The eye is made up of three layers, the internal nervous tunic, the vascular (uvea) tunic, & the outer fibrous tunic. The outer fibrous tunic is made up of the cornea & sclera. The cornea is the anterior 1/6th & the sclera is the posterior 5/6th’s. The cornea is thin, transparent & avascular, no lymphatic drainage occurs here. The cornea reflects 80% of the light that hits the eye, as well as helping the eye to focus. It is made up of five layers: the corneal epithelium, bowman’s membrane, substantia propria, descemet’s membrane, corneal endothelium. The epithelial is made up of flat, stratified columnar cells. The bowman’s membrane is acellular & has interwoven collagen fibrils. It is approx 8-12µm long. The substantia propria is the thickest layer (50-60µm) & is fibrous & transparent. Descemet’s membrane is made up of type IV collagen fibrils & is about 10µm long. The last layer, corneal endothelium, is a single layer of flattened cells & is important for corneal hydration (acts as a barrier for active transport). The corneal layer is nourished by the diffusion of aqueous humour which provides it with nutrients; the centre of the cornea is nourished by the oxygen in tear film which diffuses through. The cornea is innervated by V1 of the trigeminal nerve & mainly long ciliary nerves, the axons have no Schwann cells. (Axons only become myelinated after piercing the lamina cribrosa & leaving the actual eyeball itself).
The sclera is joined to the cornea at the corneoscleral junction, & is the tough, fibrous, protective layer of the eyeball. It’s opaque – the random distribution of fibres contributes to this. It is covered with fascial sheath & conjunctiva. The sclera preserves the shape of the eyeball & protects the intraocular contents from mechanical displacement & trauma. The sclera is smooth except for where the extraocular muscles attach or which it provides rigid insertions). It has three layers: the epithelium, the scleral stroma & the lamina fuscia. The epithelium is loose connective tissue & has a rich blood supply. The scleral stroma is dense fibrous tissue & the lamina fuscia contains melanocytes & forms part of the perichoroidal space – space between the sclera & the choroid. Check for our sponsors at Double Glazing Glasgow.
The vascular tunic (uveal layer – uva is latin for grape, & this layer resembles a grape when dissected) is made up of the (superficial to deep) iris, ciliary body & choroid. The choroid lines the inner surface of the sclera & extends from the optic nerve to the ciliary body (it’s thickest at its posterior pole). The inner part of it is attached to the RPE of the retina & is smooth, the outer surface is rough (fibrous). The choroid is made up of three layers: the vessel layer, the capillary layer, bruch’s membrane. The vessel layer is made up of loose connective tissue & lots of blood vessels. The capillary layer is made up of a big network of capillaries. Bruch’s membrane is made up of collagen & elastic fibres. Arterial supply is from the long & short posterior ciliary arteries. Venous drainage is through the vorticose veins (four or five) which pierce the sclera to join the ophthalmic veins, after draining into these they drain into the cavernous sinus. The choroid receives its nerve supply from the frontal nerve, the long ciliary nerve (branch of the nasociliary nerve of V1) & the short ciliary nerves via the ciliary ganglion. The function of the choroid is to nourish the outer layer of the retina & to prevent excess reflection by absorbing excess light via pigment cells.
The ciliary body is triangular in cross section & is continuous posteriorly with the choroid & anteriorly with the iris. On the anterior surface of the ciliary body there is the pars plicata which has projections (ciliary processes) which produce aqueous humour. On the posterior side is the pars plana which is smooth & flat. The ciliary body has three layers, the epithelium, stroma & the muscle. The epithelium consists of two layers, the outer pigmented layer & the inner non-pigmented layer, which apically face each other. (The basal part of the non-pigmented layer faces the inner limiting membrane of the neural retina, whereas the basal part of the outer pigmented layer faces the stroma of the ciliary body). The stroma is made up of loose connective tissue, has lots of blood vessels, contains melanocytes & contains embedded ciliary muscle. The muscle layer is smooth muscle with three sets of muscle fibres, longitudinal, circular & oblique. This muscle is innervated by the short ciliary nerves.