It is an inter retinal detachment
• Silicone sponge is put over detached retina, which keeps the retina in place 

Watch lectures on YouTube:

Inner Tunic of Eyeball - The Retina |Parts |Optic Disc |Macula Lutea |Layers of retina |Blood supply

Lens, Vitreous Body, Development

 The lens:

• It is transparent
• It is a biconvex structure.
• It is placed between the anterior and posterior segments of the eyeball.
• It is circular in outline
• It has a diameter of 1cm
• The central points of anterior and posterior surfaces are known as anterior and posterior poles.
• The line connecting the poles constitutes axis of lens
• The marginal circumference is termed as equator

• The chief advantage of the lens is that it can vary its dioptric power.
• It contributes about 15 diopters to the total of 58 dioptric power of the eye.
• A diopter is the inverse of the focal lengths in meters.
• A lens having a focal length of half a meter has a power of 2 diopters.

• The posterior surface of the lens is more convex than the anterior.
• The anterior surface is kept flattened by the tension of suspensory ligaments
• When the ligament is relaxed by the contraction of the ciliary muscles, the anterior surface becomes more convex due to the elasticity of the lens substance.

• The lens is enclosed in a transparent, structureless elastic capsule.
• The capsule is thickest anteriorly near the circumference.
• Deep to the capsule, the anterior surface of the lens is covered by a capsular epithelium.
• At the centre of the anterior surface, the epithelium is made up of a single layer of cuboidal cells.
• At the periphery, the cells elongate to produce the fibres of the lens.
• The fibres are concentrically arranged to form the lens substance.
• The centre of the lens is the nucleus of the lens, which is firm.
• The periphery is the cortex of the lens, which is soft and is made up of more recently formed fibres.

• The suspensory ligament of the lens or zonule of Zinn retains the lens in position
• Its tension keeps the anterior surface of the lens flattened.
• The ligament is made up of a series of fibres which are attached peripherally to the ciliary process, to the furrows between the ciliary processes and to the ora serrata.
• Centrally the fibres are attached to the lens, mostly in front and few behind the equator.

Clinical Anatomy:

• Lens become opaque with increasing age - Cataract. Since the opacities cause difficulty I vision. Here the lens has to be replaced
• The central artery of retina is an end artery. Blockage of the artery leads to sudden blindness.
• Third nerve paralysis causes partial ptosis and dilated pupil. The cornea is turned downwards and outwards.
• Horner’s syndrome results in partial ptosis and miosis
• In brainstem death, both pupils are dilated and fixed.
• Eye sees everyone. One can see the interior of the eye by ophthalmoscope. Through the ophthalmoscope, one can see the small vessels in the retina and judges the changes in diabetes and hypertension. In addition, one can also examine the optic disc for evidence of papilloedema, caused by raised intracranial pressure.

Vitreous Body:

• It is colourless
• It is a jelly-like transparent mass which fills the posterior segment of the eyeball (posterior 4/5th of eyeball)
• It is enclosed in a delicate homogenous hyaloid membrane. 
• Behind it is attached to the optic disc and in front to ora serrata.
• In between it is free and lies in contact with the retina.
• The anterior surface of the vitreous body is indented by the lens and ciliary processes

Development:

• Optic vesicle forms the optic cup.
• It is an out pouching from the forebrain vesicle.
• Lens from lens placode ( ectodermal )
• Retina  - Pigment layer  from the outer layer of optic cup.
• Nervous layer from the inner layer of optic cup.
• Coroid, sclera from mesoderm.
• Cornea from the surface ectoderm forms the epithelium, other layers develop from mesoderm.

Eyeball - Middle coat

 Choroid:

• It is a thin pigmented layer
• It separates the posterior part of the sclera from the retina.
• Anteriorly - It ends at ora serrata by merging with ciliary body.
• Posteriorly - It is perforated by the optic nerve, to which it is firmly attached.
• Its outer surface is separated from the sclera by suprachoroidal lamina, which is traversed by the ciliary vessels and nerves.
• Its attachment to sclera is loose, so that it can be easily stripped 
• Its inner surface is firmly united to the retina.

Histology:

It consists of 

1. Suprachoroidal lamina

2. Vascular lamina

3. The choriopapillary lamina.

4. The inner basal lamina or Membrane of Bruch.

Ciliary Body:

• It is a thickened part of the uveal tract lying just posterior to the corneal limbus.
• It is continuous 

Anteriorly with iris

Posteriorly with choroid.

• It suspends the lens.
• It helps lens in accommodation for near vision.

Ciliary body:

• It is triangular in cross section.
• It is thick in front and thin behind.
• Its scleral surface contains ciliary muscle.
• Its posterior part of vitreous surfaces smooth and black - Pars plana.
• Its anterior part is ridged anteriorly - pars plicata to form about 70 ciliary processes.
• The central ends of the processes are free and rounded.

Ciliary zonule is thickened vitreous membrane fitted to the posterior surfaces of ciliary processes.

The posterior layer lines hyaloid fossa and anterior thick layer form the suspensory ligament of lens.

• The ciliary muscle is a ring of unstriped muscle which are 
• longitudinal or meridional , 
• radial and 
• circular.
• The longitudinal or meridionial fibres arise from  a from a projection of sclera scleral spur near the limbus.
• They radiate backwards to the suprachoroidal lamina.
• The radial fibres are obliquely placed and get continuous with circular fibres.

• The circular fibres lie within the anterior part of the ciliary body
• They are nearest to the lens
• Contraction of all parts relaxes the suspensory ligament so that the lens becomes more convex  
• All the parts of the muscle are supplied by parasympathetic nerves.
• The pathway involves the Edinger - Westpal nucleus, oculomotor nerve and ciliary ganglion.

Iris:

• This is the anterior part of the uveal tract.
• It forms a circular curtain with an opening in the centre called the pupil.
• By adjusting the size of the pupil, it controls the amount of light entering the eye
• Thus it behaves like an adjustable diaphragm.
• It is placed vertically between the cornea and the lens.
• It divides the anterior compartment of eye into anterior and posterior chambers, both containing aqueous humour.

• Its peripheral margin is attached to the middle of the anterior surface of the ciliary body.
• It is separated from the cornea by iridocorneal angle or angle of the anterior chamber.
• The central free margin forming the boundary of the pupil rests against the lens.

• The anterior surface of the iris is covered by a single layer of mesothelium.
• The posterior surface is covered by a double layer of deeply pigmented cells, which are continuous with those of ciliary body.
• The main bulk of the iris is formed by stroma  made up of blood vessels and loose connective tissue in which there are pigment cells.
• The long posterior and the anterior ciliary arteries join to form the major arterial circle at the periphery of th iris.
• From this circle vessels converge towards the free margins of the iris and join together to form the minor arterial circle of iris.

• The colour of the iris is determined by the number of pigment cells in its connective tissue.
• If the pigment cells are absent, the iris is blue in colour due to diffusion of the light in front of the black posterior surface.

• The iris contains well - developed ring of the muscle called the sphincter pupillae.
• It lies near the margin of the pupil.
• Its nerve supply is parasympathetic.
• The dilator papillae is an ill-defined sheet of radial muscle fibres placed near the posterior surface of the iris.
• It is supplied by sympathetic nerves

Clinical Anatomy:

• While looking at infinite far the light rays run parallel; ciliary muscle is relaxed, suspensory ligament is tense and lens is flat.
• While reading a book, the ciliary muscles contract and suspensory ligament is relaxed making the lens more convex.
• Human vision is coloured, binocular and three dimensional.

Watch lectures on YouTube:

Intermediate Tunic of Eyeball Choroid, Ciliary Body & Iris | Structure | Blood & Nerve supply