Difference between revisions of "Book - Buchanan's Manual of Anatomy including Embryology 16"

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=Chapter XV! The Eye=
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CHAPTER XVI THE EYE[edit]
 
 
The eyeball is almost spherical. It consists of the segments of two spheres—namely, a large posterior or sclerotic segment, which is opaque, and a small anterior or corneal segment, which is transparent. The sclerotic segment forms five-sixths of the eyeball, and the corneal segment one-sixth. The centre of the corneal segment is called the anterior pole, and the centre of the sclerotic segment is known as the posterior pole. The sagittal (antero-posterior) axis, or axis of vision, of the eyeball is represented by a line connecting the anterior and posterior poles. The equator is represented by a line encircling the centre of the eyeball in a coronal plane, the diameter of the circle being about I inch. The plane of this circle would therefore divide the eyeball into two halves—an anterior half, consisting of the corneal and the front part of the sclerotic segment, and a posterior half, consisting of the back part of the sclerotic segment. The meridian is represented by a line encircling the eyeball horizontally at right angles to the equator, and passing through the anterior and posterior poles.
 
The eyeball is almost spherical. It consists of the segments of two spheres—namely, a large posterior or sclerotic segment, which is opaque, and a small anterior or corneal segment, which is transparent. The sclerotic segment forms five-sixths of the eyeball, and the corneal segment one-sixth. The centre of the corneal segment is called the anterior pole, and the centre of the sclerotic segment is known as the posterior pole. The sagittal (antero-posterior) axis, or axis of vision, of the eyeball is represented by a line connecting the anterior and posterior poles. The equator is represented by a line encircling the centre of the eyeball in a coronal plane, the diameter of the circle being about I inch. The plane of this circle would therefore divide the eyeball into two halves—an anterior half, consisting of the corneal and the front part of the sclerotic segment, and a posterior half, consisting of the back part of the sclerotic segment. The meridian is represented by a line encircling the eyeball horizontally at right angles to the equator, and passing through the anterior and posterior poles.
  
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==External Coat==
 
 
 
 
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A MANUAL OF ANATOMY
 
 
 
 
 
External Coat.
 
  
 
Sclera (or Sclerotic Coat).—The sclera (white of the eye) is a strong white fibrous coat of great density, which surrounds the posterior five-sixths of the eyeball, and maintains the shape of the organ. Anteriorly it unites, and becomes continuous with the cornea, which it slightly overlaps. The junction of the two is indicated by a slight groove, called the sulcus sclerce, and the union is known as the corneoscleral junction . Posteriorly, as has been shown above, the sclera is pierced by the optic nerve a little below and to' the inner side of the centre. The part of the sclera corresponding to the optic entrance
 
Sclera (or Sclerotic Coat).—The sclera (white of the eye) is a strong white fibrous coat of great density, which surrounds the posterior five-sixths of the eyeball, and maintains the shape of the organ. Anteriorly it unites, and becomes continuous with the cornea, which it slightly overlaps. The junction of the two is indicated by a slight groove, called the sulcus sclerce, and the union is known as the corneoscleral junction . Posteriorly, as has been shown above, the sclera is pierced by the optic nerve a little below and to' the inner side of the centre. The part of the sclera corresponding to the optic entrance
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Rectus Inferior
 
Rectus Inferior
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
THE EYE
 
 
 
1643
 
  
  
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Cornea.—The cornea is the transparent part of the external coat of the eyeball, of which it forms the anterior sixth, and serves to transmit light. It is almost circular, its transverse measurement being slightly greater than the vertical. At its circumference it is continuous with the sclera, by which it is slightly overlapped. The anterior surface is convex. The posterior surface is concave, and forms the anterior boundary of the anterior chamber of the eye.
 
Cornea.—The cornea is the transparent part of the external coat of the eyeball, of which it forms the anterior sixth, and serves to transmit light. It is almost circular, its transverse measurement being slightly greater than the vertical. At its circumference it is continuous with the sclera, by which it is slightly overlapped. The anterior surface is convex. The posterior surface is concave, and forms the anterior boundary of the anterior chamber of the eye.
  
Structure.— The cornea consists of the following five layers, from
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Structure.— The cornea consists of the following five layers, from before backwards: .
 
 
before backwards: .
 
  
 
1. The conjunctival epithelium.
 
1. The conjunctival epithelium.
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5. A layer of endothelium.
 
5. A layer of endothelium.
  
The conjunctival epithelium is stratified, there being not less than five strata of cells, and is continuous with the epithelium, which covers
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The conjunctival epithelium is stratified, there being not less than five strata of cells, and is continuous with the epithelium, which covers Stratified Epithelium of / Conjunctiva Membrane of Bowman or Anterior Elastic Lamina
 
 
 
 
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A MANUAL OF ANATOMY
 
 
 
 
 
^ \ Stratified Epithelium of / Conjunctiva Membrane of Bowman or Anterior Elastic Lamina
 
  
  
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The anterior elastic lamina (Bowman) is probably of the same nature as the fibrous portion of the substantia propria. It is closely connected with the substantia propria, is thin, and contains no corpuscles.
 
The anterior elastic lamina (Bowman) is probably of the same nature as the fibrous portion of the substantia propria. It is closely connected with the substantia propria, is thin, and contains no corpuscles.
  
The substantia propria is composed of modified connective tissue arranged in bundles which form superimposed laminae. These laminae amount in number to about sixty. The fibres of alternate laminae cross each other at right angles, and at the circumference of the cornea
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The substantia propria is composed of modified connective tissue arranged in bundles which form superimposed laminae. These laminae amount in number to about sixty. The fibres of alternate laminae cross each other at right angles, and at the circumference of the cornea they are continuous with the fibres of the sclerotic. The successive laminae are connected by cement substance, and within this substance are branched spaces, called the corneal spaces or lacunae, which communicate with each other by very delicate canaliculi. Each of these spaces contains a nucleated connective-tissue corpuscle, called the corneal corpuscle. These corpuscles, like the spaces which they occupy, are branched, and the offsets of adjacent corpuscles communicate with one another. As seen in vertical sections of the cornea, the corpuscles are spindle - shaped, but in
 
 
they are continuous with
 
 
 
 
 
the fibres of the sclerotic. The successive laminae are connected by cement substance, and within this substance are branched spaces, called the corneal spaces or lacunae, which communicate with each other by very delicate canaliculi. Each of these spaces contains a nucleated connective-tissue corpuscle, called the corneal corpuscle. These corpuscles, like the spaces which they occupy, are branched, and the offsets of adjacent corpuscles communicate with one another. As seen in vertical sections of the cornea, the corpuscles are spindle - shaped, but in
 
  
  
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The posterior elastic lamina (or membrane of Descemet) covers the posterior surface of the substantia propria. It is thicker than the anterior elastic lamina, and .is composed of an elastic homogeneous membrane, which is very brittle. When stripped from the substantia propria it comes away in shreds, and these curl up at their ends in such a manner that the anterior or attached surface of each shred is turned inwards. At the circumference of the cornea the posterior elastic lamina becomes broken up into fibres. The most posterior of these fibres pass in a radiating manner into the iris, and they form the ligamentum pectinatum iridis, the intervals between the fibres of which represent the spaces of the irido-corneal angle.
 
The posterior elastic lamina (or membrane of Descemet) covers the posterior surface of the substantia propria. It is thicker than the anterior elastic lamina, and .is composed of an elastic homogeneous membrane, which is very brittle. When stripped from the substantia propria it comes away in shreds, and these curl up at their ends in such a manner that the anterior or attached surface of each shred is turned inwards. At the circumference of the cornea the posterior elastic lamina becomes broken up into fibres. The most posterior of these fibres pass in a radiating manner into the iris, and they form the ligamentum pectinatum iridis, the intervals between the fibres of which represent the spaces of the irido-corneal angle.
  
The layer of endothelium lines the posterior surface of the posterior
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The layer of endothelium lines the posterior surface of the posterior elastic lamina, and consists of one stratum of cells. It is continued over the front of the iris, and into the spaces of the angle.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
THE EYE 1645
 
 
 
elastic lamina, and consists of one stratum of cells. It is continued over the front of the iris, and into the spaces of the angle.
 
  
 
The cornea in the adult is non-vascular, except at the circumference, in which situation there are the conjunctival and sclerotic capillaries, which terminate in loops. Being destitute of blood-vessels, the nourishment of the cornea is maintained by the flow of lymph through its surface. It is about 1 mm. thick, slightly more peripherally.
 
The cornea in the adult is non-vascular, except at the circumference, in which situation there are the conjunctival and sclerotic capillaries, which terminate in loops. Being destitute of blood-vessels, the nourishment of the cornea is maintained by the flow of lymph through its surface. It is about 1 mm. thick, slightly more peripherally.
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Spaces of Irido-corneal Angle (or Spaces of Fontana).—These spaces represent the irregular intervals which lie between the radiating fibres of the pectinate ligament. They are lined by a prolongation of the endothelial layer of the cornea, and they communicate internally with the anterior chamber and the lymph-spaces within the iris, and externally with the sinus venosus sclerae.
 
Spaces of Irido-corneal Angle (or Spaces of Fontana).—These spaces represent the irregular intervals which lie between the radiating fibres of the pectinate ligament. They are lined by a prolongation of the endothelial layer of the cornea, and they communicate internally with the anterior chamber and the lymph-spaces within the iris, and externally with the sinus venosus sclerae.
  
Sinus Venosus Sclerse.—This canal (formerly known as the canal of Schlemm) is situated deeply in the sclerotic, close to the corneo-scleral junction. It communicates internally with the anterior chamber through the spaces of the irido-corneal angle, and externally with anterior ciliary veins of the sclera. It encircles the outer margin of the cornea, and has a little projecting rim of sclerotic on its deep surface, called the ‘ scleral spur/ from which the ciliary muscle takes
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Sinus Venosus Sclerse.—This canal (formerly known as the canal of Schlemm) is situated deeply in the sclerotic, close to the corneo-scleral junction. It communicates internally with the anterior chamber through the spaces of the irido-corneal angle, and externally with anterior ciliary veins of the sclera. It encircles the outer margin of the cornea, and has a little projecting rim of sclerotic on its deep surface, called the ‘ scleral spur/ from which the ciliary muscle takes origin.
  
origin.
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==Middle Coat==
 
 
Middle Coat.
 
  
 
1. Choroid Coat.— This is a very vascular, deeply pigmented tunic of a dark brown colour, which lies between the sclera and the retina. It extends over the posterior five-sixths of the eyeball, and reaches as far forwards as the ora serrata of the retina. Anteriorly it is connected with the circumference of the iris, and posteriorly it is pierced by the optic nerve. Its outer surface is connected to the inner surface of the sclera by means of the lamina fusca and its processes, as well as by vessels and nerves which cross the ‘ perichoroidal lymph-space. Its inner surface is in contact with the pigmentary- layer of the retina.
 
1. Choroid Coat.— This is a very vascular, deeply pigmented tunic of a dark brown colour, which lies between the sclera and the retina. It extends over the posterior five-sixths of the eyeball, and reaches as far forwards as the ora serrata of the retina. Anteriorly it is connected with the circumference of the iris, and posteriorly it is pierced by the optic nerve. Its outer surface is connected to the inner surface of the sclera by means of the lamina fusca and its processes, as well as by vessels and nerves which cross the ‘ perichoroidal lymph-space. Its inner surface is in contact with the pigmentary- layer of the retina.
  
  
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Structure. — The choroid coat consists of connective tissue, bloodvessels, and branched pigment-cells. It is composed of three layers, which are as follows, from without inwards: (1) the lamina supra choroidea; (2) the choroid proper; and (3) the lamina basalis, or membrane of Bruch.
A MANUAL OF ANATOMY
 
 
 
 
 
Structure.—The choroid coat consists of connective tissue, bloodvessels, and branched pigment-cells. It is composed of three layers, which are as follows, from without inwards: (1) the lamina supra choroidea; (2) the choroid proper; and (3) the lamina basalis, or membrane of Bruch.
 
  
 
The suprachoroid lamina is composed of delicate, non-vascular lamellae, each of which is made up of elastic fibres arranged in a reticular manner, and of branched pigment-cells.
 
The suprachoroid lamina is composed of delicate, non-vascular lamellae, each of which is made up of elastic fibres arranged in a reticular manner, and of branched pigment-cells.
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Tapetum. — This is present in certain animals. It lies between the lamina vaseulosa and the lamina chorio-capillaris in the stratum intermedium, and it gives rise to an iridescent or rainbow-like appearance. In some animals it is fibrous in structure, and in others cellular.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
THE EYE
 
 
 
 
 
1647
 
 
 
 
 
Tapetum. —This is present in certain animals. It lies between the lamina vaseulosa and the lamina chorio-capillaris in the stratum intermedium, and it gives rise to an iridescent or rainbow-like appearance. In some animals it is fibrous in structure, and in others cellular.
 
  
 
2. Ciliary Body.—The ciliary body connects the anterior part of the choroid to the circumference of the iris. It is composed of (1) the orbicularis ciliaris, (2) the ciliary processes, and (3) the ciliary muscle.
 
2. Ciliary Body.—The ciliary body connects the anterior part of the choroid to the circumference of the iris. It is composed of (1) the orbicularis ciliaris, (2) the ciliary processes, and (3) the ciliary muscle.
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of rays arranged in a circular manner, and converge as they pass inwards and forwards to the periphery of the crystalline lens on its anterior aspect. They are somewhat conical in outline.. Their bases or free extremities, which are round and prominent, lie behind the circumference of the iris upon the anterior aspect of the periphery of the crystalline lens. Their apices are connected with the orbicularis ciliaris Anteriorly they are related to the posterior chamber of the eyeball at its circumference. Posteriorly they are related to and connected with the suspensory ligament of the lens.
 
of rays arranged in a circular manner, and converge as they pass inwards and forwards to the periphery of the crystalline lens on its anterior aspect. They are somewhat conical in outline.. Their bases or free extremities, which are round and prominent, lie behind the circumference of the iris upon the anterior aspect of the periphery of the crystalline lens. Their apices are connected with the orbicularis ciliaris Anteriorly they are related to the posterior chamber of the eyeball at its circumference. Posteriorly they are related to and connected with the suspensory ligament of the lens.
  
Structure._The ciliary processes are similar in structure to the
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Structure._The ciliary processes are similar in structure to the choroid, but the pigment-cells are not so numerous. On their deep or posterior surfaces the processes are covered by the ciliary part of the retina, which is prolonged from the pigmentary layer of the retina, and is continuous with the pars iridica retinae (uvea) on the posterior surface of the iris.
 
 
choroid, but the pigment-cells are not so numerous. On their deep or posterior surfaces the processes are covered by the ciliary part of the retina, which is prolonged from the pigmentary layer of the retina, and is continuous with the pars iridica retinae (uvea) on the posterior surface of the iris.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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A MANUAL OF ANATOMY
 
 
 
  
 
The arteries of the ciliary processes are derived from those of the anterior part of the choroid, and from the anterior ciliary arteries. The veins pass to those of the choroid.
 
The arteries of the ciliary processes are derived from those of the anterior part of the choroid, and from the anterior ciliary arteries. The veins pass to those of the choroid.
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Ciliary Muscle. —This muscle is composed of unstriped fibres. It forms a greyish-white ring, about T V inch broad, which is situated at the anterior part of the choroid opposite the ciliary processes. The fibres are arranged in two sets—radial and circular. The radial fibres arise from the calcar sclerae close to the corneo-scleral junction and behind the sinus venosus of the sclera. From this origin they pass backwards in a radiating manner, and are inserted into the orbicularis ciliaris and the attached ends of the ciliary processes. The circular fibres form a ring around the circumference of the iris internal to the radial fibres.
 
Ciliary Muscle. —This muscle is composed of unstriped fibres. It forms a greyish-white ring, about T V inch broad, which is situated at the anterior part of the choroid opposite the ciliary processes. The fibres are arranged in two sets—radial and circular. The radial fibres arise from the calcar sclerae close to the corneo-scleral junction and behind the sinus venosus of the sclera. From this origin they pass backwards in a radiating manner, and are inserted into the orbicularis ciliaris and the attached ends of the ciliary processes. The circular fibres form a ring around the circumference of the iris internal to the radial fibres.
  
 
Pupil
 
  
  
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The circular fibres of the ciliary muscle are well developed in cases of hypermetropia, but are deficient in cases of myopia.
 
The circular fibres of the ciliary muscle are well developed in cases of hypermetropia, but are deficient in cases of myopia.
  
3. Iris. —The iris forms the anterior part of the middle coat of the eyeball. It is a coloured contractile diaphragm, which is suspended in the aqueous humour between the cornea and the crystalline lens. It is perforated by an almost circular aperture, called the pupil, which is situated slightly to the nasal or inner side of its centre, and serves for the transmission of light. The margin which surrounds the pupil is known as the pupillary margin. Its circumference is continuous
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3. Iris. —The iris forms the anterior part of the middle coat of the eyeball. It is a coloured contractile diaphragm, which is suspended in the aqueous humour between the cornea and the crystalline lens. It is perforated by an almost circular aperture, called the pupil, which is situated slightly to the nasal or inner side of its centre, and serves for the transmission of light. The margin which surrounds the pupil is known as the pupillary margin. Its circumference is continuous with the ciliary body, and is connected with the posterior elastic lamina of the cornea by means of the ligamentum pectinatum iridis
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
THE EYE
 
 
 
 
 
1649
 
 
 
with the ciliary body, and is connected with the posterior elastic lamina of the cornea by means of the ligamentum pectinatum iridis
 
  
  
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at the iridial angle. The circumference is known as the ciliary margin. The surfaces of the iris are anterior and posterior. The anterior
 
at the iridial angle. The circumference is known as the ciliary margin. The surfaces of the iris are anterior and posterior. The anterior
 
TO4
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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A MANUAL OF ANATOMY
 
 
  
 
surface is directed towards the cornea. Its colour varies in different individuals, and it presents a striated appearance, the striae converging towards the margin of the pupil, and being produced by the underlying vessels. The posterior surface is directed towards the crystalline lens and ciliary processes. It has a purple colour, and is covered by two layers of columnar epithelium, the cells of which contain dark pigment. These two layers of pigmented cells constitute the pars iridica retinae (uvea), which is continuous with the pars ciliaris retinae.
 
surface is directed towards the cornea. Its colour varies in different individuals, and it presents a striated appearance, the striae converging towards the margin of the pupil, and being produced by the underlying vessels. The posterior surface is directed towards the crystalline lens and ciliary processes. It has a purple colour, and is covered by two layers of columnar epithelium, the cells of which contain dark pigment. These two layers of pigmented cells constitute the pars iridica retinae (uvea), which is continuous with the pars ciliaris retinae.
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The layer of endothelium covers the anterior surface of the iris, and is continuous with the endothelium which lines the posterior elastic lamina of the cornea.
 
The layer of endothelium covers the anterior surface of the iris, and is continuous with the endothelium which lines the posterior elastic lamina of the cornea.
 
 
Long Ciliary Artery
 
 
  
  
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The muscular tissue is of the unstriped variety, and its fibres are arranged in two sets, circular and radiating. The circular fibres form a ring round the pupil, and are nearer the posterior surface than the anterior. They are known as the sphincter pupillse. The radiating fibres converge from the ciliary margin of the iris towards the pupillary margin, where they blend with the circular fibres. The radiating fibres constitute the dilator pupillse. Some authorities regard the radiating fibres as elastic, and not muscular.
 
The muscular tissue is of the unstriped variety, and its fibres are arranged in two sets, circular and radiating. The circular fibres form a ring round the pupil, and are nearer the posterior surface than the anterior. They are known as the sphincter pupillse. The radiating fibres converge from the ciliary margin of the iris towards the pupillary margin, where they blend with the circular fibres. The radiating fibres constitute the dilator pupillse. Some authorities regard the radiating fibres as elastic, and not muscular.
  
The pigment of the iris is variously situated, according to the colour of the eye. In the eyes of albinos there is no pigment. In other eyes pigment is contained in the cells of the two layers of columnar
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The pigment of the iris is variously situated, according to the colour of the eye. In the eyes of albinos there is no pigment. In other eyes pigment is contained in the cells of the two layers of columnar epithelium which line the posterior surface of the iris, and form the pars iridica retinae (uvea). In blue eyes the pigment is largely confined to this region, but in other coloured eyes it is also present in the branched cells of the connective-tissue stroma.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
THE EYE
 
 
 
 
 
1651
 
 
 
epithelium which line the posterior surface of the iris, and form the pars iridica retinae (uvea). In blue eyes the pigment is largely confined to this region, but in other coloured eyes it is also present in the branched cells of the connective-tissue stroma.
 
  
 
Blood-supply—Arteries. —The arteries of the iris are derived from (1) the long ciliary, and (2) the anterior ciliary vessels.
 
Blood-supply—Arteries. —The arteries of the iris are derived from (1) the long ciliary, and (2) the anterior ciliary vessels.
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The long ciliary arteries are two in number, and belong to the posterior ciliary group of branches from the ophthalmic artery. They pierce the back part of the sclera, one on each side of the optic nerve, and pass forwards between the sclera and the choroid towards the ciliary margin of the iris. Here each vessel divides into two branches, upper and lower, which anastomose with those of the opposite side to form an arterial ring round the ciliary margin of the iris, called the circuius arteriosus major. This ring is joined by some of the anterior ciliary arteries, and it gives offsets to the ciliary muscle
 
The long ciliary arteries are two in number, and belong to the posterior ciliary group of branches from the ophthalmic artery. They pierce the back part of the sclera, one on each side of the optic nerve, and pass forwards between the sclera and the choroid towards the ciliary margin of the iris. Here each vessel divides into two branches, upper and lower, which anastomose with those of the opposite side to form an arterial ring round the ciliary margin of the iris, called the circuius arteriosus major. This ring is joined by some of the anterior ciliary arteries, and it gives offsets to the ciliary muscle
  
 
Pupil
 
 
Iris
 
  
  
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The veins of the iris accompany the arteries, and are in communication with the sinus venosus sclerse.
 
The veins of the iris accompany the arteries, and are in communication with the sinus venosus sclerse.
  
Nerves of the Choroid Coat and Iris. —These are derived from the ciliary nerves, short and long, the former coming from the ciliary ganglion, and the latter from the naso-ciliary branch of the ophthalmic or first division of the fifth cranial nerve. They are about sixteen
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Nerves of the Choroid Coat and Iris. —These are derived from the ciliary nerves, short and long, the former coming from the ciliary ganglion, and the latter from the naso-ciliary branch of the ophthalmic or first division of the fifth cranial nerve. They are about sixteen in number, and pierce the back part of the sclera around the optic nerve. They then pass forwards between the sclerotic and choroid, giving branches to the latter coat, which become disposed in a plexiform manner amongst the bloodvessels. Having reached the corneoscleral junction, the nerves enter the ciliary muscle, in which they form a plexus. From this plexus branches enter the iris at the ciliary margin. These branches accompany the vessels, and by their subdivisions and communications they form a copious plexus of nonmedullated fibres in the connective-tissue stroma of the iris. The sphincter pupillse is supplied by fibres which are derived from the oculomotor or third cranial nerve by means of the motor root of the ciliary ganglion. The dilator pupillae is supplied by fibres which may be traced to the second thoracic ganglion through the sympathetic root of the ciliary ganglion (see p. 1637).
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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A MANUAL OF ANATOMY
 
 
 
 
 
in number, and pierce the back part of the sclera around the optic nerve. They then pass forwards between the sclerotic and choroid, giving branches to the latter coat, which become disposed in a plexiform manner amongst the bloodvessels. Having reached the corneoscleral junction, the nerves enter the ciliary muscle, in which they form a plexus. From this plexus branches enter the iris at the ciliary margin. These branches accompany the vessels, and by their subdivisions and communications they form a copious plexus of nonmedullated fibres in the connective-tissue stroma of the iris. The sphincter pupillse is supplied by fibres which are derived from the oculomotor or third cranial nerve by means of the motor root of the ciliary ganglion. The dilator pupillae is supplied by fibres which may be traced to the second thoracic ganglion through the sympathetic root of the ciliary ganglion (see p. 1637).
 
  
 
Membrana Pupillaris. —During intra-uterine life the pupil is closed by a delicate membrane, called the membrana pupillaris. This disappears shortly before birth, but remnants of it are sometimes found.
 
Membrana Pupillaris. —During intra-uterine life the pupil is closed by a delicate membrane, called the membrana pupillaris. This disappears shortly before birth, but remnants of it are sometimes found.
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Internal Coat.
 
Internal Coat.
  
Retina.—The retina is the internal or nervous tunic of the eyeball. It is soft in consistence, translucent, and of a pinkish colour. Its internal surface is in contact with the hyaloid membrane, which
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Retina. — The retina is the internal or nervous tunic of the eyeball. It is soft in consistence, translucent, and of a pinkish colour. Its internal surface is in contact with the hyaloid membrane, which
  
  
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encloses the vitreous body, and its external surface is in contact with the choroid coat. Posteriorly it receives the fibres of the optic nerve. Anteriorly it extends almost to the ciliary body, where there is a notched border, called the ora serrata. Here its nervous elements cease, but its pigmentary layer is continued over the deep or posterior
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encloses the vitreous body, and its external surface is in contact with the choroid coat. Posteriorly it receives the fibres of the optic nerve. Anteriorly it extends almost to the ciliary body, where there is a notched border, called the ora serrata. Here its nervous elements cease, but its pigmentary layer is continued over the deep or posterior surfaces of the ciliary processes on to the posterior surface of the iris, forming, with the addition of a layer of columnar epithelial cells, the pars ciliaris retinae and pars iridica retinae (uvea) respectively. The retina diminishes in thickness from behind forwards.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
THE EYE
 
 
 
 
 
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surfaces of the ciliary processes on to the posterior surface of the iris, forming, with the addition of a layer of columnar epithelial cells, the pars ciliaris retinae and pars iridica retinae (uvea) respectively. The retina diminishes in thickness from behind forwards.
 
  
 
The external surface is formed by a stratum of hexagonal pigmentcells, which send processes into the adjacent layer. When the choroid is separated from the retina these processes are torn, and the stratum of pigment-cells remains attached to the choroid, being apparently a part of it. The pigmentary layer, however, really belongs to the retina.
 
The external surface is formed by a stratum of hexagonal pigmentcells, which send processes into the adjacent layer. When the choroid is separated from the retina these processes are torn, and the stratum of pigment-cells remains attached to the choroid, being apparently a part of it. The pigmentary layer, however, really belongs to the retina.
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Structure of the Retina.— The retina consists of eight superimposed layers, seven of which are nervous and one pigmentary.
 
Structure of the Retina.— The retina consists of eight superimposed layers, seven of which are nervous and one pigmentary.
  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
A MANUAL OF ANATOMY
 
 
 
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Membrana Limitans Interna
 
Membrana Limitans Interna
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
THE EYE
 
 
 
1655
 
  
  
Line 909: Line 460:
 
6 Outer Nuclear or Granular Layer.—This consists of granules,
 
6 Outer Nuclear or Granular Layer.—This consists of granules,
  
which are of two kinds—rod-granules and cone-granules. The rodgranules are the more numerous, and are oval enlargements m the course of the rod-fibres, as these pass to the outer plexiform layer'• Each rod-fibre has only one rod-granule, and the granules lie at different
+
which are of two kinds—rod-granules and cone-granules. The rodgranules are the more numerous, and are oval enlargements m the course of the rod-fibres, as these pass to the outer plexiform layer'• Each rod-fibre has only one rod-granule, and the granules lie at different levels. Each granule has a nucleus, which has transverse striations, there being at least two clear bands. The external process of each rod-granule is continuous with one of the rods of the bacillary layer, and the internal process passes into the outer plexiform layer, where it comes into relation with the arborizations of the external process of a rod-bipolar.
 
 
 
 
1656
 
 
 
 
 
A MANUAT, OF ANATOMY
 
 
 
 
 
levels. Each granule has a nucleus, which has transverse striations, there being at least two clear bands. The external process of each rod-granule is continuous with one of the rods of the bacillary layer, and the internal process passes into the outer plexiform layer, where it comes into relation with the arborizations of the external process of a rod-bipolar.
 
  
 
The cone-granules are larger than the rod-granules, but not so numerous, and each contains an oval nucleus. Situated in the outermost part of the outer nuclear layer, they lie close to the membrana limitans externa. The outer end of each granule is continuous with one of the cones of the bacillary layer. The inner end is prolonged into a cone-fibre, which passes into the outermost part of the outer
 
The cone-granules are larger than the rod-granules, but not so numerous, and each contains an oval nucleus. Situated in the outermost part of the outer nuclear layer, they lie close to the membrana limitans externa. The outer end of each granule is continuous with one of the cones of the bacillary layer. The inner end is prolonged into a cone-fibre, which passes into the outermost part of the outer
Line 924: Line 466:
  
  
Fig. 1016. —Scheme of the Horizontal Cells and Spongioblasts of the
+
Fig. 1016. —Scheme of the Horizontal Cells and Spongioblasts of the Retina (Ramon y Cajal).
 
 
Retina (Ramon y Cajal).
 
  
  
Line 943: Line 483:
 
7. Layer of Rods and Cones consists of rods and cones, the former being cylindrical, and the latter flask-shaped. The rods are much more numerous, longer, and narrower than the cones, and both are placed perpendicularly.
 
7. Layer of Rods and Cones consists of rods and cones, the former being cylindrical, and the latter flask-shaped. The rods are much more numerous, longer, and narrower than the cones, and both are placed perpendicularly.
  
Each rod and cone consists of two segments—outer and inner. In the case of the rods the two segments are of almost equal length the inner segment being rather larger than the outer. The outer segment is the only seat of the colouring matter known as visual purple or rhodopsin In the case of the flask-shaped cones, the inner segment of each forms two-thirds of the cone, and is of large size; whilst the
+
Each rod and cone consists of two segments—outer and inner. In the case of the rods the two segments are of almost equal length the inner segment being rather larger than the outer. The outer segment is the only seat of the colouring matter known as visual purple or rhodopsin In the case of the flask-shaped cones, the inner segment of each forms two-thirds of the cone, and is of large size; whilst the outer forms one-third, is narrow, and represents the tapering part of the flask. The outer segments of both rods and cones have faint transverse striations. The inner segments of both are subdivided. The outer part is composed of delicate fibrils longitudinally arranged, and therefore presents a longitudinally striated appearance. The inner part is faintly granular. The rods and cones are continued at their inner ends through the membrana limitans externa into the rod-fibres and cone-fibres, which belong to the outer nuclear layer. The outer ends of the rods project into the pigmentary layer.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
THE EYE
 
 
 
 
 
1657
 
 
 
outer forms one-third, is narrow, and represents the tapering part of the flask. The outer segments of both rods and cones have faint transverse striations. The inner segments of both are subdivided. The outer part is composed of delicate fibrils longitudinally arranged, and therefore presents a longitudinally striated appearance. The inner part is faintly granular. The rods and cones are continued at their inner ends through the membrana limitans externa into the rod-fibres and cone-fibres, which belong to the outer nuclear layer. The outer ends of the rods project into the pigmentary layer.
 
  
 
8 . Pigmentary Layer.—The most external layer of the retina is in close contact with the choroid coat. It consists of a single layer of hexagonal epithelial cells, which contain pigment. The deep surfaces of the cells give off processes which extend into the intervals between the outer ends of the rods and
 
8 . Pigmentary Layer.—The most external layer of the retina is in close contact with the choroid coat. It consists of a single layer of hexagonal epithelial cells, which contain pigment. The deep surfaces of the cells give off processes which extend into the intervals between the outer ends of the rods and
Line 1,015: Line 522:
  
 
These fibres form the supporting tissue of the retina, and extend from its internal surface to the boundary-line between the outer nuclear layer and the layer of rods and cones. The inner ends of the fibres are expanded, and blend at their edges to present the appearance of a distinct retinal layer, which is called the membrana limitans interna. Their outer ends, which are very numerous owing to the breaking up of the fibres, also expand and form the membrana limitans externa, which lies between the outer nuclear layer and the layer of rods and cones. (The membrana limitans interna and externa are sometimes considered layers of the retina, under which circumstances the retinal layers would be ten in number, instead of eight.) From the membrana limitans externa delicate offsets enter the layer of rods and cones, in the innermost part of which they form fibre-baskets in connection with the deep ends of the rods and cones. As the sustentacular fibres pass through the inner nuclear layer each has an oval nucleus, which contains a nucleolus. This nucleus is variously described as being situated on one side of the fibre, or as involving its whole circumference. Throughout their course the sustentacular fibres give off lateral offsets, which increase in number from within outwards.
 
These fibres form the supporting tissue of the retina, and extend from its internal surface to the boundary-line between the outer nuclear layer and the layer of rods and cones. The inner ends of the fibres are expanded, and blend at their edges to present the appearance of a distinct retinal layer, which is called the membrana limitans interna. Their outer ends, which are very numerous owing to the breaking up of the fibres, also expand and form the membrana limitans externa, which lies between the outer nuclear layer and the layer of rods and cones. (The membrana limitans interna and externa are sometimes considered layers of the retina, under which circumstances the retinal layers would be ten in number, instead of eight.) From the membrana limitans externa delicate offsets enter the layer of rods and cones, in the innermost part of which they form fibre-baskets in connection with the deep ends of the rods and cones. As the sustentacular fibres pass through the inner nuclear layer each has an oval nucleus, which contains a nucleolus. This nucleus is variously described as being situated on one side of the fibre, or as involving its whole circumference. Throughout their course the sustentacular fibres give off lateral offsets, which increase in number from within outwards.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
1658
 
 
 
A MANUAL OF ANATOMY
 
 
  
 
Structure of the Macula Lutea and Fovea Centralis.—The chief structural characters of the macula lutea and fovea centralis may be stated in the following tabular manner:
 
Structure of the Macula Lutea and Fovea Centralis.—The chief structural characters of the macula lutea and fovea centralis may be stated in the following tabular manner:
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Blood-supply of the Retina.—The retina is supplied with blood by the arteria centralis retinae, a branch of the ophthalmic artery. Within the orbit the artery pierces the under aspect of the optic nerve a little behind the eyeball, and passes forwards in the centre of the nerve. At the centre of the optic disc it divides into two branches, upper and lower. Each of these breaks up into two branches, nasal or medial,
 
Blood-supply of the Retina.—The retina is supplied with blood by the arteria centralis retinae, a branch of the ophthalmic artery. Within the orbit the artery pierces the under aspect of the optic nerve a little behind the eyeball, and passes forwards in the centre of the nerve. At the centre of the optic disc it divides into two branches, upper and lower. Each of these breaks up into two branches, nasal or medial,
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
THE EYE
 
 
 
1659
 
 
  
 
and temporal or lateral. The temporal branches keep clear of the macula lutea, but give small twigs to it, which, however, do not enter the fovea centralis, this part being non-vascular. As the branches pass inwards and outwards respectively towards the periphery of the retina they ramify freely, and end at last in capillary networks. The arteries do not extend farther outwards than the inner nuclear layer. No anastomoses take place between the branches of the arteria centralis retinae.
 
and temporal or lateral. The temporal branches keep clear of the macula lutea, but give small twigs to it, which, however, do not enter the fovea centralis, this part being non-vascular. As the branches pass inwards and outwards respectively towards the periphery of the retina they ramify freely, and end at last in capillary networks. The arteries do not extend farther outwards than the inner nuclear layer. No anastomoses take place between the branches of the arteria centralis retinae.
Line 1,301: Line 668:
 
In the foetus the arteria centralis retinae sends a branch to the posterior part of the capsule of the crystalline lens, which reaches it through the ‘ canal of Cloquet in the vitreous body.
 
In the foetus the arteria centralis retinae sends a branch to the posterior part of the capsule of the crystalline lens, which reaches it through the ‘ canal of Cloquet in the vitreous body.
  
The veins are ultimately collected into two vessels, upper and lower, which pass through the optic disc, one above and the other below' the artery. They then form one vessel which opens into the superior ophthalmic vein. The veins of the retina are destitute of muscular tissue, the wall of each being formed by a single layer of endothelial cells, external to which there is a perivascular lymph
+
The veins are ultimately collected into two vessels, upper and lower, which pass through the optic disc, one above and the other below' the artery. They then form one vessel which opens into the superior ophthalmic vein. The veins of the retina are destitute of muscular tissue, the wall of each being formed by a single layer of endothelial cells, external to which there is a perivascular lymph space, this in turn being limited by another layer of endothelial cells. These lymph-spaces are in communication with those of the optic nerve.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
i66o
 
 
 
 
 
A MANUAL OF ANATOMY
 
 
 
 
 
space, this in turn being limited by another layer of endothelial cells. These lymph-spaces are in communication with those of the optic nerve.
 
  
 
Relation of the Retinal Layers to One Another.—The only two layers which are in direct continuity are the stratum opticum and ganglionic layer, some fibres of the former being the axons of the cells of the latter. As regards most of the strata, the constituent elements of successive layers are brought into communication by means of the interlacements which take place between the arborizations formed by their various processes. These interlacements occur in the inner and outer plexiform layers.
 
Relation of the Retinal Layers to One Another.—The only two layers which are in direct continuity are the stratum opticum and ganglionic layer, some fibres of the former being the axons of the cells of the latter. As regards most of the strata, the constituent elements of successive layers are brought into communication by means of the interlacements which take place between the arborizations formed by their various processes. These interlacements occur in the inner and outer plexiform layers.
  
  
Levator Palpebrae Superioris
 
 
Hyaloid Canal >
 
 
(Canal of Cloquet)
 
 
 
Hyaloid Membrane
 
 
 
Retina
 
 
 
Sinus Venosus S clerae
 
 
Posterior Chamber
 
 
Anterior Chamber
 
 
 
Choroid
 
  
 
Rectus Superior s „ Sclera
 
 
 
Arteria
 
 
, l /Centralis
 
 
' '■ / Retinae
 
 
 
Cornea
 
 
Ciliary / y.
 
 
Processes'
 
 
Zonular Space
 
 
 
Rectus Inferior
 
 
 
Fornix Conjunctivas
 
  
 
Fig. 1020.—Vertical Sagittal Section of the Eye and its Appendages (Hirschfeld and Leveille).
 
Fig. 1020.—Vertical Sagittal Section of the Eye and its Appendages (Hirschfeld and Leveille).
Line 1,398: Line 680:
 
In the inner plexiform layer there are several strata of interlacements, by means of which the dendrites of the cells of the ganglionic layer are brought into communication with the internal processes of the bipolar cells of the inner nuclear layer. In the outer plexi orm layer there is a free intermingling between the external processes of the bipolar cells of the inner nuclear layer and the rod-fibres and conefibres.
 
In the inner plexiform layer there are several strata of interlacements, by means of which the dendrites of the cells of the ganglionic layer are brought into communication with the internal processes of the bipolar cells of the inner nuclear layer. In the outer plexi orm layer there is a free intermingling between the external processes of the bipolar cells of the inner nuclear layer and the rod-fibres and conefibres.
  
Nerve-cells of the Retina.—These are arranged in three strata, and communicate with one another through interlacing arborizations. The outermost stratum consists of the rods and cones; the middle stratum is formed by the bipolar cells; and the innermost stratum represents the cells of the ganglionic layer. The axons of the gang
+
Nerve-cells of the Retina.—These are arranged in three strata, and communicate with one another through interlacing arborizations. The outermost stratum consists of the rods and cones; the middle stratum is formed by the bipolar cells; and the innermost stratum represents the cells of the ganglionic layer. The axons of the ganglionic cells enter the stratum opticum as centripetal fibres, which pass in the optic nerve to the brain. The centrifugal fibres of the stratum opticum ramify in the inner plexiform or inner nuclear layer.
  
  
 
+
==Refracting Media==
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
THE EYE
 
 
 
 
 
1661
 
 
 
 
 
lionic cells enter the stratum opticum as centripetal fibres, which pass in the optic nerve to the brain. The centrifugal fibres of the stratum opticum ramify in the inner plexiform or inner nuclear layer.
 
 
 
Refracting Media.
 
  
 
Aqueous Humour and Chambers of the Eye.—The aqueous humour occupies the space between the cornea and the front of the crystalline lens, which is divided by the iris into two chambers, anterior and posterior. It is a clear fluid having an alkaline reaction, and is composed of H ? 0 , holding in solution a very small amount of sodium chloride and traces of albumen.
 
Aqueous Humour and Chambers of the Eye.—The aqueous humour occupies the space between the cornea and the front of the crystalline lens, which is divided by the iris into two chambers, anterior and posterior. It is a clear fluid having an alkaline reaction, and is composed of H ? 0 , holding in solution a very small amount of sodium chloride and traces of albumen.
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Crystalline Lens.—The crystalline lens is situated directly behind the pupil and iris, from which latter it is separated by the posterior chamber. It is a solid, transparent, biconvex disc, the posterior surface being more convex than the anterior, and is enclosed within a A ’ 0 fi n br e e 4° e f ) . th ^°L ( man homogeneous, transparent envelope, called the fibres (seen on end). capsule of the lens. The centre of the anterior
 
Crystalline Lens.—The crystalline lens is situated directly behind the pupil and iris, from which latter it is separated by the posterior chamber. It is a solid, transparent, biconvex disc, the posterior surface being more convex than the anterior, and is enclosed within a A ’ 0 fi n br e e 4° e f ) . th ^°L ( man homogeneous, transparent envelope, called the fibres (seen on end). capsule of the lens. The centre of the anterior
  
surface is called the anterior pole, and that of the posterior surface the posterior pole. The line connecting these two poles constitutes the axis of the lens, and a line surrounding the periphery represents the equator. The transverse measurement of the lens, is about •it inch, and its axis measures about inch. The . anterior surface at its central part faces the pupil. External to this, the pupillary margin of the iris rests upon it, and external to this again is the posterior chamber, with part of the aqueous humour. The posterior surface is received into the ‘ patellar fossa on the anterior aspect of the vitreous body. The periphery is related to the suspensory ligament, the zonular spaces present in this ligament, and the ciliary processes. From the anterior and posterior poles delicate lines radiate
+
surface is called the anterior pole, and that of the posterior surface the posterior pole. The line connecting these two poles constitutes the axis of the lens, and a line surrounding the periphery represents the equator. The transverse measurement of the lens, is about •it inch, and its axis measures about inch. The . anterior surface at its central part faces the pupil. External to this, the pupillary margin of the iris rests upon it, and external to this again is the posterior chamber, with part of the aqueous humour. The posterior surface is received into the ‘ patellar fossa on the anterior aspect of the vitreous body. The periphery is related to the suspensory ligament, the zonular spaces present in this ligament, and the ciliary processes. From the anterior and posterior poles delicate lines radiate towards the equator. In early life these are three on each surface. Those on the posterior surface form an inverted while those on the anterior form an erect Y. These lines represent the free margins of septa within the lens upon which the ends of the lens-fibres terminate.
  
  
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1662
 
 
 
A MANUAL OF ANATOMY
 
 
 
towards the equator. In early life these are three on each surface. Those on the posterior surface form an inverted while those on the anterior form an erect Y. These lines represent the free margins of septa within the lens upon which the ends of the lens-fibres terminate.
 
  
 
Structure.—The lens is laminar in structure. The outer laminae are soft in consistence, but the succeeding ones gradually become firmer, and the central portion, which constitutes the nucleus, is very firm and hard. The laminae are arranged concentrically, and after boiling or immersion in alcohol they may be peeled off, like the coats of an onion. The fibres of which the laminae are composed terminate upon septa within the lens, of which the radiating lines on the surfaces, already referred to, are the free margins. The concentric laminae are therefore not continuous all round, but are split up along these lines. The lens-fibres, which are disposed in a curved manner, are of small size, and have serrated edges, which fit closely to each other. In transverse section the fibres appear as hexagonal prisms. The fibres are the elongated cells which line the posterior part of the ectodermal vesicle (lens vesicle) from which the lens is developed. In early life each fibre has a nucleus, but after the lens has attained its full development only the outermost fibres are nucleated.
 
Structure.—The lens is laminar in structure. The outer laminae are soft in consistence, but the succeeding ones gradually become firmer, and the central portion, which constitutes the nucleus, is very firm and hard. The laminae are arranged concentrically, and after boiling or immersion in alcohol they may be peeled off, like the coats of an onion. The fibres of which the laminae are composed terminate upon septa within the lens, of which the radiating lines on the surfaces, already referred to, are the free margins. The concentric laminae are therefore not continuous all round, but are split up along these lines. The lens-fibres, which are disposed in a curved manner, are of small size, and have serrated edges, which fit closely to each other. In transverse section the fibres appear as hexagonal prisms. The fibres are the elongated cells which line the posterior part of the ectodermal vesicle (lens vesicle) from which the lens is developed. In early life each fibre has a nucleus, but after the lens has attained its full development only the outermost fibres are nucleated.
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Vitreous Body.—This body occupies about four-fifths of the space within the eyeball, and is situated between the crystalline lens and the retina. It is transparent and gelatinous, and is composed of water, holding in solution a small quantity of sodium chloride and albuminous matter. It is surrounded by a transparent, homogeneous envelope, called the hyaloid membrane. This membrane is in contact with the retina, except anteriorly, where there is an excavation called the fossa
 
Vitreous Body.—This body occupies about four-fifths of the space within the eyeball, and is situated between the crystalline lens and the retina. It is transparent and gelatinous, and is composed of water, holding in solution a small quantity of sodium chloride and albuminous matter. It is surrounded by a transparent, homogeneous envelope, called the hyaloid membrane. This membrane is in contact with the retina, except anteriorly, where there is an excavation called the fossa
  
 
/
 
  
  
THE EYE 1663
 
  
patellaris , into which the posterior surface of the crystalline lens is received.
+
Fig. 1022. — Meridional Section through the Anterior Portion of the Eye (magnified 16X1) (Fuchs).
 
 
 
 
Anterior Wall of Capsule of Lens __I
 
 
 
 
 
Sphincter Pupillae_
 
 
 
 
 
Membrane of Descemet
 
 
 
 
 
Epithelium of Cornea
 
 
 
 
 
Suspensory Ligament Middle Portion of Suspensory Ligament
 
 
 
Posterior Portion of Suspensory Ligament
 
 
 
 
 
Sinus Venosus Sclera
 
 
 
Margin of Cornea
 
 
 
 
 
Conjunctiva
 
 
 
 
 
Ciliary Muscle 'Radiating Fibres)
 
 
 
 
 
Fig. 1022.—Meridional Section through the Anterior Portion of
 
 
 
the Eye (magnified 16X1) (Fuchs).
 
  
 
C.P., C.P., zonular spaces.
 
C.P., C.P., zonular spaces.
  
  
Towards its circumference the vitreous body is laminated, the laminae being arranged concentrically. Laminae are also said to radiate
+
Towards its circumference the vitreous body is laminated, the laminae being arranged concentrically. Laminae are also said to radiate from its antero-posterior axis towards the circumference. Scattered throughout the vitreous body there are some amoeboid corpuscles, and it is traversed from behind forwards by a minute passage called the hyaloid canal (canal of Cloquet, canal of Stilling). This extends from the centre of the optic disc to the posterior wall of the capsule of the lens, and posteriorly it communicates with the lymph-spaces of the optic nerve. In the foetus the canal transmits a branch of the arteria centralis retinae, called the hyaloid artery , which supplies the lens.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
1664
 
 
 
 
 
A MANUAL OF ANATOMY
 
 
 
 
 
from its antero-posterior axis towards the circumference. Scattered throughout the vitreous body there are some amoeboid corpuscles, and it is traversed from behind forwards by a minute passage called the hyaloid canal (canal of Cloquet, canal of Stilling). This extends from the centre of the optic disc to the posterior wall of the capsule of the lens, and posteriorly it communicates with the lymph-spaces of the optic nerve. In the foetus the canal transmits a branch of the arteria centralis retinae, called the hyaloid artery , which supplies the lens.
 
  
 
No vessels enter the vitreous body, its nutrition being derived from the vessels of the retina and ciliary processes.
 
No vessels enter the vitreous body, its nutrition being derived from the vessels of the retina and ciliary processes.
Line 1,608: Line 756:
 
Behind the suspensory ligament of the lens there is a sacculated lymph-space, called the zonular space, which surrounds the equator of the lens.
 
Behind the suspensory ligament of the lens there is a sacculated lymph-space, called the zonular space, which surrounds the equator of the lens.
  
Development of the Eye.
+
==Development of the Eye==
  
 
The retina, optic nerve, and crystalline lens are developed from the ectoderm, the retina and optic nerve being derived from the ectoderm of the anterior primary cerebral vesicle, whilst the crystalline lens is developed from the ectoderm of the side of the head. The accessories of the eye— e.g., the sclera, cornea, choroid, ciliary body, and iris—are all developed in mesoderm, but ectoderm, as will be seen, is also employed in some of these. The vitreous body, though developed to a certain extent from the mesoderm, is principally formed from the ectoderm.
 
The retina, optic nerve, and crystalline lens are developed from the ectoderm, the retina and optic nerve being derived from the ectoderm of the anterior primary cerebral vesicle, whilst the crystalline lens is developed from the ectoderm of the side of the head. The accessories of the eye— e.g., the sclera, cornea, choroid, ciliary body, and iris—are all developed in mesoderm, but ectoderm, as will be seen, is also employed in some of these. The vitreous body, though developed to a certain extent from the mesoderm, is principally formed from the ectoderm.
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The earliest indication of the future eye is in the form of a shallow marginal groove on each side in the widely open cerebral plate of embryos with a few somites. As the region grows these grooves become deepened by the upgrowth of the lateral margins, which ultimately fuse in the middle line, in continuity with the fusion of the edges of the medullary folds further back. In this way the open grooves are converted into recesses or lateral pockets of the closed fore-brain, each pocket being in contact from the beginning with the ectoderm of the surface.
 
The earliest indication of the future eye is in the form of a shallow marginal groove on each side in the widely open cerebral plate of embryos with a few somites. As the region grows these grooves become deepened by the upgrowth of the lateral margins, which ultimately fuse in the middle line, in continuity with the fusion of the edges of the medullary folds further back. In this way the open grooves are converted into recesses or lateral pockets of the closed fore-brain, each pocket being in contact from the beginning with the ectoderm of the surface.
  
The pocket formed in this way is termed the optic recess, and becomes the optic vesicle very soon by its rounded enlargement under the surface ectoderm; such enlargement is mainly at its distal part, its connection with the brain
+
The pocket formed in this way is termed the optic recess, and becomes the optic vesicle very soon by its rounded enlargement under the surface ectoderm; such enlargement is mainly at its distal part, its connection with the brain being slightly constricted, forming a ' neck ' for the vesicle. As the development goes on this neck is drawn out into a definite stalk, which connects the vesicle with the fore-brain. Stages in these changes can be seen in Fig. 1025. The vesicle is hollow, its cavity being carried into the stalk, and, through this, communicating with that of the fore-brain, which will be the third ventricle. The
 
 
 
 
THE EYE 1665
 
 
 
being slightly constricted, forming a ' neck ' for the vesicle. As the development goes on this neck is drawn out into a definite stalk, which connects the vesicle with the fore-brain. Stages in these changes can be seen in Fig. 1025. The vesicle is hollow, its cavity being carried into the stalk, and, through this, communicating with that of the fore-brain, which will be the third ventricle. The
 
 
 
  
Lens Pit
 
  
  
Ectoderm
+
Fig. 1023.—Development of Crystalline Lens and Optic Vesicle (Scheme).
 
 
srjer
 
 
 
Optic Stalk
 
 
 
 
 
Optic Vesicle
 
 
 
prifT&yKS-'Ka.
 
 
 
ilM /
 
 
 
 
 
Lens
 
 
 
Cavity of Vesicle a?- m
 
 
 
6i ; w'sn'j.;! . ..£31 «
 
 
 
 
 
Optic Stalk
 
 
 
 
 
Pigmentary Layer of I Optic Cup Retinal Layer of Optic Cup
 
 
 
 
 
Fig. 1023.—Development of Crystalline Lens and Optic Vesicle
 
 
 
(Scheme).
 
  
 
The lens is lying in the optic cup.
 
The lens is lying in the optic cup.
Line 1,665: Line 777:
  
  
Fig. 1024.—Diagram showing (see Text) the Conversion of Optic
+
Fig. 1024.—Diagram showing (see Text) the Conversion of Optic Vesicle into Optic Cup.
 
 
Vesicle into Optic Cup.
 
  
 
crystalline lens is differentiated. The lens vesicle now becomes completely separated from the surface ectoderm, with which it was originally continuous
 
crystalline lens is differentiated. The lens vesicle now becomes completely separated from the surface ectoderm, with which it was originally continuous
  
^ Fl The outer wall of the vesicle, facing the rudiment of the lens, is invaginated so as to obliterate the cavity of the vesicle, which is now converted into the oi>tic cut> Fig 1024 gives diagrammatic sections which may help in the comprehension of this change. The middle vertical row of figures here shows sections
+
^ Fl The outer wall of the vesicle, facing the rudiment of the lens, is invaginated so as to obliterate the cavity of the vesicle, which is now converted into the oi>tic cut> Fig 1024 gives diagrammatic sections which may help in the comprehension of this change. The middle vertical row of figures here shows sections along the length of the optic outgrowth; the simple optic vesicle is seen at the top, the commencing invagination of its lower lateral wall is seen next, while the completed invagination is shown in the lowest section. It can be seen that the invagination extends into the optic stalk also. On the left side the invagination is shown by transverse sections of the vesicle, corresponding more or less with the stages of the middle column. Observe that the cavity (V) of the optic
 
 
105
 
 
 
 
 
 
 
1666
 
 
 
 
 
A MANUAL OF ANATOMY
 
 
 
  
along the length of the optic outgrowth; the simple optic vesicle is seen at the top, the commencing invagination of its lower lateral wall is seen next, while the completed invagination is shown in the lowest section. It can be seen that the invagination extends into the optic stalk also. On the left side the invagination is shown by transverse sections of the vesicle, corresponding more or less with the stages of the middle column. Observe that the cavity (V) of the optic
 
  
  
  
 
+
Fig. 1025. — Different Stages in the Development of the Eye (from Reconstruction Models at St. Mary’s Hospital).
Fig. 1025.—Different Stages in the Development of the Eye (from Reconstruction Models at St. Mary’s Hospital).
 
  
 
A piece of the wall of the optic vesicle has been removed in the first specimen, showing the cavity of the vesicle ; the lens thickening of the ectoderm is beginning to be depressed. In the second the optic outgrowth is entire, and the lens depression is projecting into the cavity of the optic cup. In the third figure removal of part of the wall has opened the cavity of the vesicle,and also the cavity of the cup, in which the lens vesicle is lying, still attached to the ectoderm, its cavity opened by the section. The figures also show the formation of the stalk of the vesicle and the extension into it of the cleft continuous with the cavity of the optic cup.
 
A piece of the wall of the optic vesicle has been removed in the first specimen, showing the cavity of the vesicle ; the lens thickening of the ectoderm is beginning to be depressed. In the second the optic outgrowth is entire, and the lens depression is projecting into the cavity of the optic cup. In the third figure removal of part of the wall has opened the cavity of the vesicle,and also the cavity of the cup, in which the lens vesicle is lying, still attached to the ectoderm, its cavity opened by the section. The figures also show the formation of the stalk of the vesicle and the extension into it of the cleft continuous with the cavity of the optic cup.
Line 1,694: Line 792:
 
vesicle is being obliterated, replaced by the cavity (C) of the optic cup, which is still open in front and below; the last section in the middle column has gone along this interval between the two sides of the cup. The interval is termed the choroidal or foetal fissure, and extends into the stalk. It closes later by the apposition and rapid fusion of its lips, so completing the optic cup. The righthand column of sections is made from the distal end towards the brain; they show the concavity in the vesicle, and in the stalk, lost in the last section.
 
vesicle is being obliterated, replaced by the cavity (C) of the optic cup, which is still open in front and below; the last section in the middle column has gone along this interval between the two sides of the cup. The interval is termed the choroidal or foetal fissure, and extends into the stalk. It closes later by the apposition and rapid fusion of its lips, so completing the optic cup. The righthand column of sections is made from the distal end towards the brain; they show the concavity in the vesicle, and in the stalk, lost in the last section.
  
 
 
 
 
 
THE EYE
 
 
 
1667
 
  
 
The lens vesicle, when it separates from the surface ectoderm, lies in the opening of the optic cup. Vascular mesoderm extends into the cavity of the cup through the choroidal fissure, behind and below the lens vesicle; when the fissure closes, the mesoderm within the cavity of the cup loses its connection with the outer mesoderm, except at the end of the fissure, where a relatively large vessel persists, and becomes ultimately the central artery of the retina. Since the end of the fissure is in the optic stalk, which becomes the optic nerve, this artery passes in the terminal piece of the nerve to enter the eye. The artery, when first formed, is known as the hyaloid artery, and is distributed over the posterior surface of the lens.
 
The lens vesicle, when it separates from the surface ectoderm, lies in the opening of the optic cup. Vascular mesoderm extends into the cavity of the cup through the choroidal fissure, behind and below the lens vesicle; when the fissure closes, the mesoderm within the cavity of the cup loses its connection with the outer mesoderm, except at the end of the fissure, where a relatively large vessel persists, and becomes ultimately the central artery of the retina. Since the end of the fissure is in the optic stalk, which becomes the optic nerve, this artery passes in the terminal piece of the nerve to enter the eye. The artery, when first formed, is known as the hyaloid artery, and is distributed over the posterior surface of the lens.
Line 1,718: Line 807:
 
As development proceeds, additional lens-fibres are formed by the proliferation of cells at the equator of the lens. These fibres are laid down in successive layers, which are arranged concentrically.
 
As development proceeds, additional lens-fibres are formed by the proliferation of cells at the equator of the lens. These fibres are laid down in successive layers, which are arranged concentrically.
  
Capsule of the Crystalline Lens.— At an early period in its development the lens becomes invested by a mesodermic capsule, freely supplied with bloodvessels derived from the hyaloid artery and anterior ciliary arteries. This capsule is known as the tunica vasculosa. It persists throughout the period of active growth of the lens, and then undergoes retrogression to form the permanent lens capsule. The portion of the tunica vasculosa which covers the front part of the lens is called the membrana papillaris, but this usually disappears prior to birth. It may, however, be present at birth, giving rise to the condition
+
Capsule of the Crystalline Lens.— At an early period in its development the lens becomes invested by a mesodermic capsule, freely supplied with bloodvessels derived from the hyaloid artery and anterior ciliary arteries. This capsule is known as the tunica vasculosa. It persists throughout the period of active growth of the lens, and then undergoes retrogression to form the permanent lens capsule. The portion of the tunica vasculosa which covers the front part of the lens is called the membrana papillaris, but this usually disappears prior to birth. It may, however, be present at birth, giving rise to the condition known as atresia pupillce. Towards the end of intra-uterine life the tunica vasculosa undergoes retrogression and becomes transformed, as stated, into the permanent lens capsule, which is a transparent, homogeneous, elastic membrane.
 
 
 
 
i668
 
 
 
 
 
A MANUAL OF ANATOMY
 
 
 
 
 
known as atresia pupillce. Towards the end of intra-uterine life the tunica vasculosa undergoes retrogression and becomes transformed, as stated, into the permanent lens capsule, which is a transparent, homogeneous, elastic membrane.
 
  
 
This mesodermal pupillary membrane is a continuation across the open mouth of the cup of the plane of the choroidal layer. It is, therefore, on the outer surface of the developing iris, of which it forms the mesodermal base, the muscles being derived from the actual ectodermal or retinal layer itself.
 
This mesodermal pupillary membrane is a continuation across the open mouth of the cup of the plane of the choroidal layer. It is, therefore, on the outer surface of the developing iris, of which it forms the mesodermal base, the muscles being derived from the actual ectodermal or retinal layer itself.
Line 1,738: Line 818:
  
 
The optic stalk is transformed into the optic nerve. The stalk is at first hollow, its cavity communicating with that of the optic vesicle on the one hand, and with the third ventricle of the brain on the other. As stated, the choroidal fissure involves the under surface of the optic stalk near the optic vesicle, as well as the under surface of the optic vesicle itself. When the choroidal fissure undergoes closure, the hyaloid artery, which passed through that fissure, becomes enclosed within the optic stalk, and forms the arteria centralis retincc of adult life. By the closure of the choroidal fissure, and the consequent enclosure of the hyaloid artery, the cavity of the distal portion of the optic stalk becomes obliterated. Inasmuch as the ventral or lower wall of this part of the stalk has been previously invaginated, the wall of the stalk is now composed of two layers—outer and inner—the inner being formed by the invaginated ventral or lower wall. The outer layer of the optic stalk is now continuous with the outer layer of the optic cup, whilst the inner layer of the optic stalk is continuous with the inner layer of the optic cup. As regards the proximal part of the optic stalk, its cavity becomes gradually closed. The wall of the optic stalk becomes thickened, its cells proliferate, and they give rise to the neuroglial or sustentacular tissue of the future nerve. The nerve-fibres which build up the optic nerve are regarded as having two sources. The majority of them represent the axons of the ganglion cells of the retina, which pass in the optic stalk to the diencephalon and mesencephalon. These are therefore centripetal fibres. Other fibres are regarded as being centrifugal, these arising in connection with the diencephalon and mesencephalon.
 
The optic stalk is transformed into the optic nerve. The stalk is at first hollow, its cavity communicating with that of the optic vesicle on the one hand, and with the third ventricle of the brain on the other. As stated, the choroidal fissure involves the under surface of the optic stalk near the optic vesicle, as well as the under surface of the optic vesicle itself. When the choroidal fissure undergoes closure, the hyaloid artery, which passed through that fissure, becomes enclosed within the optic stalk, and forms the arteria centralis retincc of adult life. By the closure of the choroidal fissure, and the consequent enclosure of the hyaloid artery, the cavity of the distal portion of the optic stalk becomes obliterated. Inasmuch as the ventral or lower wall of this part of the stalk has been previously invaginated, the wall of the stalk is now composed of two layers—outer and inner—the inner being formed by the invaginated ventral or lower wall. The outer layer of the optic stalk is now continuous with the outer layer of the optic cup, whilst the inner layer of the optic stalk is continuous with the inner layer of the optic cup. As regards the proximal part of the optic stalk, its cavity becomes gradually closed. The wall of the optic stalk becomes thickened, its cells proliferate, and they give rise to the neuroglial or sustentacular tissue of the future nerve. The nerve-fibres which build up the optic nerve are regarded as having two sources. The majority of them represent the axons of the ganglion cells of the retina, which pass in the optic stalk to the diencephalon and mesencephalon. These are therefore centripetal fibres. Other fibres are regarded as being centrifugal, these arising in connection with the diencephalon and mesencephalon.
 
 
THE EYE
 
 
 
1669
 
  
  
Line 1,769: Line 843:
  
 
cup (to form the ectodermal portion of the iris) is accompanied by the appearance of a more fibrillar vitreous formation corresponding with it; this is sometimes referred to as the tertiary vitreous, and the fibrils of the suspensory ligament of the lens are developed in this formation.
 
cup (to form the ectodermal portion of the iris) is accompanied by the appearance of a more fibrillar vitreous formation corresponding with it; this is sometimes referred to as the tertiary vitreous, and the fibrils of the suspensory ligament of the lens are developed in this formation.
 
 
1670
 
 
 
A MANUAL OF ANATOMY
 
  
  

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I have decided to take early retirement in September 2020. During the many years online I have received wonderful feedback from many readers, researchers and students interested in human embryology. I especially thank my research collaborators and contributors to the site. The good news is Embryology will remain online and I will continue my association with UNSW Australia. I look forward to updating and including the many exciting new discoveries in Embryology!

Frazer JE. Buchanan's Manual of Anatomy, including Embryology. (1937) 6th Edition. Bailliere, Tindall And Cox, London.

Buchanan's Manual of Anatomy: I. Terminology and Relative Positions | II. General Embryology | III. Osteology | IV. Bones of Trunk | V. Bones of Head | VI. Bones of Upper Limb | VII. Bones of Lower Limb | VIII. Joints | IX. The Upper Limb | X. Lower Limb | XI. The Abdomen | XII. The Thorax | XIII. Development of Vascular Systems | XIV. The Head and Neck | XV. The Nervous System | XVI. The Eye | XVII. The Ear | Glossary
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Chapter XV! The Eye

The eyeball is almost spherical. It consists of the segments of two spheres—namely, a large posterior or sclerotic segment, which is opaque, and a small anterior or corneal segment, which is transparent. The sclerotic segment forms five-sixths of the eyeball, and the corneal segment one-sixth. The centre of the corneal segment is called the anterior pole, and the centre of the sclerotic segment is known as the posterior pole. The sagittal (antero-posterior) axis, or axis of vision, of the eyeball is represented by a line connecting the anterior and posterior poles. The equator is represented by a line encircling the centre of the eyeball in a coronal plane, the diameter of the circle being about I inch. The plane of this circle would therefore divide the eyeball into two halves—an anterior half, consisting of the corneal and the front part of the sclerotic segment, and a posterior half, consisting of the back part of the sclerotic segment. The meridian is represented by a line encircling the eyeball horizontally at right angles to the equator, and passing through the anterior and posterior poles.

Posteriorly the eyeball receives the optic nerve, which pierces the sclerotic coat at a point about J inch to the inner side of and about -f T inch below the posterior pole.

The eyeball is composed of three coats concentrically arranged: (i) an external coat, consisting of an opaque part, called the sclera, and a transparent part, called the cornea; (2) a middle coat, which is vascular, pigmented, and muscular, and consists of (a) a posterior part, called the choroid coat, (b) an anterior part, the iris, and (c) an intermediate part, representing the ciliary body; and (3) an internal

coat , called the retina. .

These three coats enclose the following refracting media : (1) a fluid, called the aqueous humour, which lies between the cornea and the crystalline lens, where it occupies the anterior and posterior chambers, into which this region is divided by the iris; (2) a solid body, called the crystalline lens, which lies behind the aqueous humour, and (3) a soft gelatinous body, called the vitreous body, which occupies the laige space behind the crystalline lens.


Coats of the Eyeball.

External=sclera and cornea.

Middle =choroid, ciliary body, and iris. Internal = retina.


Refracting Media.

Aqueous humour. Crystalline lens. Vitreous body.


External Coat

Sclera (or Sclerotic Coat).—The sclera (white of the eye) is a strong white fibrous coat of great density, which surrounds the posterior five-sixths of the eyeball, and maintains the shape of the organ. Anteriorly it unites, and becomes continuous with the cornea, which it slightly overlaps. The junction of the two is indicated by a slight groove, called the sulcus sclerce, and the union is known as the corneoscleral junction . Posteriorly, as has been shown above, the sclera is pierced by the optic nerve a little below and to' the inner side of the centre. The part of the sclera corresponding to the optic entrance

Levatoi Palpebras Superioris


Fornix Conjunctive

Fig. 1005.—Vertical Sagittal Section of the Eye and its Appendages (Hirschfeld and Leveille).

is pierced by a number of openings for the passage of the fasciculi of the optic nerve, and hence is called the lamina cribrosa.

Around the optic entrance there are numerous minute openings for the ciliary vessels and nerves, and here the dura matral sheath of the optic nerve blends with the sclerotic coat. About midway between the optic entrance and the corneo-scleral junction the sclera is pierced by four openings for the passage of the vence vorticosce of the choroid.

The sclera is thickest posteriorly around the optic entrance. It is also thick near the sclero-corneal junction, where it receives the insertions of the recti muscles.

The outer surface of the sclera is covered by a membranous investment, called the fascial sheath of the eyeball (fascia bulbi or capsule of Tenon), and between the two there is the episcleral lymph-space (or


Hyaloid Canal (Canal of Cloquet)


Hyaloid Membrane


Retina

>


Sinus Venosus Sclera?


Anterior Chamber \


. Choroid


Rectus Superior i „--Sclera


Arteria z Centralis


/ Retinas


Cornea

Ciliary / y. Processes'’ ft

Zonuiar Space


Rectus Inferior


Tenon’s space), which is broken up into a reticulum by processes of connective tissue which pass between the sheath and the sclera. This space communicates with the subdural and subarachnoid spaces. The inner surface of the sclerotic coat is dark brown, and has grooves for the ciliary vessels and nerves. It is lined with connective tissue containing pigment-cells, forming the lamina fusca. Processes from this layer pass to the choroid coat, and these, together with vessels and nerves, traverse an interval, which represents the perichoroidal lymph-space. This space communicates with the episcleral lymphspace through the vascular openings in the sclera. Anteriorly the sclera blends with the cornea at the sclero-corneal junction, the sclera slightly overlapping the cornea. Posteriorly around the optic entrance the sclera blends with the dura matral sheath of the optic nerve.

Structure.—The sclera is composed of fibrous tissue mixed with elastic fibres, and contains many connective-tissue corpuscles. The fibres are arranged in bundles, which are disposed longitudinally and transversely, and interlace with one another. The connective-tissue corpuscles occupy spaces between the fibres, which may be regarded as lymph-spaces.

Arteries.—These are the short ciliary group of posterior ciliary arteries, and the anterior ciliary arteries, which are branches of the ophthalmic. The vessels belonging to the former group are disposed in the form of capillary networks; whilst the vessels derived from the latter form a ring near the sclero-corneal junction beneath the conjunctiva, to which ring they converge in the substance of the sclerotic coat.

The sclerotic veins open into the anterior ciliary veins, and into the vencB vorticosce of the choroid. There is also a slight drainage into the sinus venosus sclerce , a minute channel running deeply at the sclero-corneal junction.

Nerve-supply.—The ciliary nerves.

Cornea.—The cornea is the transparent part of the external coat of the eyeball, of which it forms the anterior sixth, and serves to transmit light. It is almost circular, its transverse measurement being slightly greater than the vertical. At its circumference it is continuous with the sclera, by which it is slightly overlapped. The anterior surface is convex. The posterior surface is concave, and forms the anterior boundary of the anterior chamber of the eye.

Structure.— The cornea consists of the following five layers, from before backwards: .

1. The conjunctival epithelium.

2. The anterior elastic lamina.

3. The substantia propria.

4. The posterior elastic lamina.

5. A layer of endothelium.

The conjunctival epithelium is stratified, there being not less than five strata of cells, and is continuous with the epithelium, which covers Stratified Epithelium of / Conjunctiva Membrane of Bowman or Anterior Elastic Lamina


the free surface of the conjunctiva. The cells of the deepest stratum are columnar; succeeding these there are layers of polygonal cells; and these in turn are overlaid by layers of squamous cells.

The anterior elastic lamina (Bowman) is probably of the same nature as the fibrous portion of the substantia propria. It is closely connected with the substantia propria, is thin, and contains no corpuscles.

The substantia propria is composed of modified connective tissue arranged in bundles which form superimposed laminae. These laminae amount in number to about sixty. The fibres of alternate laminae cross each other at right angles, and at the circumference of the cornea they are continuous with the fibres of the sclerotic. The successive laminae are connected by cement substance, and within this substance are branched spaces, called the corneal spaces or lacunae, which communicate with each other by very delicate canaliculi. Each of these spaces contains a nucleated connective-tissue corpuscle, called the corneal corpuscle. These corpuscles, like the spaces which they occupy, are branched, and the offsets of adjacent corpuscles communicate with one another. As seen in vertical sections of the cornea, the corpuscles are spindle - shaped, but in


Substantia Propria


Posterior Elastic Lamina or

Membrane of Descemet 'Single Layer of Squamous Epithelium lining Descemet’s Membrane


Fig. 1006.—Vertical Section of the Cornea

(magnified) .


horizontal sections they appear flattened out, and give off their branches.

The posterior elastic lamina (or membrane of Descemet) covers the posterior surface of the substantia propria. It is thicker than the anterior elastic lamina, and .is composed of an elastic homogeneous membrane, which is very brittle. When stripped from the substantia propria it comes away in shreds, and these curl up at their ends in such a manner that the anterior or attached surface of each shred is turned inwards. At the circumference of the cornea the posterior elastic lamina becomes broken up into fibres. The most posterior of these fibres pass in a radiating manner into the iris, and they form the ligamentum pectinatum iridis, the intervals between the fibres of which represent the spaces of the irido-corneal angle.

The layer of endothelium lines the posterior surface of the posterior elastic lamina, and consists of one stratum of cells. It is continued over the front of the iris, and into the spaces of the angle.

The cornea in the adult is non-vascular, except at the circumference, in which situation there are the conjunctival and sclerotic capillaries, which terminate in loops. Being destitute of blood-vessels, the nourishment of the cornea is maintained by the flow of lymph through its surface. It is about 1 mm. thick, slightly more peripherally.

Nerve-supply.—The nerves are derived from the ciliary nerves, and are very numerous. They enter the deep surface of the anterior part of the sclera, and form a plexus round the corneo-scleral junction. Offsets from this plexus enter the cornea, and form what is known as the plexus annularis. From this plexus delicate offsets are given off, which traverse the substance of the cornea and pass through the anterior elastic lamina. They then give rise to a fine plexus upon the surface of that lamina, called the subepithelial plexus. From this plexus, in turn, minute fibrils are given off, which pass amongst the cells of the conjunctival epithelium, and almost reach the surface, forming an intra-epithelial plexus.

Pectinate Ligament of Iris.—It has been seen that the posterior elastic lamina at its circumference breaks up into fibres. The most posterior of these pass in a radiating manner into the iris, constitute the ligamentum pectinatum iridis, and are covered by a prolongation of the endothelial layer of the cornea.

Spaces of Irido-corneal Angle (or Spaces of Fontana).—These spaces represent the irregular intervals which lie between the radiating fibres of the pectinate ligament. They are lined by a prolongation of the endothelial layer of the cornea, and they communicate internally with the anterior chamber and the lymph-spaces within the iris, and externally with the sinus venosus sclerae.

Sinus Venosus Sclerse.—This canal (formerly known as the canal of Schlemm) is situated deeply in the sclerotic, close to the corneo-scleral junction. It communicates internally with the anterior chamber through the spaces of the irido-corneal angle, and externally with anterior ciliary veins of the sclera. It encircles the outer margin of the cornea, and has a little projecting rim of sclerotic on its deep surface, called the ‘ scleral spur/ from which the ciliary muscle takes origin.

Middle Coat

1. Choroid Coat.— This is a very vascular, deeply pigmented tunic of a dark brown colour, which lies between the sclera and the retina. It extends over the posterior five-sixths of the eyeball, and reaches as far forwards as the ora serrata of the retina. Anteriorly it is connected with the circumference of the iris, and posteriorly it is pierced by the optic nerve. Its outer surface is connected to the inner surface of the sclera by means of the lamina fusca and its processes, as well as by vessels and nerves which cross the ‘ perichoroidal lymph-space. Its inner surface is in contact with the pigmentary- layer of the retina.


Structure. — The choroid coat consists of connective tissue, bloodvessels, and branched pigment-cells. It is composed of three layers, which are as follows, from without inwards: (1) the lamina supra choroidea; (2) the choroid proper; and (3) the lamina basalis, or membrane of Bruch.

The suprachoroid lamina is composed of delicate, non-vascular lamellae, each of which is made up of elastic fibres arranged in a reticular manner, and of branched pigment-cells.

The choroid proper consists principally of bloodvessels and pigmentcells supported by connective tissue. The bloodvessels are arranged partly as arteries and veins, and partly as capillaries. The choroid proper is therefore composed of two layers—external or lamina vasculosa, and internal or lamina chorio-capillaris.

The lamina vasculosa (arterio-venous layer) is composed of (1) branches of the short ciliary group of the posterior ciliary arteries, which pass forwards before they turn inwards to end in capillaries;

Suprachoroid Lamina


Arterio-Venous Layer


Membrana Chorio-capillaris

Basal Lamina (Bruch’s Memb.)

Pigmentary Layer of the Retina

Fig. 1007.—Vertical Section of the Choroid Coat.

The pigmentary layer of the retina is also shown.

and (2) veins, which form the chief part of the lamina vasculosa, and are called the vense vorticosae. These veins are very closely set, and are arranged in a whorled manner. They ultimately converge and form four or five vessels, which pierce the sclerotic nearly midway between the optic entrance and the corneo-scleral junction at points equally distant from each other. Scattered throughout the lamina vasculosa are branched pigment-cells.

The lamina chorio-capillaris is composed of a plexus of capillary bloodvessels, the arteries leading to it being derived from the short ciliary arteries.

The lamina vasculosa and lamina chorio-capillaris are connected by fine elastic fibres, which form what is known as the stratum intermedium.

The lamina basalis, or membrane of Bruch, is situated on the inner surface of the lamina chorio-capillaris, which it separates from the pigmentary layer of the retina. It is a very delicate membrane without any very definite structure.


Tapetum. — This is present in certain animals. It lies between the lamina vaseulosa and the lamina chorio-capillaris in the stratum intermedium, and it gives rise to an iridescent or rainbow-like appearance. In some animals it is fibrous in structure, and in others cellular.

2. Ciliary Body.—The ciliary body connects the anterior part of the choroid to the circumference of the iris. It is composed of (1) the orbicularis ciliaris, (2) the ciliary processes, and (3) the ciliary muscle.

The orbicularis ciliaris, or ciliary ring, is a narrow zone which lies immediately in front of the anterior part of the choroid, with which it is continuous. In it are folds which are radially disposed, and it separates the ciliary processes from the ora serrata of the retina.

The ciliary processes, about seventy in number, are infoldings (Fig. 1005) of the anterior part of the choroid, and consist of the choroid proper and the basal membrane (of Bruch). They constitute a series


Conjunctiva

Choroid

Sclera


Optic Nerve


Retina


Ciliary Body Iris

Cornea " Lens

'Anterior Chamber Posterior Chamber

— Sinus Venosus Scleras . Ora Serrata


Vitreous


Fig. 1008.—Diagram of Section through the Eyeball to show the Main Layers mentioned in the Description.

S, S, suspensory ligament of lens.


of rays arranged in a circular manner, and converge as they pass inwards and forwards to the periphery of the crystalline lens on its anterior aspect. They are somewhat conical in outline.. Their bases or free extremities, which are round and prominent, lie behind the circumference of the iris upon the anterior aspect of the periphery of the crystalline lens. Their apices are connected with the orbicularis ciliaris Anteriorly they are related to the posterior chamber of the eyeball at its circumference. Posteriorly they are related to and connected with the suspensory ligament of the lens.

Structure._The ciliary processes are similar in structure to the choroid, but the pigment-cells are not so numerous. On their deep or posterior surfaces the processes are covered by the ciliary part of the retina, which is prolonged from the pigmentary layer of the retina, and is continuous with the pars iridica retinae (uvea) on the posterior surface of the iris.

The arteries of the ciliary processes are derived from those of the anterior part of the choroid, and from the anterior ciliary arteries. The veins pass to those of the choroid.

Ciliary Muscle. —This muscle is composed of unstriped fibres. It forms a greyish-white ring, about T V inch broad, which is situated at the anterior part of the choroid opposite the ciliary processes. The fibres are arranged in two sets—radial and circular. The radial fibres arise from the calcar sclerae close to the corneo-scleral junction and behind the sinus venosus of the sclera. From this origin they pass backwards in a radiating manner, and are inserted into the orbicularis ciliaris and the attached ends of the ciliary processes. The circular fibres form a ring around the circumference of the iris internal to the radial fibres.



Fig. 1009.—The Iris and Ciliary Processes (Posterior View)

(Hirschfeld and Leveille).

The ciliary muscle is supplied by the short ciliary nerves, which are branches of the ciliary ganglion, and derive their fibres from the motor oculi nerve.

Action. —The ciliary muscle is the muscle of accommodation , and adjusts the eye to the vision of near objects. When it contracts it draws forwards the choroid and the ciliary processes; the suspensory ligament of the crystalline lens is thereby relaxed, and, as a consequence, the anterior surface of the lens is rendered convex.

The circular fibres of the ciliary muscle are well developed in cases of hypermetropia, but are deficient in cases of myopia.

3. Iris. —The iris forms the anterior part of the middle coat of the eyeball. It is a coloured contractile diaphragm, which is suspended in the aqueous humour between the cornea and the crystalline lens. It is perforated by an almost circular aperture, called the pupil, which is situated slightly to the nasal or inner side of its centre, and serves for the transmission of light. The margin which surrounds the pupil is known as the pupillary margin. Its circumference is continuous with the ciliary body, and is connected with the posterior elastic lamina of the cornea by means of the ligamentum pectinatum iridis


Anterior Wall of Capsule of Lens..(I


Sphincter Pupillae ...


Membrane of Descemet —


Epithelium . of Cornea


Sinus Venosus Scleras


Suspensory Ligament Middle Portion of Suspensory Ligament

Posterior Portion of Suspensory Ligament


Margin of Cornea

Conjunctiva —


Ciliary Muscle "Radiating Fibres)


Fig. ioio.—Meridional Section through the Anterior Portion of

the Eye (magnified 16X1) (Fuchs).

C.P., C.P., zonular spaces.

at the iridial angle. The circumference is known as the ciliary margin. The surfaces of the iris are anterior and posterior. The anterior

surface is directed towards the cornea. Its colour varies in different individuals, and it presents a striated appearance, the striae converging towards the margin of the pupil, and being produced by the underlying vessels. The posterior surface is directed towards the crystalline lens and ciliary processes. It has a purple colour, and is covered by two layers of columnar epithelium, the cells of which contain dark pigment. These two layers of pigmented cells constitute the pars iridica retinae (uvea), which is continuous with the pars ciliaris retinae.

_ The iris divides the space between the cornea and the crystalline lens into two compartments, the anterior chamber and posterior chamber, both of which contain the aqueous humour.

Structure. —The component parts of the iris are (1) a layer of endothelium; (2) a connective-tissue stroma, with branched pigmentcells; (3) muscular tissue; and (4) pigment.

The layer of endothelium covers the anterior surface of the iris, and is continuous with the endothelium which lines the posterior elastic lamina of the cornea.


Fig. ioii.—The Arteries of the Choroid and Iris (Lateral View).

The connective-tissue stroma is composed of fibres which for the most part pass in a radiating manner towards the pupillary margin. Some, however, are disposed circularly at the ciliary margin. They support the bloodvessels and nerves, and scattered between their bundles there are branched cells. These cells contain pigment in darkcoloured eyes, but in blue eyes there is little pigment here.

The muscular tissue is of the unstriped variety, and its fibres are arranged in two sets, circular and radiating. The circular fibres form a ring round the pupil, and are nearer the posterior surface than the anterior. They are known as the sphincter pupillse. The radiating fibres converge from the ciliary margin of the iris towards the pupillary margin, where they blend with the circular fibres. The radiating fibres constitute the dilator pupillse. Some authorities regard the radiating fibres as elastic, and not muscular.

The pigment of the iris is variously situated, according to the colour of the eye. In the eyes of albinos there is no pigment. In other eyes pigment is contained in the cells of the two layers of columnar epithelium which line the posterior surface of the iris, and form the pars iridica retinae (uvea). In blue eyes the pigment is largely confined to this region, but in other coloured eyes it is also present in the branched cells of the connective-tissue stroma.

Blood-supply—Arteries. —The arteries of the iris are derived from (1) the long ciliary, and (2) the anterior ciliary vessels.

The long ciliary arteries are two in number, and belong to the posterior ciliary group of branches from the ophthalmic artery. They pierce the back part of the sclera, one on each side of the optic nerve, and pass forwards between the sclera and the choroid towards the ciliary margin of the iris. Here each vessel divides into two branches, upper and lower, which anastomose with those of the opposite side to form an arterial ring round the ciliary margin of the iris, called the circuius arteriosus major. This ring is joined by some of the anterior ciliary arteries, and it gives offsets to the ciliary muscle


Fig. 1012. —The Choroid and Iris, showing the Ven,e Vorticose and Ciliary Nerves (after Hirschfeld and Leveille).

The sclera and cornea have been removed.

and iris. The branches which enter the iris are supported by the connective-tissue stroma, and converge towards the pupillary margin, near which they form by their anastomoses another arterial ring,' called the circulus minor.

The anterior ciliary arteries are about six in number, and are derived from the muscular and lacrimal branches of the ophthalmic artery. They are of small size, and pierce the anterior part of the sclera close to the corneo-scleral junction. Some of them supply the ciliary processes, and others join the circulus major (see Fig. ion).

The veins of the iris accompany the arteries, and are in communication with the sinus venosus sclerse.

Nerves of the Choroid Coat and Iris. —These are derived from the ciliary nerves, short and long, the former coming from the ciliary ganglion, and the latter from the naso-ciliary branch of the ophthalmic or first division of the fifth cranial nerve. They are about sixteen in number, and pierce the back part of the sclera around the optic nerve. They then pass forwards between the sclerotic and choroid, giving branches to the latter coat, which become disposed in a plexiform manner amongst the bloodvessels. Having reached the corneoscleral junction, the nerves enter the ciliary muscle, in which they form a plexus. From this plexus branches enter the iris at the ciliary margin. These branches accompany the vessels, and by their subdivisions and communications they form a copious plexus of nonmedullated fibres in the connective-tissue stroma of the iris. The sphincter pupillse is supplied by fibres which are derived from the oculomotor or third cranial nerve by means of the motor root of the ciliary ganglion. The dilator pupillae is supplied by fibres which may be traced to the second thoracic ganglion through the sympathetic root of the ciliary ganglion (see p. 1637).

Membrana Pupillaris. —During intra-uterine life the pupil is closed by a delicate membrane, called the membrana pupillaris. This disappears shortly before birth, but remnants of it are sometimes found.


Internal Coat.

Retina. — The retina is the internal or nervous tunic of the eyeball. It is soft in consistence, translucent, and of a pinkish colour. Its internal surface is in contact with the hyaloid membrane, which


Fig. 1013.—The Posterior Portion of the Right Retina

(Anterior View).


encloses the vitreous body, and its external surface is in contact with the choroid coat. Posteriorly it receives the fibres of the optic nerve. Anteriorly it extends almost to the ciliary body, where there is a notched border, called the ora serrata. Here its nervous elements cease, but its pigmentary layer is continued over the deep or posterior surfaces of the ciliary processes on to the posterior surface of the iris, forming, with the addition of a layer of columnar epithelial cells, the pars ciliaris retinae and pars iridica retinae (uvea) respectively. The retina diminishes in thickness from behind forwards.

The external surface is formed by a stratum of hexagonal pigmentcells, which send processes into the adjacent layer. When the choroid is separated from the retina these processes are torn, and the stratum of pigment-cells remains attached to the choroid, being apparently a part of it. The pigmentary layer, however, really belongs to the retina.

The internal surface shows, in the line of the visual axis of the eyeball, the macula lutea or yellow spot, where vision is most distinct. This spot is transversely oval, and measures about X V inch from side


Fig. 1014.—Longitudinal Section through the Head of the Optic

Nerve (14X1)


r. Retina

b. Centre of Porus Opticus ch. Choroid

s. Sclera

so. Outer Part of Sclera si. Inner part of Sclera ci. Ciliary Artery (in longitudinal section) sd. Subdural Space

nasal, Medial Side


(Fuchs).

sa. Subarachnoid Space n. Bundles of Nerve-fibres se. Septa between the Nerve-bundles a. Arteria Centralis Retinae v. Vena Centralis Retinae p. Sheath formed by Pia Mater ar. Sheath formed by Arachnoid du. Sheath formed by Dura Mater

temporal, Lateral Side


to side. At its centre is a slight depression, called the fovea centralis. In this situation the retina is thinnest, and the dark colour of the hexagonal pigment-cells is visible through it, giving it the appearance of a foramen. About inch to the inner side of the posterior pole of the eyeball, and about iucb below its level, is the porus opticus, or optic disc. This is circular in outline, and its circumference is slightly elevated. It is the point of entrance of the fibres of the optic nerve, and the centre of the disc is pierced by the arteria centralis retinae which immediately divides into two branches upper and lower. * The optic disc consists entirely of nerve-fibres, and is known as the ‘ blind spot,’ vision being absent in this situation.

Structure of the Retina.— The retina consists of eight superimposed layers, seven of which are nervous and one pigmentary.


In addition to these, there are sustentacular fibres. The eight layers are as follows, from within outwards:


1. Stratum opticum, or layer of nerve-fibres.

2. Ganglionic layer, or layer of nerve-cells.

3. Inner plexiform (inner molecular) layer.

4. Inner nuclear or granular layer.

5. Outer plexiform (outer molecular) layer.

6. Outer nuclear or granular layer.

7. Layer of rods and cones.

8. Pigmentary layer.


Pigmentary Layer


1 Layer of Rods and Cones


A. Membrana Limitans Externa


> Outer Nuclear Layer


„_Outer Plexiform Layer


. Inner Nuclear Layer


In addition to the foregoing layers, there are two very delicate membranes, which really belong to the sustentacular fibres of the

retina, but are known as the membrana limitans interna and externa. The membrana limitans interna covers the retina on its internal surface, and the membrana limitans externa intervenes between the outer nuclear layer and that of the rods and cones. The layers of the retina are supported by fibres called the sustentacular fibres.

1. Stratum Opticum.

—This layer consists of the fibres of the optic nerve, and it extends from the optic disc to the ora serrata. The fibres are non-medullated, and are chiefly centripetal, but some are centrifugal. The

centripetal fibres arise Fus ioi 5. Diagrammatic Section of the Human mainly as the axons of

• R =mIT LTZE) COPIED FR0M QUAIN ’ S the “ lls of the S an 8 lionic layer. The centrifugal fibres pass towards the inner plexiform and inner nuclear layers.

2. Ganglionic Layer.—This consists of large, somewhat flaskshaped, multipolar ganglion-cells, which for the most part form a single layer. In the macula lutea, however, they form several layers.


> Inner Plexiform Layer


Layer of Nerve-cells (Ganglionic Layer)

I Layer of Nerve-fibres

Membrana Limitans Interna


The round ends of the cells rest upon the stratum opticum, and from each of these ends an axon is given off, which enters the stratum opticum obliquely, and forms one of its component fibres. The tapering end of each cell sends off several dendrites, which enter the inner plexiform layer, within which they arborize.

3. Inner Plexiform (Inner Molecular) Layer contains the arborizations of the dendrites of (1) the cells of the ganglionic layer, and (2) the bipolar cells of the inner nuclear layer. The intercommunications between these two sets of dendrites give rise to five strata, according to Ramon y Cajal. Besides these, there are the arborizations. of the processes of the spongioblasts of the inner nuclear layer, which are likewise arranged in strata.

4. Inner Nuclear or Granular Layer.—This layer consists of cells which are arranged in three groups: (1) bipolar cells, (2) horizontal cells, and (3) spongioblasts, or amacrine cells. The bipolar cells are the most numerous, and are nucleated. Each cell gives off two processes—internal and external. The internal processes of the cells enter the inner plexiform layer, and end at different levels in arborizations. The external processes pass into the outer plexiform layer, and form arborizations in its outermost part, which are closely related to the terminal parts of the rods and cones of the bacillary layer. According to Cajal, the bipolar cells are of two kinds—rod-bipolars and conebipolars. The external processes of the rod-bipolars ramify round the terminal parts of the rod-fibres, and the internal processes arborize round the cells of the ganglionic layer. The external processes of the cone-bipolars form horizontal arborizations round the ends of the cone-fibres, and the internal processes terminate in arborizations in the inner plexiform layer at different levels.

The horizontal cells occupy the outer part of the inner nuclear layer. Their dendrites enter the outer plexiform layer, and come into relation with the terminal parts of the cone-fibres, whilst their

axons run in a horizontal direction. .

The spongioblasts are situated in the innermost part of the inner nuclear layer. They are destitute of axons, and ha\ e been called amacrine cells, because each cell is ‘ without a long fibre or process. Their dendrites enter the inner plexiform layer, and end in arboriza

tions, which are arranged in strata.

=; Outer Plexiform (Outer Molecular) Layer.—This layer is composed of the following structures: (i) the external processes of the rod-bipolars and cone-bipolars of the inner nuclear layer; (2) the dendrites of the horizontal cells of the inner nuclear layer; and (3) the terminal parts of the rod-fibres, and filaments from the foot-plates

of the cone-fibres. ,

6 Outer Nuclear or Granular Layer.—This consists of granules,

which are of two kinds—rod-granules and cone-granules. The rodgranules are the more numerous, and are oval enlargements m the course of the rod-fibres, as these pass to the outer plexiform layer'• Each rod-fibre has only one rod-granule, and the granules lie at different levels. Each granule has a nucleus, which has transverse striations, there being at least two clear bands. The external process of each rod-granule is continuous with one of the rods of the bacillary layer, and the internal process passes into the outer plexiform layer, where it comes into relation with the arborizations of the external process of a rod-bipolar.

The cone-granules are larger than the rod-granules, but not so numerous, and each contains an oval nucleus. Situated in the outermost part of the outer nuclear layer, they lie close to the membrana limitans externa. The outer end of each granule is continuous with one of the cones of the bacillary layer. The inner end is prolonged into a cone-fibre, which passes into the outermost part of the outer


Fig. 1016. —Scheme of the Horizontal Cells and Spongioblasts of the Retina (Ramon y Cajal).


A. Rod-fibres

B. Cone-fibres

1. Outer Plexiform Layer a, b. Horizontal Cells, with arborizations c. Horizontal Cell, with deep processes


2. Inner Plexiform Layer /» S, h , f Spongioblasts extending to j, l. 1 different depths m, n. Spongioblasts with diffuse processes o. Ganglionic Nerve-cell


plexiform layer, where it expands into a foot-plate, from which filaments are given off. These filaments come into relation with the arborizations of the external process of a cone-bipolar cell.

7. Layer of Rods and Cones consists of rods and cones, the former being cylindrical, and the latter flask-shaped. The rods are much more numerous, longer, and narrower than the cones, and both are placed perpendicularly.

Each rod and cone consists of two segments—outer and inner. In the case of the rods the two segments are of almost equal length the inner segment being rather larger than the outer. The outer segment is the only seat of the colouring matter known as visual purple or rhodopsin In the case of the flask-shaped cones, the inner segment of each forms two-thirds of the cone, and is of large size; whilst the outer forms one-third, is narrow, and represents the tapering part of the flask. The outer segments of both rods and cones have faint transverse striations. The inner segments of both are subdivided. The outer part is composed of delicate fibrils longitudinally arranged, and therefore presents a longitudinally striated appearance. The inner part is faintly granular. The rods and cones are continued at their inner ends through the membrana limitans externa into the rod-fibres and cone-fibres, which belong to the outer nuclear layer. The outer ends of the rods project into the pigmentary layer.

8 . Pigmentary Layer.—The most external layer of the retina is in close contact with the choroid coat. It consists of a single layer of hexagonal epithelial cells, which contain pigment. The deep surfaces of the cells give off processes which extend into the intervals between the outer ends of the rods and


Layer of Rods and Cones


Membrana Limitans Externa


Outer Nuclear Layer


Outer Plexiform Layer Inner Nuclear Layer


Inne’ Plexiform Layer


Ganglionic Layer


1 1 Nerve-fibre Layer ■/.Membrana Limitans Interna

Fig. 1017.—Section of the Retina as seen

UNDER THE MICROSCOPE (MAGNIFIED).



cones.

Sustentacular Fibres (or Fibres of Muller).—

These fibres form the supporting tissue of the retina, and extend from its internal surface to the boundary-line between the outer nuclear layer and the layer of rods and cones. The inner ends of the fibres are expanded, and blend at their edges to present the appearance of a distinct retinal layer, which is called the membrana limitans interna. Their outer ends, which are very numerous owing to the breaking up of the fibres, also expand and form the membrana limitans externa, which lies between the outer nuclear layer and the layer of rods and cones. (The membrana limitans interna and externa are sometimes considered layers of the retina, under which circumstances the retinal layers would be ten in number, instead of eight.) From the membrana limitans externa delicate offsets enter the layer of rods and cones, in the innermost part of which they form fibre-baskets in connection with the deep ends of the rods and cones. As the sustentacular fibres pass through the inner nuclear layer each has an oval nucleus, which contains a nucleolus. This nucleus is variously described as being situated on one side of the fibre, or as involving its whole circumference. Throughout their course the sustentacular fibres give off lateral offsets, which increase in number from within outwards.

Structure of the Macula Lutea and Fovea Centralis.—The chief structural characters of the macula lutea and fovea centralis may be stated in the following tabular manner:


Macula Lutea.

1. Cones only.

2. Outer nuclear layer has only cone fibres disposed obliquely.

3. Ganglionic layer very thick, cells being

several layers deep.

4. Stratum opticum not continuously

disposed.

B


Fovea Centralis.

1. Thinnest part of the retina.

2. Pigmentary layer thick.

3. Cones only.

4. Outer nuclear layer has only

cone-fibres.

5. Ganglionic layer absent.

6. Stratum opticum absent.

A


Membrana Limitansv Externa N, “


Fibrous Basket-work


Outer Plexiform Layer


Nucleus of one of Sustentacular Fibres


Inner Plexiform_

Layer


Sustentacular Fibres' Limitans Interna 1 ' Membrana


Centrifugal Nerve-fibre


Rods and Cones


Outer Nuclear Layer


Subepithelial

Ganglion-cell

Stellate Ganglioncell

Bipolar Q.anglioncell

Multipolar

Ganglion-cell


Multipolar Ganglion-cell Layer of Nervefibres


Fig. 1018.—Diagram of the Elements of the Retina (Wiedersheim,

AFTER PH. StoHR).

A, nervous elements; B, supporting elements.


Structure of the Ora Serrata.—At the ora serrata the nervous elements of the retina end, and its pigmentary layer is continued over the deep or posterior surfaces of the ciliary processes. Here is added to its deep or posterior surface a layer of columnar epithelial cells, and the two layers form the pars ciliaris retinas, which is continued into the pars iridica retinae (uvea). In the latter the cells of both layers are pigmented.

Blood-supply of the Retina.—The retina is supplied with blood by the arteria centralis retinae, a branch of the ophthalmic artery. Within the orbit the artery pierces the under aspect of the optic nerve a little behind the eyeball, and passes forwards in the centre of the nerve. At the centre of the optic disc it divides into two branches, upper and lower. Each of these breaks up into two branches, nasal or medial,

and temporal or lateral. The temporal branches keep clear of the macula lutea, but give small twigs to it, which, however, do not enter the fovea centralis, this part being non-vascular. As the branches pass inwards and outwards respectively towards the periphery of the retina they ramify freely, and end at last in capillary networks. The arteries do not extend farther outwards than the inner nuclear layer. No anastomoses take place between the branches of the arteria centralis retinae.


Fig. 1019._Scheme of the Retina, showing the Connection between

the Layer of Rods and Cones and the Ganglionic Layer (Ramon


y Cajal).

A. Layer of Rods and Cones

B. Outer Nuclear Layer

C. Outer Plexiform Layer

E. Inner Nuclear Layer

F. Inner Plexiform Layer

G. Ganglionic Layer

H. Layer of Nerve-fibres M. Sustentacular fibre

a. Rods

b. Cones

c. Granule of Cones

d. Granule of Rods


e. Bipolar Cells of Rods

f. Bipolar Cells of Cones

g, h, i, \ Ganglionic Corpuscles ramifying at different j, k. f levels in Inner Plexiform Layer r, r'. Deep arborizations of Bipolar Cells

s. Centrifugal Nerve-fibre

t. Nucleus of Sustentacular Fibre

X. Deep ends of Rod-fibres amongst superficial arborizations of Bipolar Cells Z. Meeting of arborizations of Cones and Bipolar Cells


In the foetus the arteria centralis retinae sends a branch to the posterior part of the capsule of the crystalline lens, which reaches it through the ‘ canal of Cloquet in the vitreous body.

The veins are ultimately collected into two vessels, upper and lower, which pass through the optic disc, one above and the other below' the artery. They then form one vessel which opens into the superior ophthalmic vein. The veins of the retina are destitute of muscular tissue, the wall of each being formed by a single layer of endothelial cells, external to which there is a perivascular lymph space, this in turn being limited by another layer of endothelial cells. These lymph-spaces are in communication with those of the optic nerve.

Relation of the Retinal Layers to One Another.—The only two layers which are in direct continuity are the stratum opticum and ganglionic layer, some fibres of the former being the axons of the cells of the latter. As regards most of the strata, the constituent elements of successive layers are brought into communication by means of the interlacements which take place between the arborizations formed by their various processes. These interlacements occur in the inner and outer plexiform layers.



Fig. 1020.—Vertical Sagittal Section of the Eye and its Appendages (Hirschfeld and Leveille).


In the inner plexiform layer there are several strata of interlacements, by means of which the dendrites of the cells of the ganglionic layer are brought into communication with the internal processes of the bipolar cells of the inner nuclear layer. In the outer plexi orm layer there is a free intermingling between the external processes of the bipolar cells of the inner nuclear layer and the rod-fibres and conefibres.

Nerve-cells of the Retina.—These are arranged in three strata, and communicate with one another through interlacing arborizations. The outermost stratum consists of the rods and cones; the middle stratum is formed by the bipolar cells; and the innermost stratum represents the cells of the ganglionic layer. The axons of the ganglionic cells enter the stratum opticum as centripetal fibres, which pass in the optic nerve to the brain. The centrifugal fibres of the stratum opticum ramify in the inner plexiform or inner nuclear layer.


Refracting Media

Aqueous Humour and Chambers of the Eye.—The aqueous humour occupies the space between the cornea and the front of the crystalline lens, which is divided by the iris into two chambers, anterior and posterior. It is a clear fluid having an alkaline reaction, and is composed of H ? 0 , holding in solution a very small amount of sodium chloride and traces of albumen.

The anterior chamber is bounded anteriorly by the cornea, and posteriorly by the iris and the central portion of the crystalline lens enclosed within its capsule. The anterior chamber communicates with the irido-corneal spaces, through them with the sinus venosus sclerae, and through this canal with the veins of the sclera.

The posterior chamber, which is of limited extent, is bounded anteriorly by the iris, and posteriorly by the peripheral part of the crystalline lens and its suspensory ligament, and by the ciliary processes. The anterior and posterior chambers communicate with each other through the pupil; with lymph-spaces in the iris; and through the latter spaces with the perichoroidal lymph-space.

Crystalline Lens.—The crystalline lens is situated directly behind the pupil and iris, from which latter it is separated by the posterior chamber. It is a solid, transparent, biconvex disc, the posterior surface being more convex than the anterior, and is enclosed within a A ’ 0 fi n br e e 4° e f ) . th ^°L ( man homogeneous, transparent envelope, called the fibres (seen on end). capsule of the lens. The centre of the anterior

surface is called the anterior pole, and that of the posterior surface the posterior pole. The line connecting these two poles constitutes the axis of the lens, and a line surrounding the periphery represents the equator. The transverse measurement of the lens, is about •it inch, and its axis measures about inch. The . anterior surface at its central part faces the pupil. External to this, the pupillary margin of the iris rests upon it, and external to this again is the posterior chamber, with part of the aqueous humour. The posterior surface is received into the ‘ patellar fossa on the anterior aspect of the vitreous body. The periphery is related to the suspensory ligament, the zonular spaces present in this ligament, and the ciliary processes. From the anterior and posterior poles delicate lines radiate towards the equator. In early life these are three on each surface. Those on the posterior surface form an inverted while those on the anterior form an erect Y. These lines represent the free margins of septa within the lens upon which the ends of the lens-fibres terminate.


Fig. 1021.—Fibres of the Crystalline Lens (highly magnified) (after Kolliker).



Structure.—The lens is laminar in structure. The outer laminae are soft in consistence, but the succeeding ones gradually become firmer, and the central portion, which constitutes the nucleus, is very firm and hard. The laminae are arranged concentrically, and after boiling or immersion in alcohol they may be peeled off, like the coats of an onion. The fibres of which the laminae are composed terminate upon septa within the lens, of which the radiating lines on the surfaces, already referred to, are the free margins. The concentric laminae are therefore not continuous all round, but are split up along these lines. The lens-fibres, which are disposed in a curved manner, are of small size, and have serrated edges, which fit closely to each other. In transverse section the fibres appear as hexagonal prisms. The fibres are the elongated cells which line the posterior part of the ectodermal vesicle (lens vesicle) from which the lens is developed. In early life each fibre has a nucleus, but after the lens has attained its full development only the outermost fibres are nucleated.

Capsule of the Lens.—This is a transparent, homogeneous, elastic and brittle membrane, which surrounds and encloses the lens. Its anterior wall is thicker and more elastic than the posterior. In the adult the lens and its capsule are non-vascular, but in the foetus they receive the hyaloid branch of the arteria centralis retinae, which reaches it through the hyaloid canal in the vitreous body.

Epithelium of the Lens.—The posterior surface of the lens is devoid of epithelium, and is in direct contact with the posterior wall of the capsule. The anterior surface is covered by a single layer of columnar cells, which intervenes between the anterior surface and the anterior wall of the capsule. Towards the equator these cells become elongated, and pass into short fibres, which become continuous with the superficial lens-fibres.

Crystalline Lens at Different Ages.—The characters of the lens at different ages are as follows:


Foetal Lens.

Almost spherical. Pinkish colour. Semitransparent. Soft in consistence.


Adult Lens.

Biconvex

Colourless.

Transparent.

Firm in consistence.


Lens in Old Age.

Flattened.

Amber colour.

Opaque, more or less. Very firm in consistence.


Vitreous Body.—This body occupies about four-fifths of the space within the eyeball, and is situated between the crystalline lens and the retina. It is transparent and gelatinous, and is composed of water, holding in solution a small quantity of sodium chloride and albuminous matter. It is surrounded by a transparent, homogeneous envelope, called the hyaloid membrane. This membrane is in contact with the retina, except anteriorly, where there is an excavation called the fossa



Fig. 1022. — Meridional Section through the Anterior Portion of the Eye (magnified 16X1) (Fuchs).

C.P., C.P., zonular spaces.


Towards its circumference the vitreous body is laminated, the laminae being arranged concentrically. Laminae are also said to radiate from its antero-posterior axis towards the circumference. Scattered throughout the vitreous body there are some amoeboid corpuscles, and it is traversed from behind forwards by a minute passage called the hyaloid canal (canal of Cloquet, canal of Stilling). This extends from the centre of the optic disc to the posterior wall of the capsule of the lens, and posteriorly it communicates with the lymph-spaces of the optic nerve. In the foetus the canal transmits a branch of the arteria centralis retinae, called the hyaloid artery , which supplies the lens.

No vessels enter the vitreous body, its nutrition being derived from the vessels of the retina and ciliary processes.

Zonula ciliaris, or zonule of Zinn, is the thickened portion of the hyaloid membrane which is situated in front of the ora serrata of the retina. From this point it extends inwards behind the ciliary processes towards the periphery of the crystalline lens. Behind the ciliary processes are radial folds with intervening depressions. The depressions receive the ciliary processes, and the radial folds are separated from the intervals between the ciliary processes by lymphspaces, which communicate with the posterior chamber of the eye.

Suspensory Ligament of the Lens, and Zonular Spaces.—The ciliary zonule, as it approaches the periphery of the lens, divides into three layers—posterior, middle, and anterior. The posterior layer lines the fossa patellaris in front of the hyaloid membrane. The middle layer consists of a few scattered fibres which pass to the equator of the lens. The anterior layer is the thickest, and forms the suspensory ligament of the lens, which is attached to the anterior wall of its capsule not far from the equator (see Fig. 1022). When the radiating fibres of the ciliary muscle contract the suspensory ligament is relaxed, and the convexity of the anterior surface of the lens is increased.

Behind the suspensory ligament of the lens there is a sacculated lymph-space, called the zonular space, which surrounds the equator of the lens.

Development of the Eye

The retina, optic nerve, and crystalline lens are developed from the ectoderm, the retina and optic nerve being derived from the ectoderm of the anterior primary cerebral vesicle, whilst the crystalline lens is developed from the ectoderm of the side of the head. The accessories of the eye— e.g., the sclera, cornea, choroid, ciliary body, and iris—are all developed in mesoderm, but ectoderm, as will be seen, is also employed in some of these. The vitreous body, though developed to a certain extent from the mesoderm, is principally formed from the ectoderm.

The earliest indication of the future eye is in the form of a shallow marginal groove on each side in the widely open cerebral plate of embryos with a few somites. As the region grows these grooves become deepened by the upgrowth of the lateral margins, which ultimately fuse in the middle line, in continuity with the fusion of the edges of the medullary folds further back. In this way the open grooves are converted into recesses or lateral pockets of the closed fore-brain, each pocket being in contact from the beginning with the ectoderm of the surface.

The pocket formed in this way is termed the optic recess, and becomes the optic vesicle very soon by its rounded enlargement under the surface ectoderm; such enlargement is mainly at its distal part, its connection with the brain being slightly constricted, forming a ' neck ' for the vesicle. As the development goes on this neck is drawn out into a definite stalk, which connects the vesicle with the fore-brain. Stages in these changes can be seen in Fig. 1025. The vesicle is hollow, its cavity being carried into the stalk, and, through this, communicating with that of the fore-brain, which will be the third ventricle. The


Fig. 1023.—Development of Crystalline Lens and Optic Vesicle (Scheme).

The lens is lying in the optic cup.


enlargement formed by the optic vesicle lies deep to, and in contact with, the ectoderm of the lateral surface of the head (Fig. 1025).

The ectoderm in relation with the optic vesicle becomes thickened and depressed, this depressed portion constituting the lens area. The depressed ectoderm is deepened and converted into a kind of cup, and, the mouth of the fossa becoming constricted, its lips unite. In this manner the lens area becomes transformed into a closed ectodermic sac, called the lens vesicle, from which the


Fig. 1024.—Diagram showing (see Text) the Conversion of Optic Vesicle into Optic Cup.

crystalline lens is differentiated. The lens vesicle now becomes completely separated from the surface ectoderm, with which it was originally continuous

^ Fl The outer wall of the vesicle, facing the rudiment of the lens, is invaginated so as to obliterate the cavity of the vesicle, which is now converted into the oi>tic cut> Fig 1024 gives diagrammatic sections which may help in the comprehension of this change. The middle vertical row of figures here shows sections along the length of the optic outgrowth; the simple optic vesicle is seen at the top, the commencing invagination of its lower lateral wall is seen next, while the completed invagination is shown in the lowest section. It can be seen that the invagination extends into the optic stalk also. On the left side the invagination is shown by transverse sections of the vesicle, corresponding more or less with the stages of the middle column. Observe that the cavity (V) of the optic



Fig. 1025. — Different Stages in the Development of the Eye (from Reconstruction Models at St. Mary’s Hospital).

A piece of the wall of the optic vesicle has been removed in the first specimen, showing the cavity of the vesicle ; the lens thickening of the ectoderm is beginning to be depressed. In the second the optic outgrowth is entire, and the lens depression is projecting into the cavity of the optic cup. In the third figure removal of part of the wall has opened the cavity of the vesicle,and also the cavity of the cup, in which the lens vesicle is lying, still attached to the ectoderm, its cavity opened by the section. The figures also show the formation of the stalk of the vesicle and the extension into it of the cleft continuous with the cavity of the optic cup.

vesicle is being obliterated, replaced by the cavity (C) of the optic cup, which is still open in front and below; the last section in the middle column has gone along this interval between the two sides of the cup. The interval is termed the choroidal or foetal fissure, and extends into the stalk. It closes later by the apposition and rapid fusion of its lips, so completing the optic cup. The righthand column of sections is made from the distal end towards the brain; they show the concavity in the vesicle, and in the stalk, lost in the last section.


The lens vesicle, when it separates from the surface ectoderm, lies in the opening of the optic cup. Vascular mesoderm extends into the cavity of the cup through the choroidal fissure, behind and below the lens vesicle; when the fissure closes, the mesoderm within the cavity of the cup loses its connection with the outer mesoderm, except at the end of the fissure, where a relatively large vessel persists, and becomes ultimately the central artery of the retina. Since the end of the fissure is in the optic stalk, which becomes the optic nerve, this artery passes in the terminal piece of the nerve to enter the eye. The artery, when first formed, is known as the hyaloid artery, and is distributed over the posterior surface of the lens.

In cases of non-closure of the choroidal fissure the region of the fissure remains unpigmented, and one of two congenital deficiencies in the eye is met with, each being known by the general term coloboma. If the patent fissure affects the ventral wall of the optic cup, then the deficiency in pigment affects the choroid, and the condition is known as coloboma choroidea. If the patent fissure affects the lower margin of the optic cup, then the deficiency affects the lower part of the iris, and the condition is known as coloboma iridis.

As stated, the wall of the optic cup consists of two layers. The outer layer, which is comparatively simple, gives rise to the pigmentary layer of the retina. The inner layer is, on the other hand, very complicated. After much differentiation it gives rise to all the other layers of the retina. The mesodermic tissue, which invests the optic cup, gives rise to the sclera, cornea, choroid, ciliary body (including the ciliary processes and ciliary muscle), and iris. The ciliary processes are covered by layers from the (ectodermal) walls of the cup.

Crystalline Lens. —The lens is of ectodermic origin. The surface ectoderm on the lateral aspect of the head opposite the optic vesicle becomes thickened and depressed to form, as stated, the lens area. The depressed ectoderm is deepened and converted into a kind of cup. The mouth of the cup becomes constricted, and its lips unite. In this manner the lens area becomes transformed into a closed ectodermic sac, called the lens vesicle, from which the crystalline lens is differentiated. The lens vesicle becomes completely separated from the surface ectoderm, with which it was originally continuous. It is now received into the optic cup, which has been formed in connection with the optic vesicle, its position being just within the mouth of the cup, the circumference of the margin projecting slightly in advance of the vesicle.

The anterior and posterior walls of the lens vesicle at this stage consist of several layers of cylindrical cells, and the vesicle contains a small central cavity. The anterior wall becomes gradually thin, and is ultimately formed of one layer of flattened cells, these cells constituting the anterior epithelium of the adult crystalline lens. The cells of the posterior wall become elongated in a forward direction, obliterating the cavity of the vesicle, and coming into contact with the anterior wall. By this process of cell elongation the lens-fibres are formed. At the equator of the lens the cells of the anterior and posterior walls merge gradually into one another through the medium of a transitional zone of columnar cells.

At this stage in its development the crystalline lens consists of (1) an anterior epithelial wall, and (2) a posterior wall composed of elongated cells forming the lens-fibres.

As development proceeds, additional lens-fibres are formed by the proliferation of cells at the equator of the lens. These fibres are laid down in successive layers, which are arranged concentrically.

Capsule of the Crystalline Lens.— At an early period in its development the lens becomes invested by a mesodermic capsule, freely supplied with bloodvessels derived from the hyaloid artery and anterior ciliary arteries. This capsule is known as the tunica vasculosa. It persists throughout the period of active growth of the lens, and then undergoes retrogression to form the permanent lens capsule. The portion of the tunica vasculosa which covers the front part of the lens is called the membrana papillaris, but this usually disappears prior to birth. It may, however, be present at birth, giving rise to the condition known as atresia pupillce. Towards the end of intra-uterine life the tunica vasculosa undergoes retrogression and becomes transformed, as stated, into the permanent lens capsule, which is a transparent, homogeneous, elastic membrane.

This mesodermal pupillary membrane is a continuation across the open mouth of the cup of the plane of the choroidal layer. It is, therefore, on the outer surface of the developing iris, of which it forms the mesodermal base, the muscles being derived from the actual ectodermal or retinal layer itself.

Development of the Optic Cup and Optic Stalk. —The optic cup, as stated, is formed by the invagination of the distal or outer wall of the optic vesicle, the invagination also affecting the ventral wall of the optic vesicle and the ventral wall of the part of the optic stalk which is adjacent to the optic vesicle, thereby giving rise to the choroidal fissure. The mouth of the optic cup is directed towards the lateral aspect of the head, and the lens vesicle lies just within the mouth. That the invagination of the optic cup is not caused by the growth of the lens vesicle has been proved by experimental transplantations on amphibian embryos. The margin of the cup projects slightly over the lens vesicle, and the circumference of this margin represents the outline of the pupil. The wall of the cup consists of two layers —namely, inner and outer, the inner representing the distal or outer wall of the optic vesicle, which has now become invaginated, or folded inwards. The cup is divisible into two regions—namely, (i) the ciliary region, adjoining the margin of the cup; and (2) the fundus. The line of separation between these two regions corresponds to the ora serrata of the adult eye.

The ciliary region of the optic cup is associated with the ciliary body (including the ciliary processes and ciliary muscle) and the iris, which are developed from the thickened anterior part of the choroid. The outer layer of the ciliary portion, as elsewhere, forms the pigmentary layer of the retina. The inner layer of the ciliary portion, which is very thin, forms (1) the pars ciliaris retinae on the posterior surfaces of the ciliary processes, and (2) the pigmented pars iridica retinae [uvea) on the posterior surface of the iris.

The fundus of the optic cup is the proper retinal region. The outer layer forms, as in the ciliary region, the pigmentary layer of the retina. The inner or retinal layer becomes differentiated into all the layers of the retina except the pigmentary layer. The changes which it undergoes are very complicated. Its thickness is considerably increased, and it subdivides into two layers— outer and inner—from which the various retinal strata (except the outer pigmentary layer) are specialized.

The optic stalk is transformed into the optic nerve. The stalk is at first hollow, its cavity communicating with that of the optic vesicle on the one hand, and with the third ventricle of the brain on the other. As stated, the choroidal fissure involves the under surface of the optic stalk near the optic vesicle, as well as the under surface of the optic vesicle itself. When the choroidal fissure undergoes closure, the hyaloid artery, which passed through that fissure, becomes enclosed within the optic stalk, and forms the arteria centralis retincc of adult life. By the closure of the choroidal fissure, and the consequent enclosure of the hyaloid artery, the cavity of the distal portion of the optic stalk becomes obliterated. Inasmuch as the ventral or lower wall of this part of the stalk has been previously invaginated, the wall of the stalk is now composed of two layers—outer and inner—the inner being formed by the invaginated ventral or lower wall. The outer layer of the optic stalk is now continuous with the outer layer of the optic cup, whilst the inner layer of the optic stalk is continuous with the inner layer of the optic cup. As regards the proximal part of the optic stalk, its cavity becomes gradually closed. The wall of the optic stalk becomes thickened, its cells proliferate, and they give rise to the neuroglial or sustentacular tissue of the future nerve. The nerve-fibres which build up the optic nerve are regarded as having two sources. The majority of them represent the axons of the ganglion cells of the retina, which pass in the optic stalk to the diencephalon and mesencephalon. These are therefore centripetal fibres. Other fibres are regarded as being centrifugal, these arising in connection with the diencephalon and mesencephalon.


Vitreous Body. —This body is formed within the optic cup, for the most part posterior to the lens vesicle. It is principally developed from the ectoderm, but the mesoderm also takes part in its formation. The ectodermic fibres are derived from those cells which pertain to the sustentacular fibres of the retina.

These ectodermal fibres form a very delicate reticulum (Fig. 1026) connecting the lens vesicle and the inner layer of the optic cup. Mesodermal ingrowth through the choroidal fissure brings in vessels which ramify to some extent between the ectodermal connecting strands, but for the most part pass forward to the back of the growing lens, over which the vessels spread, with their thin mesodermal surrounding. The main vessel thus reaching the lens is the hyaloid artery, and this with its surrounding fine mesoderm occupies at first a large part of the small cavity of the cup, enclosed by ectodermal processes, more or less avascular in the more peripheral parts of the cup. This is the state known as the primary vitreous, characterized by ectodermal formations connected in origin with both retina and lens, and associated fairly intimately with vascular mesoderm. The central hyaloid artery is distributed over the back of the lens, its terminal branches meeting, at the periphery of this structure, vessels which enter the cup from the outside, turning round its rim.

The primary vitreous is gradually succeeded and replaced by the secondary vitreous. The time of the beginning of the change is usually considered to be about the fifth to sixth week, when the posterior hyaloid capsule of the lens makes its appearance; after this the slowly increasing ectodermal element can be produced only by the retina. It is this element which, by its growth, occupies the extra space resulting from the increasing size of the eyeball, so that it gradually comes about that the original vascu- C, wall of fore-brain ; OP.V., points to lar mesodermo-ectodermal formation cavity of optic vesicle; OC, to cavity


Fig. 1026.—Vertical Section through Eye in 5 Mm. Embryo.


of optic cup; L.P., lens pit; ECT., surface ectoderm. Protoplasmic processes connect the lens pit with the inner wall of the cup.


(primary vitreous) is surrounded and enclosed by an increasing mass of ectodermal secondary vitreous ; this is largely non-vascular, but does not become completely avascular until the hyaloid artery atrophies. The vessels are contained in a central funnel-shaped ‘ space ’ in this stage, surrounded by the secondary vitreous, which does not compress them in any way; the broad end of the funnel is behind the lens, over which the vessels extend as before, making a vascular capsule for the structure, and joining round the periphery with vessels reaching its anterior surface. The anterior part of this tunica vasculosa has been seen already to form the pupillary

m The bloodvessels atrophy and disappear in the latter part of foetal life, when the interval in which they lay persists as the hyaloid (or vitreous) canal, or canal of Cloquet, the remaining ectodermal substance, now avascular, being the

definite vitreous. r , r J ,

About the end of the third month the growth forward of the nm of the optic

cup (to form the ectodermal portion of the iris) is accompanied by the appearance of a more fibrillar vitreous formation corresponding with it; this is sometimes referred to as the tertiary vitreous, and the fibrils of the suspensory ligament of the lens are developed in this formation.


That part of the hyaloid artery which lies in the fissure in the optic stalk remains as the extra-ocular part of the arteria retinae centralis. The actual arteries of the retina are secondary and late branches which extend into that layer from the hyaloid artery as this enters the eyeball; when the lentine part of the vessel atrophies, these retinal branches remain and enlarge.

Derivatives of the Mesodermic Envelope of the Optic Cup. —These are as follows: (1) Sclera, (2) cornea, (3) choroid, (4) ciliary body (including the ciliary processes and ciliary muscle), and (5) iris.

The mesoderm which invests the outer surface of the optic cup is disposed in two layers —outer and inner. The outer layer has a fibrous character, and gives rise to the sclera, of which the cornea is a forward extension. The inner layer is vascular, and gives rise to the choroid, and mesodermal bases of the ciliary body and iris. The outer dense fibrous layer of the mesoderm of the outer surface of the optic cup, as stated, gives rise to the sclera. From its anterior margin a thick lamina of mesoderm is prolonged between the lens vesicle and the surface ectoderm. This lamina shows two layers—superficial and deep. The superficial layer becomes differentiated into the cornea, which is thus genetically continuous with the sclera. The deep layer becomes the pupillary membrane (see above). Between these two layers there is an interval, which represents the aqueous chamber.

The inner vascular layer of the mesoderm of the outer surface of the optic cup, as stated, gives rise to the choroid. The anterior margin of the choroid, which adjoins the margin of the optic cup (ciliary region) becomes thickened, and gives rise to the ciliary body, in connection with which the ciliary processes and ciliary muscle are developed. The ciliary processes become covered posteriorly by the pars ciliaris retince, which is a thin retinal expansion from the ciliary region of the optic cup. The iris is also developed at the anterior margin of the choroid in the form of a ring of mesoderm. In this mesoderm the fibres forming the dilator pupillce and sphincter pupillce muscles are formed by proliferation of the ectodermal cells of the edge of the optic cup, which has extended forward in front of the lens, and the back of the iris receives a pigmentary covering {uvea) from the pars iridica retince.