For details of structure and working, the eye of a mammal is described first. The eye forms an almost spherical eyeball lodged in the orbit.
Attached to the eye for moving it in the orbit are six extrinsic muscles, they are anterior, superior, inferior and posterior rectus muscles, and superior and inferior oblique muscles, they are named according to the region of the eyeball where each muscle is inserted.
These muscles are innervated by three motor cranial nerves, namely the oculomotor, trochlear, and abducens.
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The oculomotor (III) supplies the inferior oblique muscle and anterior, superior, and inferior rectus muscles.
The trochlear (IV) goes to the superior oblique muscles, and abducens (VI) innervates the posterior rectus muscle. The eyeball is made of three coats.
The external coat is a selerotic made of dense fibrous connective tissue which is opaque, but in front the sclerotic forms a transparent cornea of connective tissue.
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The cornea is covered by a thin transparent but vascular conjunctiva which is formed by modified epidermis and continuous with the lining of eyelids, to distinguish the two, the conjunctiva is more specifically termed the bulbar conjunctiva and the lining of eyelids is called palpebral conjunctiva.
The second coat or choroid is made of highly vascular connective tissue, pigment cells, and some smooth muscles.
In some mammals, .especially the nocturnal mammals, the choroid has silvery connective tissue which reflects light rays causing the eyes to shine at night.
The choroid closely lines the sclerotic, but it separates in front to form an iris which is perforated by a rounded pupil.
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The iris seen through the cornea is pigmented in mammals including man, it may be blackish, dark brown, light brown, blue, green, gray, or yellow.
The iris contains ectodermal intrinsic circular and radial smooth muscles, the circular muscles contract the pupil, while radial muscles dilate it.
Along the peripheral margin of the iris the choroid mainly and to some extent the outer part of the retina form a ring-like ciliary body which is a thick folded band containing smooth ciliary muscles, radiating ciliary processes to which suspensory ligament fibres are attached, blood vessels, and glands.
The circular and radial muscles of the iris and ciliary muscles are under the control of the autonomic nervous system receiving sympathetic and parasympathetic fibres.
The innermost coat of the eyeball is a thin, light-sensitive nervous layer called retina. The retina lines the choroid and extends in front over the ciliary body to form a posterior lining of the iris up to the pupil.
Only the posterior part of the retina called pars optica is sensory, while ends at the ciliary body along an irregular line termed ora serrata.
The anterior part of the retina over the ciliary body and on the inner surface of the iris is thin, non-sensory, and has a simple structure.
The retina has a complicated structure and is made of two layers, an outer pigment layer and an inner nervous layer.
The pigment layer is made ofcuboidal cells containing dark brown pigments, the cells have fringe-like protoplasmic processes, it is this layer alone which is continued forward beyond the ora serrata.
The nervous layer is confined to the pars optica, it is transparent but is very complex and has an outermost photosensitive layer of visual cells called rods and cones.
Rods respond to low intensities of light but not to colours, while cones are sensitive to bright light and to various colours.
The rods bear a long thin cylinder, each of which contains a purple pigment rhodopsin made of a protein and vitamin A, it is bleached and destroyed by light but is formed again in darkness by vitamin A.
A violet pigment iodopsin is found in cones, but very little is known about it. Recently three other pigments have been found in the human eye, they are erythrolabe chlorolabe, and cyanolabe, they are sensitive respectively to red, green and blue light, these pigments appear to be located in cones and may be responsible ior colour vision.
Rods detect differences in the intensity of light but they do not discriminate finer details of objects, hence the eyes of nocturnal animals and those living in dark places and deep water (moles, whales) have only rods in the retina.
The cones bear a short tapering process each, they are concerned both with discrimination of acute details of structure in bright light and perception of colours.
The rods and cones are nucleated cells and their inner ends are continuous with slender nerve fibres.
The retina has mixture of rods and cones, towards the periphery there are more rods than cones, but towards the posterior side there are more cones than rods, and in a small depression called area centralis (in primates it is called macula lutea because it contains yellow pigment) there is a smaller depression in the centre known as fovea centralis which contains only cones.
The fovea centralis lies at the posterior end of the eyeball on the main visual axis and is an area of most acute vision for discriminating the minutest details of structure and colour.
Internal to the rods and cones the retina has a layer of bipolar ueurons whose dendrites form synapses with nerve fibres of rods and cones.
The innermost layer of the retina is a ganglionic layer of neurons, the ganglionic neurons by their dendrites form synapses with the axons of bipolar neurons, the axons of ganglionic neurons pass along the inner surface of the retina and then bend sharply to run parallel to each other, they become medullated and form an optic nerve. In higher mammals the optic nerves decussate only partially, these forms have binocular and stereoscopic vision.
The optic nerve pierces through the retina, choroid, and sclerotic at a blind spot and then joins the diencephalon, there are no visual cells at the blind spot.
The retina also contains neuroglia cells and Muller’s fibres which fill the spaces between the visual cells and nervous layers.
In order to reach the photosensitive rods and cones light must pass through the layers of nerve fibres, ganglionic layer and bipolar neurons, a retina of this type with visual cells away from the source of light is characteristic of the vertebrate eye and is spoken of as an “inverted retina”.
Behind the iris is a transparent biconvex lens enclosed in a transparent and elastic membrane called lens capsule.
The lens is attached to the ciliary processes of the ciliary body by very line fibrous suspensory ligaments.
The space between the cornea and iris is an anterior chamber, and the narrow space between the iris and the lens is a posterior chamber, both these chambers are filled with a watery aqueous humour.
The large cavity of the eye ball behind the lens is a vitreous chamber filled with a transparent jelly-like vitreous humour or body.
A lymphatic hyaloid canal pisses through the vitreous humour from the lens to the blind spot, in the embryo it contained a hyaloid artery which disappears later and its place is taken by a retinal artery which capillarizes in the choroid.
There are two movable eyelids as transverse folds of skin, but in some mammals only the upper eyelid is movable, the eyelids are protective and close the eye.
A third eyelid is a transparent nictitating membrane which is movable and can cover the entire cornea.
In man the nictitating membrane is reduced to a vestigial plica semilunaris lying as a reddish patch in the inner corner of the eye.
Below the outer angle of the upper eyelid are lacrimal glands which produce a salty secretion or tears continuously to keep the surface of the eye moist and free from dust and to provide nourishment for the cornea, the tears are drained from the eye into a nasal sinus by a lacrimal duct.
In emotional stress or injury excess tears are secreted so that they overflow over the lower eyelid and cheeks.
In man the edge of eyelids have rows of hairs called eyelashes, modified sweat glands known as glands of Moll open into the follicles of eyelashes.
On the margins of both eyelids are openings of tarsal or Meibomian glands regarded as modified sebaceous glands, they produce an oily secretion which forms a film over tears to hold them “evenly over the surface of the eyeball.
In some aquatic mammals (whales) and mice there is a large Harderian gland lying behind the eyeball which secretes an oily fluid for lubricating the nictitating membrane, it is absent in most mammals.