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Equine Special Senses - Horse Anatomy (view source)
Revision as of 16:07, 22 November 2012
, 16:07, 22 November 2012→The Transparent Media (conjunctiva, cornea, lens, and vitreous and aqueous humour)
====Aqueous Humour====
====Aqueous Humour====
Aqueous humour is produced by '''ciliary processes''' of the '''ciliary body'''. It is a transparent ultra-filtrate of plasma. Its continual flow is vital for the delivery of nutrients and removal of waste products to the avascular structures it bathes; lens, inner portion of the cornea (including corneal endothelium) and trabecular meshwork. Once produced aqueous traverses the posterior chamber, passes through the pupil and enters the anterior chamber where it can then drain at the '''iridiocorneal angle'''. The aqueous is optically clear under normal conditions, a property important for vision. An opacity in the aqueous humour is usually the result of an inflammatory process. The '''blood-aqueous barrier''' to free diffusion of molecules is formed by the tight junction between non-pigmented epithelial cells of the ciliary body. Breakdown of this barrier occurs in many conditions, including inflammation, trauma and vascular disease. Interventions such as paracentesis and surgery also causes breakdown, as does topical administration of drugs such as pilocarpine. Typically, the aqueous humour becomes turbid (aqueous flare) because of leakage of plasma proteins into the anterior and posterior chambers due to the damaged barrier and loss of vessel integrity. Fibrinogen and other proteins render the aqueous ‘plasmoid’, as its protein content approaches that of plasma, and when inflammation is severe fibrin clots may form. The cellular component varies according to the underlying aetiology. A preponderance of white blood cells is termed '''hypopyon''' and a preponderance of red blood cells is called '''hyphaema'''. It also maintains intraocular pressure (25mm.Hg), and is replaced several times a day (2µl/min). An increase in intraocular pressure can cause [[Glaucoma - Donkey|glaucoma]].
Aqueous humour is produced by '''ciliary processes''' of the '''ciliary body'''. It is a transparent ultra-filtrate of plasma. Its continual flow is vital for the delivery of nutrients and removal of waste products to the avascular structures it bathes; lens, inner portion of the cornea (including corneal endothelium) and trabecular meshwork. Once produced aqueous traverses the posterior chamber, passes through the pupil and enters the anterior chamber where it can then drain at the '''iridiocorneal angle'''. The aqueous is optically clear under normal conditions, a property important for vision. An opacity in the aqueous humour is usually the result of an inflammatory process. The '''blood-aqueous barrier''' to free diffusion of molecules is formed by the tight junction between non-pigmented epithelial cells of the ciliary body. Breakdown of this barrier occurs in many conditions, including inflammation, trauma and vascular disease. Interventions such as paracentesis and surgery also causes breakdown, as does topical administration of drugs such as pilocarpine. Typically, the aqueous humour becomes turbid (aqueous flare) because of leakage of plasma proteins into the anterior and posterior chambers due to the damaged barrier and loss of vessel integrity. Fibrinogen and other proteins render the aqueous ‘plasmoid’, as its protein content approaches that of plasma, and when inflammation is severe fibrin clots may form. The cellular component varies according to the underlying aetiology. A preponderance of white blood cells is termed '''hypopyon''' and a preponderance of red blood cells is called '''hyphaema'''. It also maintains intraocular pressure (25mm.Hg), and is replaced several times a day (2µl/min). An increase in intraocular pressure can cause [[Glaucoma - Donkey|glaucoma]].
====Lens====
The lens is about 65% water; protein is the other main component. The basement membrane of the lens epithelial cells creates the lens capsule, which envelops the body of the lens. The capsule is an elastic membrane, which is semi-permeable and allows passage of metabolites. The anterior lens capsule is much thicker than the posterior lens capsule.
Throughout life, lens epithelial cells at the equator of the lens undergo mitosis and are pushed towards the ‘lens bow’ where they elongate producing layers of lens fibers that extend into the anterior and posterior cortex. The fibers are continually pressed inward by newly formed fibers and then lose their nuclei. As each fiber elongates, it curves around the deeper and older fibers. The tips of the lens fibers from different regions of the equator meet in a radiating pattern of lines known as '''sutures'''. These lines usually form a ‘y’ on the anterior surface of the lens and an ‘inverted y’ on the posterior surface. The suture line can sometimes be helpful in determining the location of an opacity within the lens.
The mature lens is transparent and biconvex. The centers of the lens surface are referred to as the anterior and posterior '''poles'''. The circumference of the lens is known as the '''equator'''. The center of the lens itself is referred to as the '''nucleus''', which is surrounded by the lens '''cortex'''. Attached to the lens equator are '''zonular ligaments'''; the zonules attach the lens to the ciliary body and mediate accommodation. Contraction of the ciliary body muscles relieves tension on the zonules, which in turn, allows the elastic capsule to retract; this causes the lens to become more convex. This change in the shape of the lens changes the focal length of the lens.
The lens metabolizes mostly through anaerobic glycolysis; metabolism is limited by the avascular state of the lens and low oxygen supply. The lens is completely dependent on the aqueous humor for nutrients. Pathological changes within the lens result in deposition of lens protein, disruption of lens fibres and hence loss of lens transparency known as a '''[[Cataracts - Donkey|cataract]]]'''.
==Ear==
==Ear==