Clinical defects of vision The sense of vision represents an interaction between our nervous system and a system of physical optics which provides it with the most basic and inportant special sense which comnects us with the outside world.Defects of this system are many and for most of us even the normal processes of ageing bring restrictions to our sight. The conjunctfva and cornea The optics of the eye demand that the cornea and its overlying conjunctiva be transparent and optically correct.To this end they are mde up of non-keratinizing stratified squanous epitheliun By far the most common defect of the comjunctiva, at least in the westerm world,is acute bacterial or allergic conjunctivitis where pain and hyperaemia demnd nedical attention before serious danage to the optics of the eye occurs.Much more serious is trachom,a long-standing chlamdial infection of the cormea where an inflaned mass replaces the superficial layers.This disease is the rost common cause of blindness in underdeveloped countries. GIaucoms Behind the cornea is the anterior chanber,filled with aqueous humor produced by the ciliary body and removed by the trahecular neshwork and the canal of Schlemm Pressure of aqueous humor should not exceed 20 mmHg.This is measured in the clinic as the pressure of a strean of air required to indent the cornea.Glaucoma is an increase in intraocular pressure sufficiemt to cause degeneration of the optic disc and nerve.It is a disorder affecting 4%of people over 40 years of age.The most common cause is obstruction of outflow of aqueous humor.This obstruction is of two major types,causing respectively primary open-angle glaucons and closed-angle glaucona. Priory open-angle glaicoms Primary open-angle glaucona is responsible for 90%of cases of raised intraocular pressure.A chronic slow rise in pressure is attributable to microscopic abnormalities in the canal of Schlemm Constriction of the pupil with drugs such
Clinical defects of vision The sense of vision represents an interaction between our nervous system and a system of physical optics which provides it with the most basic and important special sense which connects us with the outside world. Defects of this system are many and for most of us even the normal processes of ageing bring restrictions to our sight. The conjunctiva and cornea The optics of the eye demand that the cornea and its overlying conjunctiva be transparent and optically correct. To this end they are made up of non-keratinizing stratified squamous epithelium. By far the most common defect of the conjunctiva, at least in the western world, is acute bacterial or allergic conjunctivitis where pain and hyperaemia demand medical attention before serious damage to the optics of the eye occurs. Much more serious is trachoma, a long-standing chlamydial infection of the cornea where an inflamed mass replaces the superficial layers. This disease is the most common cause of blindness in underdeveloped countries. Glaucoma Behind the cornea is the anterior chamber, filled with aqueous humor produced by the ciliary body and removed by the trabecular meshwork and the canal of Schlemm. Pressure of aqueous humor should not exceed 20 mmHg. This is measured in the clinic as the pressure of a stream of air required to indent the cornea. Glaucoma is an increase in intraocular pressure sufficient to cause degeneration of the optic disc and nerve. It is a disorder affecting 4% of people over 40 years of age. The most common cause is obstruction of outflow of aqueous humor. This obstruction is of two major types, causing respectively primary open-angle glaucoma and closed-angle glaucoma. Primary open-angle glaucoma Primary open-angle glaucoma is responsible for 90% of cases of raised intraocular pressure. A chronic slow rise in pressure is attributable to microscopic abnormalities in the canal of Schlemm. Constriction of the pupil with drugs such
as pllocarpine relieves pressure on the canal and aids aqueous flow.Surgical treatment can be successful in severe cases.Treatment arrests visual loss but cannot reverse existing damage. CTosed-angle glauco Closed-angle glaucom is so called because the outflow of aqueous bumor is impeded by a change in the angle betseen the iris and cormea through which the humor leaves the anterior charber.This acute ophthalmological emergency is usually precipitated by dilatation of the pupil in preparation for fundoscopy in patients with an anatomfcal predisposition.The rapid increase in intraocular pressure causes severe pain and if not treated by osmotic agents such as mannitol and pupillary constrictors such as pilocarpine can cause complete blindness in days. The lens Refraction of light to forn focused imverted real images on the retina is the result of the light passing through media of different refractive indexes fron air through cornea,anterlor chanber.lens and posterfor chanber.Most refraction takes place at the air-corneal interface,but this is fixed.Accommodation,which focuses light from different distances by changing the thickness of the lens,is mediated by the parasympathetic branch of the oculorotor nerve.By the age of 60 most of the flexibility of the lens to accommodate has been lost.This phenomenon and the mismatch between the optical and biological lengths of the eye that results in the conditions of myopia (nearsightedness)or hyperopia (farsightedness)are so common as to hardly warrant the description pathological.They are therefore dealt with in the main section of this chapter. Cataraets Cataracts are cloudy or opaque areas which develop in the lems.They nay be congenital.inherited in an autosomal fashion,due to fetal infection,especially rubella,or associated with chromosomal abnormalities or a variety of trauas to the adult leas.Their incidence increases as the lens enlarges with age.The lens has no blood supply and receives its nutrition froa the aqueous humor.It consists of a nass of modiried epithelial cells of the ectodern from which it is derived
as pilocarpine relieves pressure on the canal and aids aqueous flow. Surgical treatment can be successful in severe cases. Treatment arrests visual loss but cannot reverse existing damage. Closed-angle glaucoma Closed-angle glaucoma is so called because the outflow of aqueous humor is impeded by a change in the angle between the iris and cornea through which the humor leaves the anterior chamber. This acute ophthalmological emergency is usually precipitated by dilatation of the pupil in preparation for fundoscopy in patients with an anatomical predisposition. The rapid increase in intraocular pressure causes severe pain and if not treated by osmotic agents such as mannitol and pupillary constrictors such as pilocarpine can cause complete blindness in days. The lens Refraction of light to form focused inverted real images on the retina is the result of the light passing through media of different refractive indexes from air through cornea, anterior chamber, lens and posterior chamber. Most refraction takes place at the air-corneal interface, but this is fixed. Accommodation, which focuses light from different distances by changing the thickness of the lens, is mediated by the parasympathetic branch of the oculomotor nerve. By the age of 60 most of the flexibility of the lens to accommodate has been lost. This phenomenon and the mismatch between the optical and biological lengths of the eye that results in the conditions of myopia (nearsightedness) or hyperopia (farsightedness) are so common as to hardly warrant the description pathological. They are therefore dealt with in the main section of this chapter. Cataracts Cataracts are cloudy or opaque areas which develop in the lens. They may be congenital, inherited in an autosomal fashion, due to fetal infection, especially rubella, or associated with chromosomal abnormalities or a variety of traumas to the adult lens. Their incidence increases as the lens enlarges with age. The lens has no blood supply and receives its nutrition from the aqueous humor. It consists of a mass of modified epithelial cells of the ectoderm from which it is derived
In advanced cataracts these cells break dosn and undergo dissolution,becoming opaque.High levels of glucose in diabetes produce sorbitol which exerts an osnotic effeet,damaging the cells.Climically,cataracts produce halos or spots in the visual field which result in a progressive loss of visual acuity.Current treatment involves removing the lens and replacing it with an artificial ooe. T乃o retina The delicate structure of the ret ina is susceptible to damage from a variety of sources as well as a spectrun of genetic disorders. Net inopathy of prematurity Premature infants frequently suffer from respiratory distress syndrone which requires hyperbaric oxygen.The imature retina responds to increased partial pressure of oxygen with vasospasm and proliferation of retinal vessels into the vitreous humor.Oedema and leakage of blood then leads to retinal detachnent and blindness.Careful control of oxygen therapy of the newborn is therefore essential. Retinitis pigoentosa Retinal degeneration is the hallmark of a group of imherited degenerative disorders known as retinitis pigmentosa.Beginning in early life at the peripbery of the retina.loss of rods,cones and ganglion cells procresses slowly to blindness by the age of 50. Retinal detacheent Detachment of the neuroepithelial layer of the retina froa the pigmented layer may be the result of: ·fluid exudation contraction of fibrous tissue forned as a result of haemorrhage a hole developing in the retina which allows ingress of liquefied vitreous huror between the layers. One in 10 persons over 40 years of age have such holes in their retinas.Retinal detachent deprives the neuroepithelial layer of its blood supply and causes degeneration within 4-5 weeks.This manifests itself as sudden loss of part of the
In advanced cataracts these cells break down and undergo dissolution, becoming opaque. High levels of glucose in diabetes produce sorbitol which exerts an osmotic effect, damaging the cells. Clinically, cataracts produce halos or spots in the visual field which result in a progressive loss of visual acuity. Current treatment involves removing the lens and replacing it with an artificial one. The retina The delicate structure of the retina is susceptible to damage from a variety of sources as well as a spectrum of genetic disorders. Retinopathy of prematurity Premature infants frequently suffer from respiratory distress syndrome which requires hyperbaric oxygen. The immature retina responds to increased partial pressure of oxygen with vasospasm and proliferation of retinal vessels into the vitreous humor. Oedema and leakage of blood then leads to retinal detachment and blindness. Careful control of oxygen therapy of the newborn is therefore essential. Retinitis pigmentosa Retinal degeneration is the hallmark of a group of inherited degenerative disorders known as retinitis pigmentosa. Beginning in early life at the periphery of the retina, loss of rods, cones and ganglion cells progresses slowly to blindness by the age of 50. Retinal detachment Detachment of the neuroepithelial layer of the retina from the pigmented layer may be the result of: • fluid exudation • contraction of fibrous tissue formed as a result of haemorrhage • a hole developing in the retina which allows ingress of liquefied vitreous humor between the layers. One in 10 persons over 40 years of age have such holes in their retinas. Retinal detachment deprives the neuroepithelial layer of its blood supply and causes degeneration within 4-5 weeks. This manifests itself as sudden loss of part of the
field of vision.Untreated,the detachment progresses to involve the whole retina. Laser treatment is effective in arresting the progression of visual loss. Colour blindness This rather extreme description is applied to persons who.owing to inherited or acquired factors,do not see colours in the way generally agreed.Acquired defects in colour vision can be divided into those affecting the outer retina.which relate to blue vision.and those affecting the inner retina.which affect red-greem vision. Because there are three types of cones carrying three photopigments there can be three types of defect in colour vision.The absence of the red mechanisn is called protanopia.absence of the green is deuteranopia.and absence of the blue tritanopia. Some individaals do not suffer from a frank loss of one or more of the colour mechanisns but are less sensitive to one of the primary colours.These people are said to suffer froa an ancaaly (protanonaly.deuteranomaly,tritanomaly). The gene for red-green colour blindness is recessive and located on the X chronosone.Because men have only one X chromosome,the presence of this recessive gene inevitably results in red-green colour blindness.Wooen with their XX combination of chromosomes can carry the gene but only express it if it is on both chronosomes.About 8%of European men have a red-green defect.whereas omly 0.4% of wonen show this defect.The gene for the blue cone mechanisn is autosoral. Some people lack all three cone mechanisms and are known as achromats. It has been suggested that diseases of the outer retinal layer produce tritanopia while those of the inner layer and optie nerve produce protan-deuteranopia because the larger nunber of very fine fibres which serve red and green cones will be more likely to be affected by a disease of the inmer layer. These defects are detected by a number of methods,the best known being Ishihara's Test Charts in which a number,made up of a series of dots of a specific colour,is invisible against a background of dots of other colours umless the mechanisn for detecting the nunber's colour is present. Clinical defects of vision
field of vision. Untreated, the detachment progresses to involve the whole retina. Laser treatment is effective in arresting the progression of visual loss. Colour blindness This rather extreme description is applied to persons who, owing to inherited or acquired factors, do not see colours in the way generally agreed. Acquired defects in colour vision can be divided into those affecting the outer retina, which relate to blue vision, and those affecting the inner retina, which affect red-green vision. Because there are three types of cones carrying three photopigments there can be three types of defect in colour vision. The absence of the red mechanism is called protanopia, absence of the green is deuteranopia, and absence of the blue tritanopia. Some individuals do not suffer from a frank loss of one or more of the colour mechanisms but are less sensitive to one of the primary colours. These people are said to suffer from an anomaly (protanomaly, deuteranomaly, tritanomaly). The gene for red-green colour blindness is recessive and located on the X chromosome. Because men have only one X chromosome, the presence of this recessive gene inevitably results in red-green colour blindness. Women with their XX combination of chromosomes can carry the gene but only express it if it is on both chromosomes. About 8% of European men have a red-green defect, whereas only 0.4% of women show this defect. The gene for the blue cone mechanism is autosomal. Some people lack all three cone mechanisms and are known as achromats. It has been suggested that diseases of the outer retinal layer produce tritanopia while those of the inner layer and optic nerve produce protan-deuteranopia because the larger number of very fine fibres which serve red and green cones will be more likely to be affected by a disease of the inner layer. These defects are detected by a number of methods, the best known being Ishihara's Test Charts in which a number, made up of a series of dots of a specific colour, is invisible against a background of dots of other colours unless the mechanism for detecting the number's colour is present. Clinical defects of vision
The sense of vision represents an interaction between our nervous systea and a system of physical optics which provides it with the most basic and important special sense which comnects us with the outside world.Defects of this system are many and for most of us even the normal processes of axeing bring restrictions to our sight. heGo及加0Gt18&BdG0用s The optics of the eye derand that the cornea and its owerlying conjunctiva be transparent and optically correct.To this end they are made up of non-keratinizing stratified squanous epitheliun By far the most common defect of the conjunctiva, at least in the westerm world,is acute bacterial or allergic conjunctivitis where pain and hyperaenia demind nedical attention before serious dange to the optics of the eye occurs.Much more serious is tracko,a lomg-standing chlamydial infection of the cormea where an inflaned mass replaces the superficial layers.This disease is the rost common cause of blindness in underdeveloped countries. Gl&eos圆 Behind the cornea is the anterior chanber,filled with aqueous humor produced by the cillary body and removed by the trabecular meshwork and the canal of Schlemm Pressure of aqueous humor should not exceed 20 mAlg.This is measured in the clinic as the pressure of a strean of air required to indent the cornea.Glaucoma is an increase in intraocular pressure sufficient to cause degeneration of the optic disc and nerve.It is a disorder affecting 4s of people over 40 years of age.The most common cause is obstruction of outflow of aqueous humor.This obstruction is of two major types,causing respectively primary open-angle glaucona and closed-angle glaucoma. Priory open-angle glatxcoma Primary open-angle glaucona is responsible for 90%of cases of raised intraocular pressure.A chronic slow rise in pressure is attributable to microscopic abnornalities in the canal of Schlemm Constriction of the pupil with drugs such as pilocarpine relieves pressure on the canal and aids aqueous flow.Surgical
The sense of vision represents an interaction between our nervous system and a system of physical optics which provides it with the most basic and important special sense which connects us with the outside world. Defects of this system are many and for most of us even the normal processes of ageing bring restrictions to our sight. The conjunctiva and cornea The optics of the eye demand that the cornea and its overlying conjunctiva be transparent and optically correct. To this end they are made up of non-keratinizing stratified squamous epithelium. By far the most common defect of the conjunctiva, at least in the western world, is acute bacterial or allergic conjunctivitis where pain and hyperaemia demand medical attention before serious damage to the optics of the eye occurs. Much more serious is trachoma, a long-standing chlamydial infection of the cornea where an inflamed mass replaces the superficial layers. This disease is the most common cause of blindness in underdeveloped countries. Glaucoma Behind the cornea is the anterior chamber, filled with aqueous humor produced by the ciliary body and removed by the trabecular meshwork and the canal of Schlemm. Pressure of aqueous humor should not exceed 20 mmHg. This is measured in the clinic as the pressure of a stream of air required to indent the cornea. Glaucoma is an increase in intraocular pressure sufficient to cause degeneration of the optic disc and nerve. It is a disorder affecting 4% of people over 40 years of age. The most common cause is obstruction of outflow of aqueous humor. This obstruction is of two major types, causing respectively primary open-angle glaucoma and closed-angle glaucoma. Primary open-angle glaucoma Primary open-angle glaucoma is responsible for 90% of cases of raised intraocular pressure. A chronic slow rise in pressure is attributable to microscopic abnormalities in the canal of Schlemm. Constriction of the pupil with drugs such as pilocarpine relieves pressure on the canal and aids aqueous flow. Surgical
treatment can be successful in severe cases.Treataent arrests visual loss but cannot reverse existing danage. Closed-angle glaucom Closed-angle glaucom is so called because the outflow of aqucous humor is impeded by a change in the angle between the iris and cornea through which the humor leaves the anterior chanber.This acute ophthalmological emergency is usually precipitated by dilatation of the pupil in preparation for fundoscopy in patients with an anatomical predisposition.The rapid increase in intraocular pressure causes severe pain and if not treated by osmotic agents such as mannitol and pupillary constrictors such as pilocarpine can cause complete blindmess in days. The lens Refraction of light to form focused inverted real images on the retina is the result of the light passing through media of different refractive indexes from air through cornea,anterior chamher.lens and posterior chanber.lost refraction takes place at the air-corneal interface,but this is fixed.Accommodation,which focuses light from different distances by changing the thickness of the lens,is mediated by the parasympathetic branch of the oculonotor nerve.By the age of 60 most of the flexibility of the lens to accommodate has been lost.This phenomenon and the mismatch between the optical and biological lengths of the eye that results in the conditions of myopia (nearsightedness)or hyperopia (farsightedness)are so common as to hardly warrant the description pathological.They are therefore dealt with in the main section of this chapter. Cataracts Cataracts are cloudy or opaque areas which develop in the lems.They ray be congenital.inherited in an autosomal fashion.due to fetal infection.especially ruhella,or associated with chromosomal abnormalities or a variety of traunas to the adult lems.Their incidence increases as the lens enlarges with age.The lens has no blood supply and receives its nutrition froa the aqueous humor.It consists of a mass of modified epithelial cells of the ectodern froa which it is derived. In advanced cataracts these cells break domn and undergo dissolution,becoming
treatment can be successful in severe cases. Treatment arrests visual loss but cannot reverse existing damage. Closed-angle glaucoma Closed-angle glaucoma is so called because the outflow of aqueous humor is impeded by a change in the angle between the iris and cornea through which the humor leaves the anterior chamber. This acute ophthalmological emergency is usually precipitated by dilatation of the pupil in preparation for fundoscopy in patients with an anatomical predisposition. The rapid increase in intraocular pressure causes severe pain and if not treated by osmotic agents such as mannitol and pupillary constrictors such as pilocarpine can cause complete blindness in days. The lens Refraction of light to form focused inverted real images on the retina is the result of the light passing through media of different refractive indexes from air through cornea, anterior chamber, lens and posterior chamber. Most refraction takes place at the air-corneal interface, but this is fixed. Accommodation, which focuses light from different distances by changing the thickness of the lens, is mediated by the parasympathetic branch of the oculomotor nerve. By the age of 60 most of the flexibility of the lens to accommodate has been lost. This phenomenon and the mismatch between the optical and biological lengths of the eye that results in the conditions of myopia (nearsightedness) or hyperopia (farsightedness) are so common as to hardly warrant the description pathological. They are therefore dealt with in the main section of this chapter. Cataracts Cataracts are cloudy or opaque areas which develop in the lens. They may be congenital, inherited in an autosomal fashion, due to fetal infection, especially rubella, or associated with chromosomal abnormalities or a variety of traumas to the adult lens. Their incidence increases as the lens enlarges with age. The lens has no blood supply and receives its nutrition from the aqueous humor. It consists of a mass of modified epithelial cells of the ectoderm from which it is derived. In advanced cataracts these cells break down and undergo dissolution, becoming
opaque.High levels of glucose in diabetes produce sorbitol which exerts an osnotic effect,damaging the cells.Climically,cataracts produce halos or spots in the visual field which result in a progressive loss of visual acuity.Current treatment involves renoving the lens and replacing it with an artificial ome. The retina The delicate structure of the ret ina is susceptible to danage from a variety of sources as well as a spectrum of genetic disorders. Retinopathy of premsturity Prerature infants frequently suffer fron respiratory distress syndrone which requires hyperharic oxygem.The immature retina responds to increased partial pressure of oxygen with vasospasm and proliferation of retinal vessels into the vitreous humor.Oedema and leakage of blood then leads to retinal detachnent and blindness.Careful control of oxygen therapy of the newhorn is therefore essential. Retinitis pigeontosa Retinal degeneration is the hallmark of a group of inherited degenerative disorders known as retinitis pignentosa.Beginning in early life at the periphery of the retina.loss of rods,cones and ganglion cells progresses slowly to blindness by the age of 50. Retinal detacheent Detachment of the neuroepithellal layer of the retina froa the pigmented layer may be the result of: fluid exudation contraction of fibrous tissue forned as a result of haenorrhace a hole developing in the retina which allows ingress of liquefied vitreous humor between the layers. One in 10 persons over 40 years of age have such holes in their retinas.Retinal detachaent deprives the neuroepithelial layer of its blood supply and causes degeneration within 4-5 weeks.This manifests itself as sudden loss of part of the field of vision.Untreated.the detachment progresses to involve the whole retina. Laser treatment is effective in arresting the progression of visual loss
opaque. High levels of glucose in diabetes produce sorbitol which exerts an osmotic effect, damaging the cells. Clinically, cataracts produce halos or spots in the visual field which result in a progressive loss of visual acuity. Current treatment involves removing the lens and replacing it with an artificial one. The retina The delicate structure of the retina is susceptible to damage from a variety of sources as well as a spectrum of genetic disorders. Retinopathy of prematurity Premature infants frequently suffer from respiratory distress syndrome which requires hyperbaric oxygen. The immature retina responds to increased partial pressure of oxygen with vasospasm and proliferation of retinal vessels into the vitreous humor. Oedema and leakage of blood then leads to retinal detachment and blindness. Careful control of oxygen therapy of the newborn is therefore essential. Retinitis pigmentosa Retinal degeneration is the hallmark of a group of inherited degenerative disorders known as retinitis pigmentosa. Beginning in early life at the periphery of the retina, loss of rods, cones and ganglion cells progresses slowly to blindness by the age of 50. Retinal detachment Detachment of the neuroepithelial layer of the retina from the pigmented layer may be the result of: • fluid exudation • contraction of fibrous tissue formed as a result of haemorrhage • a hole developing in the retina which allows ingress of liquefied vitreous humor between the layers. One in 10 persons over 40 years of age have such holes in their retinas. Retinal detachment deprives the neuroepithelial layer of its blood supply and causes degeneration within 4-5 weeks. This manifests itself as sudden loss of part of the field of vision. Untreated, the detachment progresses to involve the whole retina. Laser treatment is effective in arresting the progression of visual loss
Colour blindness This rather extreme description is applied to persons who.owing to inherited or acquired factors,do not see colours in the way gemerally agreed.Acquired defects in colour vision can be divided into those affecting the outer retina.which relate to blue vision,and those affecting the inner retina,which affect red-greem vision. Because there are three types of cones carrying three photopignents there can be three types of defect in colour vision.The absence of the red mechanism is called protanopia.absence of the green is deuteranopia.and ahsence of the blue tritanopia. Some individaals do not suffer from a frank loss of one or more of the colour mechanisns but are less sensitive to one of the primary colours.These people are said to suffer froa an anomaly (protancnaly.deuteranomly,tritanomaly). The gene for red-green colour blindness is recessive and located om the X chronosone.Because men have only one X chromosome,the presence of this recessive gene inevitably results in red-green colour blindness.Nomen with their XX combination of chromosomes can carry the gene but only express it if it is on both chromosomes.About 8%of European men have a red-green defect,whereas only 0. of wonen show this defect.The gene for the blue cone mechanisn is autosonal. Some people lack all three cone nechanisms and are known as achromats. It has been suggested that diseases of the outer retinal layer produce tritanopia while those of the inner layer and optic nerve produce protan-deuteranopia because the larger number of very fine fibres which serve red and green cones will be more likely to be affected by a disease of the inner layer. These defects are detected by a number of methods,the best known being Ishihara's Test Charts in which a number,made up of a series of dots of a specific colour,is invisible against a background of dots of other colours unless the mochanisn for detecting the nunber's colour is present
Colour blindness This rather extreme description is applied to persons who, owing to inherited or acquired factors, do not see colours in the way generally agreed. Acquired defects in colour vision can be divided into those affecting the outer retina, which relate to blue vision, and those affecting the inner retina, which affect red-green vision. Because there are three types of cones carrying three photopigments there can be three types of defect in colour vision. The absence of the red mechanism is called protanopia, absence of the green is deuteranopia, and absence of the blue tritanopia. Some individuals do not suffer from a frank loss of one or more of the colour mechanisms but are less sensitive to one of the primary colours. These people are said to suffer from an anomaly (protanomaly, deuteranomaly, tritanomaly). The gene for red-green colour blindness is recessive and located on the X chromosome. Because men have only one X chromosome, the presence of this recessive gene inevitably results in red-green colour blindness. Women with their XX combination of chromosomes can carry the gene but only express it if it is on both chromosomes. About 8% of European men have a red-green defect, whereas only 0.4% of women show this defect. The gene for the blue cone mechanism is autosomal. Some people lack all three cone mechanisms and are known as achromats. It has been suggested that diseases of the outer retinal layer produce tritanopia while those of the inner layer and optic nerve produce protan-deuteranopia because the larger number of very fine fibres which serve red and green cones will be more likely to be affected by a disease of the inner layer. These defects are detected by a number of methods, the best known being Ishihara's Test Charts in which a number, made up of a series of dots of a specific colour, is invisible against a background of dots of other colours unless the mechanism for detecting the number's colour is present