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==Introduction==

==Introduction==

The eye is a paired organ, the organ of vision. The eye is made up of various components, which enable it to receive light stimuli from the environment, and deliver this stimuli to the brain in the form of an electrical signal. Vision involves all components of the eye.

The eye is a paired organ, the organ of vision. The eye is made up of various components, which enable it to receive light stimuli from the environment, and deliver this stimuli to the brain in the form of an electrical signal. Vision involves all components of the eye.

==Structure==

==Structure==

The eye is contained within the bony orbit of the head. The bony orbit is a cavity, comprising parts of the lacrimal bone (includes fossa for nasolacrimal duct) and the maxilla (includes caudal foramen of infraorbital canal). It is continuous with the temporal bone and the pterygopalatine fossa caudally. The bony orbit is located laterally in herbivores, but is set forward in hunting animals, that is carnivores.

The eye is contained within the bony orbit of the head. The bony orbit is a cavity, comprising parts of the lacrimal bone (includes fossa for nasolacrimal duct) and the maxilla (includes caudal foramen of infraorbital canal). It is continuous with the temporal bone and the pterygopalatine fossa caudally. The bony orbit is located laterally in herbivores, but is set forward in hunting animals, that is carnivores.

[[Image:Detailed globe.jpg|thumb|right|150px|The Globe - Copyright David Bainbridge]]

[[Image:Detailed globe.jpg|thumb|right|200px|The Globe - Copyright David Bainbridge]]

Movement of the eyeball is achieved through the contraction and relaxation of six extraocular muscles:

Movement of the eyeball is achieved through the contraction and relaxation of six extraocular muscles:

3. Thick transparent fibrous layer

3. Thick transparent fibrous layer

4. Descement’s membrane

4. Descemet’s membrane

5. Endothelium (inner lining of the cornea)

5. Endothelium (inner lining of the cornea)

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 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.

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|150px|Layers of the Retina - Copyright David Bainbridge]]

[[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:

Layers of the retina, from vitreous humour to choroid:

===Around the Eye===

===Around the Eye===

[[Image:Extraocular Muscles.jpg|thumb|right|150px|Extraocular Muscles - Copyright David Bainbridge]]

[[Image:Extraocular Muscles.jpg|thumb|right|200px|Extraocular Muscles - Copyright David Bainbridge]]

{| style="width:75%; height:200px" border="1"

{| style="width:75%; height:200px" border="1"

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|-

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[[Image: eye gross structure.JPG |thumb|right|150px|Dissected canine eye- Copyright C. Clarkson and T.F. Fletcher, University of Minnesota]]

[[Image: eye gross structure.JPG |thumb|right|200px|Dissected canine eye- Copyright C. Clarkson and T.F. Fletcher, University of Minnesota]]

All of the extraocular muscles originate at the equator of the eyeball.

All of the extraocular muscles originate at the equator of the eyeball.

[[Image:Adnexa.jpg|thumb|right|150px|Adnexa - Copyright David Bainbridge]]

[[Image:Adnexa.jpg|thumb|right|200px|Adnexa - Copyright David Bainbridge]]

*'''Adnexa''': accessory structures of the eye, including the eyelids and the lacrimal apparatus. It contains three layers of the tear film:

*'''Adnexa''': accessory structures of the eye, including the eyelids and the lacrimal apparatus. It contains three layers of the tear film:

==Vision==

==Vision==

===Optics===

===Optics===

Refractive indices:

Refractive indices:

==Central Visual Pathways==

==Central Visual Pathways==

[[Image:Central Visual Pathway.jpg|thumb|right|150px|Central Visual Pathway - Copyright David Bainbridge]]

[[Image:Central Visual Pathway.jpg|thumb|right|200px|Central Visual Pathway - Copyright David Bainbridge]]

The optic nerve ([[Cranial Nerves - Anatomy & Physiology|CN II]]) is a paired nerve that carries visual information from the retina to the brain. The ganglion cell axons leave the retina and information passes through the optic nerve to the optic chiasm, where some nerve fibres cross over. The optic tract (the optic nerve fibres) wraps around the cerebral peduncles of the midbrain, where it passes into the lateral geniculate nucleus, which is part of the thalamus. Most of the optic tract axons synapse here, and the remaining fibres branch off and synapse in the pretectal nuclei of the superior colliculi. The integrated visual information is then passed via nerve fibres to the cerebral cortex.

The optic nerve ([[Cranial Nerves - Anatomy & Physiology|CN II]]) is a paired nerve that carries visual information from the retina to the brain. The ganglion cell axons leave the retina and information passes through the optic nerve to the optic chiasm, where some nerve fibres cross over. The optic tract (the optic nerve fibres) wraps around the cerebral peduncles of the midbrain, where it passes into the lateral geniculate nucleus, which is part of the thalamus. Most of the optic tract axons synapse here, and the remaining fibres branch off and synapse in the pretectal nuclei of the superior colliculi. The integrated visual information is then passed via nerve fibres to the cerebral cortex.

==Histology==

==Histology==

[[Image:Retina numbered.jpg|thumb|right|150px|Retina - Copyright John Bredl]]

 

Image:Retina numbered.jpg|Retina - John Bredl

[[Image:Retina of the Dog.jpg|thumb|right|150px|Retina of the Dog - Copyright John Bredl]]

Image:Retina of the Dog.jpg|Retina of the Dog - John Bredl

 

Image:Stroma.jpg|Corneal Stroma - John Bredl

[[Image:Stroma.jpg|thumb|right|150px|Corneal Stroma - Copyright John Bredl]]

Image:Iris.jpg|Iris - John Bredl

 

Image:Iris in detail.jpg|Iris in detail - John Bredl

[[Image:Iris.jpg|thumb|right|150px|Iris - Copyright John Bredl]]

 

[[Image:Iris in detail.jpg|thumb|right|150px|Iris in detail - Copyright John Bredl]]

===Cornea:===

===Cornea:===

*The cornea has an important role in image formation, it forms a primary refractive element in the eye.

*The cornea has an important role in image formation, it forms a primary refractive element in the eye.

*Bulbar conjunctiva is lined by stratified columnar epithelium.

*Bulbar conjunctiva is lined by stratified columnar epithelium.

*Contains goblet cells.

*Contains goblet cells.

==Species Differences==

==Species Differences==

*The eye glands of many reptiles are excretory organs for salt.

*The eye glands of many reptiles are excretory organs for salt.

*Some chelonians (for example, tortoise, turtle or terrapin) do not have eye glands.

*Some chelonians (for example, tortoise, turtle or terrapin) do not have eye glands.

*Some reptiles have what is termed a parietal eye, sometimes called their third eye. Although it is called an eye, the reptiles cannot actually see out of it, although it can be used to detect light and dark, meaning that is is photoreceptive. It is part of the epithalamus, and is associated with the pineal gland. It can be used to determine circadian rhythms. Circadian rhythms are the cycle of light and dark - these cycles have a big impact on feeding and sleeping habits, and they also have an impact on thermoregulation.

*Some reptiles have what is termed a [[Lizard Eye#Parietal Eye|parietal eye]], sometimes called their third eye. Although it is called an eye, the reptiles cannot actually see out of it, although it can be used to detect light and dark, meaning that is is photoreceptive. It is part of the epithalamus, and is associated with the pineal gland. It can be used to determine circadian rhythms. Circadian rhythms are the cycle of light and dark - these cycles have a big impact on feeding and sleeping habits, and they also have an impact on thermoregulation.

===Fish===

===Fish===

*As water has a refractive index similar to the cornea, aqueous humour and vitreous humour, most refraction is carried out by the lens itself, which often has an extremely high refractive index. The lens is often extremely spherical, and accommodation is effected by moving it rather than distorting it.

*As water has a refractive index similar to the cornea, aqueous humour and vitreous humour, most refraction is carried out by the lens itself, which often has an extremely high refractive index. The lens is often extremely spherical, and accommodation is effected by moving it rather than distorting it.

==Links==

==Links==

[[Neuro Exam of The Horse - Pathology|Eye Pathology]] (Section 5.2)

[[Neurological Eye Examination - Horse|Neurological examination of the equine eye]]  

 

 

 

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