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Equine Special Senses - Horse Anatomy (view source)
Revision as of 15:55, 22 November 2012
, 15:55, 22 November 2012→The Transparent Media (conjunctiva, cornea, lens, and vitreous and aqueous humour)
===The Transparent Media (conjunctiva, cornea, lens, and vitreous and aqueous humour)===
===The Transparent Media (conjunctiva, cornea, lens, and vitreous and aqueous humour)===
====Conjunctiva====
The conjunctiva is continuous with the skin of the eyelids. The '''palpebral conjunctiva''' is the part of the conjunctiva that covers the inner surface of the eyelid, the '''bulbar conjunctiva''' covers the surface of the eyeball. It is lined by stratified squamous epithelium, and contains goblet cells, which secrete the deepest, mucus, layer of tear film, which adheres to the surface of the globe. It is highly vascular.
====Cornea====
The cornea and sclera together make up the tough outer, supportive structure of the eye. Although the cornea has the same composition as the sclera, it is transparent. The main function of the cornea is to refract light. The hallmark of corneal disease is opacity. The transparency of the cornea is partially due to the organization of stromal collagen, but the endothelium plays a critical role as it keeps the cornea in a constant state of relative dehydration. It is avascular, so nutrients and oxygen are obtained from the aqueous humour, and oxygen is also obtained from air. The cornea joins with the sclera at the point of the limbus; the sclera is vascular, but otherwise similar to the cornea.
The layers of the cornea are:
1. (conjunctiva)
2. Bowman’s membrane (basal lamina)
3. Thick transparent fibrous layer
4. Descement’s membrane
5. Endothelium (inner lining of the cornea)
The corneal '''epithelium''' consists of an outer layer of squamous cells, a middle layer of polyhedral cells and an inner single layer of columnar cells that produce the basement membrane. The epithelium is attached to the basement membrane by hemidesmosomes and fine fibrils link the basal epithelium, basement membrane and subepithelial stroma.
The '''stroma''' (substantia propria)comprises the bulk of the cornea, consisting primarily of keratinocytes, regularly arranged collagen fibrils and ground substance. Fibrils are arranged in sheet-like bundles (lamellae) that extend across the entire width of the cornea, from limbus to limbus; this arrangement helps to maintain transparency. The ground substance of proteoglycans and glycosaminoglycans (GAGs) & glycoproteins occupies the space between lamellae. The keratinocytes have a slow turnover rate in the normal adult cornea, but if there is a corneal insult they can become active fibroblasts and produce precursors of collagen and ground substance. Other cells such as leukocytes, lymphocytes and macrophages are rare in the normal cornea.
The corneal '''endothelium''' produces Descemet’s membrane throughout life, but its own powers of replication diminish in early life. When endothelial cells die, they are replaced by neighbouring cells spreading to fill the gap. Consequently, the endothelium thins and descemet’s membrane thickens with age. The endothelium is a single layer of cells of high metabolic activity and are crucial in maintaining dehydration of the cornea and thus corneal clarity. The endothelial cells actively pump fluid that can accumulate, from tear film etc, out of the cornea.
====Aqueous Humour====
Aqueous humour is produced by ciliary processes of '''ciliary body'''. It provides nutrients for lens and cornea. It also maintains intraocular pressure (25mm.Hg), and is replaced several times a day (2µl/min). An increase in intraocular pressure can cause glaucoma.
==Ear==
==Ear==