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Equine Special Senses - Horse Anatomy (view source)
Revision as of 16:26, 22 November 2012
, 16:26, 22 November 2012→The Transparent Media (conjunctiva, cornea, lens, and vitreous and aqueous humour)
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]]'''.
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]]'''.
====Vitreous Humour====
The vitreous humour is a transparent hydrogel that occupies almost 3/4 of the volume of the eye in the '''posterior segment''' of the globe; it is composed of 99% water with collagen fibrils as a skeleton for hylaronic acid. The vitreous body is attached to peripheral structures only at the edge of the optic nerve, near the ora serrata and at the posterior lens capsule. It acts as a clear medium for transmission of light between the lens and retina and provides mechanical support for the internal structures of the eye.
===The Wall (retina, uvea and sclera)===
====Retina====
===The Wall (retina, uvea and sclera)===
The retina is the inner layer of the eyeball, it develops from the '''optic vesicle''', which is an outgrowth of the diencephalon. It remains connected to the diencephalon via the '''{Equine Cranial Nerves - Horse Anatomy#Optic Nerve (II)|optic nerve]]'''. It can be divided into the non-visual retina (lines the back of the ciliary body) and the optic part of the retina.
The non-visual part of the retina is lined by inner and outer single-layers of epithelium; the outer layer is pigmented, the inner layer is neural. This neural inner layer contains photoreceptors, interneurons, ganglion cells and associated stromal cells, called MÜLLER cells. '''Rods''' and '''cones''' are photosensitive receptor cells, and are found in the optic part of the retina. Rods are mainly utilised at night, as they are highly sensitive receptors to black and white, while cones are mainly utilised during the day as they are used for colour vision. The '''fovea''' is an area of the retina that contains mostly cones, and in high numbers. Here, oxygen is obtained from the choroid (a pigmented layer that makes up part of the uveal tract - mentioned further down) by diffusion. In this region, there is one nerve fibre per cone.
[[Image:Layers of the Retina.jpg|thumb|right|200px|Layers of the Retina - Copyright David Bainbridge]]
Layers of the retina, from vitreous humour to choroid:
1. Inner limiting membrane
2. Optic nerve fibres and axons of ganglion cells
3. Ganglion cell bodies
4. Inner plexiform layer - synapse of bipolar cells with ganglion cells
5. Inner nuclear layer – nuclei of bipolar cells
6. Outer plexiform layer - synapses of photoreceptors with bipolar cells
7. Outer nuclear layer – photoreceptor cell nuclei
8. Outer limiting membrane
9. Photoreceptor layer – rods and cones
'''''Line of Detachment'''''
10. Pigment cells – retinal pigmented epithelium
====Uveal Tract====
The uveal tract is a three-part physiological and pathological unit, positioned between the sclera and the retina:
1. '''''Choroid''''': pigmented, highly vascular layer, containing:
a. tapetum lucidum (inner layer, nearest retina): reflective and coloured, so increases sensitivity to poor light
b. vascular layer: nutritive
c. black/connective layer (outer layer, nearest sclera)
2. '''''Ciliary body''''': produces aqueous humour and vitreous humour, and is involved in lens accommodation, as its muscle fibres stretch the lens into a flatter shape, allowing distant vision
3. '''''Iris''''': vascular, coloured, and contractile for pupil size
====Sclera====
The sclera is continuous with the cornea at the point of the '''limbus'''. It is similar to the cornea, except that it is vascular, and has dense, irregular, fibrous connective tissue.
==Ear==
==Ear==