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.
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