Comprehensive Ophthalmology AK Khurana, Bhawna Khurana, Aruj K Khurana
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1Anatomy and Physiology of Eye
Section Outline
  • 1. Anatomy and Development of Eye
  • 2. Physiology of Eye and Vision2

Anatomy and Development of Eye1

CHAPTER OUTLINE
  • ANATOMY OF EYE
  • • Eyeball
  • • Visual pathway
  • • Orbit, extraocular muscles and appendages of the eye
  • • Blood vessels and nerves
  • DEVELOPMENT OF EYE
  • • Formation of optic vesicle and optic stalk
  • • Formation of lens vesicle
  • • Formation of optic cup
  • • Changes in the associated mesenchyme
  • • Development of various ocular structures
  • • Structures derived from the embryonic layers
  • • Important milestones in the development of eye
 
ANATOMY OF EYE
This chapter gives only a brief account of the anatomy of eyeball and its related structures. The detailed anatomy of different structures is described in the relevant chapters.
 
EYEBALL
Each eyeball (Fig. 1.1) is a cystic structure kept distended by the pressure inside it.
  • Shape. Although, generally referred to as a globe, the eyeball is not a sphere but an oblate spheroid.
  • Poles. The central point on the maximal convexities of the anterior and posterior curvatures of the eyeball is called the anterior and posterior pole, respectively.
  • Equator of the eyeball lies at the mid plane between the two poles (Fig. 1.2).
 
Dimensions of an adult eyeball
  • Anteroposterior diameter
24 mm
  • Horizontal diameter
23.5 mm
  • Vertical diameter
23 mm
  • Circumference
75 mm
  • Volume
6.5 ml
  • Weight
7 gm
 
Coats of the eyeball
The eyeball comprises three coats: outer (fibrous coat), middle (vascular coat) and inner (nervous coat).
1. Fibrous coat. It is a dense strong wall which protects the intraocular contents. Anterior 1/6th of this fibrous coat is transparent and is called cornea. Posterior 5/6th opaque part is called sclera. Cornea is set into the sclera like a watch glass. Junction of the cornea and sclera is called limbus. Conjunctiva is firmly attached at the limbus.
2. Vascular coat (uveal tissue). It supplies nutrition to the various structures of the eyeball. It consists of three parts, from anterior to posterior, which are: iris, ciliary body and choroid.
3. Nervous coat (retina). It is concerned with visual functions and projects to visual cortex through the visual pathway.
 
Segments and chambers of the eyeball
The eyeball can be divided into two segments: anterior and posterior.
1. Anterior segment. It includes crystalline lens (which is suspended from the ciliary body by zonules), and structures anterior to it, viz., iris, cornea and two aqueous humour-filled spaces : anterior and posterior chambers.
■ Anterior chamber. It is bounded anteriorly by the back of cornea, and posteriorly by the anterior surface of iris and part of ciliary body. The anterior chamber is about 2.5 mm deep in the centre in normal adults. It is slightly shallower in hypermetropes and deeper in myopes, but is almost equal in the two eyes of the same individual. It contains about 0.25 ml of the aqueous humour. It communicates with posterior chamber through the pupil.4
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Fig. 1.1: Gross anatomy of the eyeball
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Fig. 1.2: Poles and equators of the eyeball
Posterior chamber. It is a triangular space containing 0.06 ml of aqueous humour. It is bounded anteriorly by the posterior surface of iris and part of ciliary body, posteriorly by the crystalline lens and its zonules, and laterally by the ciliary body.
2. Posterior segment. It includes the structures posterior to lens, viz., vitreous humour (a gel-like material which fills the space behind the lens), retina, choroid and optic disc.
 
VISUAL PATHWAY
Each eyeball acts as a camera; it perceives the images and relays the sensations to the brain (occipital cortex) via visual pathway which comprises optic nerves, optic chiasma, optic tracts, geniculate bodies and optic radiations (Fig. 1.3).
 
ORBIT, EXTRAOCULAR MUSCLES AND APPENDAGES OF THE EYE
Each eyeball is suspended by extraocular muscles and fascial sheaths in a quadrilateral pyramid-shaped bony cavity called orbit (Fig. 1.4). Each eyeball is located in the anterior orbit, nearer to the roof and lateral wall than to the floor and medial wall. Each eye is protected anteriorly by two shutters called the eyelids. The anterior part of the sclera and posterior surface of lids are lined by a thin membrane called conjunctiva. For smooth functioning, the cornea and conjunctiva are to be kept moist by tears which are produced by lacrimal gland and drained by the lacrimal passages. These structures (eyelids, eyebrows, conjunctiva and lacrimal apparatus) are collectively called ‘the appendages of the eye’.
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Fig. 1.3: Gross anatomy of the visual pathway
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Fig. 1.4: Section of the orbital cavity to demonstrate eyeball and its accessory structures
 
BLOOD VESSELS AND NERVES
 
Arteries
Ophthalmic artery, a branch of internal carotid artery, constitutes the main source of blood supply for the eyeball and other orbital structures. Blood supply of each ocular structure is described in the relevant chapters.
  • Branches of ophthalmic artery are shown in Fig. 1.5.
  • Arterial supply of eyeball is depicted in Fig. 1.6.
 
Veins
Veins draining blood from the eyeball include (Figs.1.6 and 1.7):
  • Central retinal vein which drains blood from the retina; and
  • Anterior ciliary veins, short posterior ciliary veins and venae verticosae which drain blood from the uveal tissue.
Main venous channels which ultimately get tributaries from various orbital structures include (Fig. 1.7):
  • Superior ophthalmic vein,
  • Inferior ophthalmic vein,
  • Middle ophthalmic vein,
  • Medial ophthalmic vein,
  • Angular vein, and
  • Cavernous sinus.
 
Nerves
 
Sensory nerves
 
Ophthalmic nerve
Ophthalmic nerve, smallest of the three divisions of trigeminal (5th cranial) nerve, supplies the various ocular structures through its three branches (Fig. 1.8):
Lacrimal nerve. It lies in the lateral part of the orbit and supplies lacrimal gland, conjunctiva and lateral part of upper eyelid.
Frontal nerve. It divides into two branches in the middle of orbit:
■ Supratrochlear nerve supplies the conjunctiva, middle part of upper eyelid, and skin of the forehead above the root of nose.
■ Supraorbital nerve supplies the conjunctiva, central part of upper eyelid, and part of the skin of forehead and scalp.
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Fig.1.5: Ophthalmic artery and its branches
3. Nasociliary nerve. It has following branches:
Long ciliary nerves, two in number, pierce the sclera on either side of optic nerve, run forward between sclera and choroid and supply sensory nerves to the ciliary body, iris and cornea.
Communicating branches to ciliary ganglion form its sensory root and their fibres pass along the short ciliary nerves, to supply the ciliary body, iris and cornea.
Posterior ethmoidal nerve supplies the ethmoidal and sphenoidal air sinuses.
Anterior ethmoidal nerve is a terminal branch of nasociliary nerve which leaves the orbit through the anterior ethmoidal foramen.
Infratrochlear nerve is the other terminal branch of nasociliary nerve. It runs forward and supplies the conjunctiva, lacrimal sac, caruncle, and medial part of the eyelids.
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Fig. 1.6: Blood supply of eyeball
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Fig.1.7: Schematic drawing showing orbital veins draining blood from the ocular tissues
 
Motor nerves
  • 3rd, 4th and 6th cranial nerves supply the extraocular muscles, and
  • 7th cranial nerve branches supply the orbicularis oculi muscle of the eyelids.
 
Autonomic nerves
 
Parasympathetic nerves
Edinger-Westphal nucleus, located in midbrain (Fig. 1.9), sends preganglionic fibres through the third cranial nerve to ciliary ganglion and accessory ganglion. Postganglionic nerve fibres from ciliary ganglion travel along the short ciliary nerves to supply the sphincter pupillae muscle and postganglionic fibres from the accessory ganglion supply the ciliary muscle.
Salivatory nucleus, located in pons (Fig. 1.10), sends preganglionic fibres through facial nerve to the sphenopalatine ganglion. Postganglionic secretomotor fibres finally reach the lacrimal gland through the lacrimal nerve.
Ciliary ganglion
Ciliary ganglion is a peripheral parasympathetic ganglion placed in the course of oculomotor nerve near the apex of orbit (Fig. 1.9).
Roots of ciliary ganglion include:
Sensory root, as described above, comes from the nasociliary nerve.
Sympathetic root of ciliary ganglion comes from internal carotid plexus.
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Fig.1.8: Ophthalmic nerve and its branches
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Fig.1.9: Course of parasympathetic nerve supply from Edinger-Westphal nucleus to ciliary muscle and sphincter pupillae muscle
■ These fibres also do not relay here and pass along the short ciliary nerves to supply the blood vessels of the eyeball.
Parasympathetic root of ciliary ganglion arises from the nerve to inferior oblique muscle and carries the preganglionic fibres from the Edinger–Westphal nucleus. These fibres relay here and postganglionic fibres pass through the short ciliary nerves and supply the sphincter pupillae and ciliary muscle.
Short ciliary nerves, branches of ciliary ganglion, about 10 in number, pierce the sclera around the optic nerve and run forward between the sclera and choroid and reach the ciliary muscles where the sensory fibres form a plexus and supply it.
 
Sympathetic nerves
Preganglionic fibres arise from the ciliospinal center of Budge (located in inferio mediolateral cell column of spinal cord at C8, T1 and T2 level) and go to cervical sympathetic chain to relay in the superior cervical ganglion (Fig. 1.11).
Postganglionic fibres from the superior cervical ganglion enter the skull with the internal carotid plexuses and supply following structures:
Orbital arteries receive vasomotor fibres through the plexuses around ophthalmic artery.
Dilator pupillae muscle is supplied by the sympathetic fibres from the carotid plexuses which enter the Gasserian ganglion of 5th cranial nerve and ultimately reach the dilator pupillae muscle along with long ciliary branches of nasociliary nerve (branch of ophthalmic division of 5th nerve).
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Fig.1.10: Course of parasympathetic nerve supply from lacrimatory nucleus to the lacrimal gland
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Fig.1.11: Course of ocular sympathetic nerves
Blood vessels inside the eyeball are supplied by the fibres from the internal carotid plexuses which join the ciliary ganglion as its sympathetic root and without relaying here these fibres enter inside the eyeball along the short ciliary nerves. These fibres may also supply the dilator pupillae muscle when it is not supplied by the usual course via the nasociliary nerve.
Palpebral (Muller’s) muscle of the eyelid is supplied by the postganglionic fibres forming cavernous plexuses reaching the muscle through the branches of oculomotor (3rd cranial) nerve.
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Fig.1.12: Lymphatic drainage of eyelids, conjunctiva and orbital tissues
 
Lymphatic drainage
Lymphatics draining the eyelids, conjunctiva and orbital tissues are arranged in two groups (Fig. 1.12):
  • Medial group of lymphatics drain into the submandibular lymph nodes, and
  • Lateral group of lymphatics drain into the sub-auricular lymph nodes.
 
DEVELOPMENT OF EYE
Development of eyeball can be considered to commence around day 22 when the embryo has eight pairs of somites and is around 2 mm in length. Eyeball and its related structures are derived from the following primordia:
  • Optic vesicle, an outgrowth from prosencephalon (a neuroectodermal structure),
  • Lens placode, a specialized area of surface ectoderm, and the surrounding surface ectoderm,
  • Mesenchyme surrounding the optic vesicle, and
  • Visceral mesoderm of maxillary processes.
Before going into the development of individual structure, it will be helpful to understand the formation of optic vesicle, lens placode, optic cup and changes in the surrounding mesenchyme, which play a major role in the development of the eye and its related structures.
 
FORMATION OF OPTIC VESICLE AND OPTIC STALK
The area of neural plate (Fig. 1.13A) which forms the prosencephalon develops a linear thickened area on either side (Fig. 1.13B), which soon becomes depressed to form the optic sulcus (Fig. 1.13C).9
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Fig. 1.13: Formation of the optic vesicle and optic stalk
Meanwhile the neural plate gets converted into prosencephalic vesicle. As the optic sulcus deepens, the walls of the prosencephalon overlying the sulcus bulge out to form the optic vesicle (Figs. 1.13D,E and F). The proximal part of the optic vesicle becomes constricted and elongated to form the optic stalk (Figs. 1.13G and H).
 
FORMATION OF LENS VESICLE
The optic vesicle grows laterally and comes in contact with the surface ectoderm.
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Fig. 1.14: Formation of lens vesicle and optic cup
The surface ectoderm, overlying the optic vesicle becomes thickened to form the lens placode (Fig. 1.14A) which sinks below the surface and is converted into the lens vesicle (Figs. 1.14B and C). It is soon separated from the surface ectoderm at 33rd day of gestation (Fig. 1.14D).
 
FORMATION OF OPTIC CUP
The optic vesicle is converted into a double-layered optic cup. It appears from Fig. 1.15 that this has happened because the developing lens has invaginated itself into the optic vesicle. In fact conversion of the optic vesicle to the optic cup is due to differential growth of the walls of the vesicle. The margins of optic cup grow over the upper and lateral sides of the lens to enclose it. However, such a growth does not take place over the inferior part of the lens, and therefore, the walls of the cup show deficiency in this part. This deficiency extends to some distance along the inferior surface of the optic stalk and is called the choroidal or fetal fissure (Fig. 1.15).
 
CHANGES IN THE ASSOCIATED MESENCHYME
The developing neural tube (from which central nervous system develops) is surrounded by mesenchyme, which subsequently condenses to form meninges.
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Fig. 1.15: Optic cup and stalk seen from below to show the choroidal fissure
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Fig. 1.16: Developing optic cup surrounded by mesenchyme
An extension of this mesenchyme also covers the optic vesicle. Later, this mesenchyme differentiates to form a superficial fibrous layer (corresponding to dura) and a deeper vascular layer (corresponding to pia-arachnoid) (Fig. 1.16).
With the formation of optic cup, part of the inner vascular layer of mesenchyme is carried into the cup through the choroidal fissure. With the closure of this fissure, the portion of mesenchyme which has made its way into the eye is cut off from the surrounding mesenchyme and gives rise to the hyaloid system of the vessels (Fig. 1.17).
The fibrous layer of mesenchyme surrounding the anterior part of optic cup forms the cornea. The corresponding vascular layer of mesenchyme becomes the iridopupillary membrane, which in the peripheral region attaches to the anterior part of the optic cup to form the iris. The central part of this lamina is pupillary membrane which also forms the tunica vasculosa lentis (Fig. 1.17).
In the posterior part of optic cup the surrounding fibrous mesenchyme forms sclera and extraocular muscles, while the vascular layer forms the choroid and ciliary body.
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Fig. 1.17: Derivation of various structures of the eyeball
 
DEVELOPMENT OF VARIOUS OCULAR STRUCTURES
 
Retina
Retina is developed from the two walls of the optic cup, namely:
  • Nervous retina from the inner wall, and Pigment epithelium from the outer wall (Fig. 1.18).
Nervous retina. The inner wall of the optic cup is a single-layered epithelium. It divides into several layers of cells which differentiate into the following three layers (as also occurs in neural tube):
  • Matrix cell layer. Cells of this layer form the rods and cones.
  • Mantle layer. Cells of this layer form the bipolar cells, ganglion cells, other neurons of retina and the supporting tissue.
  • Marginal layer. This layer forms the ganglion cells, axons of which form the nerve fibre layer.
Pigment epithelial layer. Cells of the outer wall of the optic cup become pigmented. Its posterior part forms the pigmented epithelium of retina and the anterior part continues forward in ciliary body and iris as their anterior pigmented epithelium.
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Fig. 1.18: Development of the retina
 
Optic nerve
Optic nerve develops in the framework of optic stalk as below:
Optic nerve fibres develop from the nerve fibre layer of retina which grow into the optic stalk by passing through the choroidal fissure (by 6th week of gestation) and pass posteriorly to the brain.
Glial system of the nerve develops from the neuroectodermal cells forming the outer wall of the optic stalk. Glial septa surrounding the nerve bundles are composed of astroglia that differentiate from the cells of the inner wall of the optic stalk.
Sheaths of optic nerve are formed from the layers of mesenchyme like meninges of other parts of central nervous system.11
Myelination of nerve fibres takes place from brain distally and reaches the lamina cribrosa just before birth and stops there. In some cases, this extends up to around the optic disc and presents as congenital opaque nerve fibres. These develop after birth.
 
Crystalline lens
The crystalline lens is developed from the surface ectoderm as below:
Lens placode and lens vesicle formation (see page 9 and Fig. 1.14).
Primary lens fibres. The cells of posterior wall of lens vesicle elongate rapidly to form the primary lens fibres which obliterate the cavity of lens vesicle. The primary lens fibres are formed upto 3rd month of gestation and are preserved as the compact core of lens, known as embryonic nucleus (Fig. 1.19).
Secondary lens fibres are formed from equatorial cells of anterior epithelium which remain active throughout life. Since the secondary lens fibres are laid down concentrically, the lens on section has a laminated appearance. Depending upon the period of development, the secondary lens fibres are named as below:
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Fig. 1.19: Development of the crystalline lens
  • Fetal nucleus (3rd to 8th month),
  • Infantile nucleus (last weeks of fetal life to puberty),
  • Adult nucleus (after puberty), and
  • Cortex (superficial lens fibres of adult lens).
Lens capsule is a true basement membrane produced by the lens epithelium on its external aspect.
 
Cornea
1. Epithelium is formed from the surface ectoderm.
2. Other layers viz. endothelium, Descemet’s membrane, pre-Descemet’s membrane, stroma and Bowman’s layer are derived from the fibrous layer of mesenchyme lying anterior to the optic cup (Fig. 1.17).
 
Sclera
Sclera is developed from the fibrous layer of mesenchyme surrounding the optic cup (corresponding to dura of CNS) (Fig. 1.17).
 
Choroid
Choroid is derived from the inner vascular layer of mesenchyme that surrounds the optic cup (Fig. 1.17).
 
Ciliary body
The two layers of epithelium of ciliary body develop from the anterior part of the two layers of optic cup (neuroectodermal).
Stroma of ciliary body, ciliary muscle and blood vessels are developed from the vascular layer of mesenchyme surrounding the optic cup (Fig. 1.17).
 
Iris
  • Both layers of epithelium are derived from the marginal region of optic cup (neuroectodermal) (Fig. 1.17).
  • Sphincter and dilator pupillae muscles are derived from the anterior epithelium (neuroectodermal).
  • Stroma and blood vessels of the iris develop from the vascular mesenchyme present anterior to the optic cup.
 
Vitreous
1. Primary or primitive vitreous is mesenchymal in origin and is a vascular structure having the hyaloid system of vessels.
2. Secondary or definitive or vitreous proper is secreted by neuroectoderm of optic cup. This is an avascular structure. When this vitreous fills the cavity, primitive vitreous with hyaloid vessels is pushed anteriorly and ultimately disappears.
3. Tertiary vitreous is developed from neuroectoderm in the ciliary region and is represented by the ciliary zonules.
 
Eyelids
Eyelids are formed by reduplication of surface ectoderm above and below the cornea (Fig. 1.20). 12 The folds enlarge and their margins meet and fuse with each other. The lids cut off a space called the conjunctival sac. The folds thus formed contain some mesoderm which would form the muscles of the lid and the tarsal plate. The lids separate after the seventh month of intrauterine life.
  • Tarsal glands are formed by ingrowth of a regularrow of solid columns of ectodermal cells from the lid margins.
  • Cilia develop as epithelial buds from lid margins.
 
Conjunctiva
Conjunctiva develops from the ectoderm lining the lids and covering the globe (Fig. 1.20).
Conjunctival glands develop as growth of the basal cells of upper conjunctival fornix. Fewer glands develop from the lower fornix.
 
The lacrimal apparatus
Lacrimal gland develops from about 8 cuneiform epithelial buds which grow by the end of 2nd month of fetal life from the superolateral side of the conjunctival sac (Fig. 1.20).
Lacrimal sac, nasolacrimal duct and canaliculi develop from the ectoderm of nasolacrimal furrow which extends from the medial angle of eye to the region of developing mouth. The ectoderm gets buried to form a solid cord which is later canalized. The upper part forms the lacrimal sac. The nasolacrimal duct is derived from the lower part as it forms a secondary connection with the nasal cavity. The ectodermal buds arise from the medial margins of eyelids which are later canalized to form the canaliculi.
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Fig. 1.20: Development of the eyelids, conjunctiva and lacrimal gland
 
Extraocular muscles
All the extraocular muscles develop in a closely associated manner by mesodermally derived mesenchymal condensation. This probably corresponds to preotic myotomes, hence the triple nerve supply (III, IV and VI cranial nerves).
 
STRUCTURES DERIVED FROM THE EMBRYONIC LAYERS
Based on the above description, the various structures derived from the embryonic layers are given below:
 
1. Surface ectoderm
  • The crystalline lens
  • Epithelium of the cornea
  • Epithelium of the conjunctiva
  • Lacrimal gland
  • Epithelium of eyelids and its derivatives viz., cilia, tarsal glands and conjunctival glands
  • Epithelium lining the lacrimal apparatus.
 
2. Neural ectoderm
  • Retina with its pigment epithelium
  • Epithelial layers of ciliary body
  • Epithelial layers of iris
  • Sphincter and dilator pupillae muscles
  • Optic nerve (neuroglia and nervous elements only)
  • Melanocytes
  • Secondary vitreous
  • Ciliary zonules (tertiary vitreous).
 
3. Associated paraxial mesenchyme
  • Blood vessels of choroid, iris, ciliary vessels, central retinal artery, other vessels
  • Primary vitreous
  • Substantia propria, Descemet’s membrane and endothelium of cornea
  • The sclera
  • Stroma of iris
  • Ciliary muscle
  • Sheaths of optic nerve
  • Extraocular muscles
  • Fat, ligaments and other connective tissue structures of the orbit
  • Upper and medial walls of the orbit13
  • Connective tissue of the upper eyelid.
 
4. Visceral mesoderm of maxillary process below the eye
  • Lower and lateral walls of orbit
  • Connective tissue of the lower eyelid.
 
IMPORTANT MILESTONES IN THE DEVELOPMENT OF EYE
 
Embryonic and fetal period
Stage of growth
Development
2.6 mm (3 weeks)
Optic pits appear on either side of cephalic end of forebrain.
3.5 mm (4 weeks)
Primary optic vesicle invaginates.
5.5 to 6 mm
Development of embryonic fissure.
10 mm (6 weeks)
Retinal layers differentiate, lens vesicle formed.
20 mm (9 weeks)
Sclera, cornea and extra ocular muscles differentiate.
25 mm (10 weeks)
Lumen of optic nerve obliterated.
50 mm (3 months)
Optic tracts completed, pars ciliaris retina grows forwards, pars iridica retina grows forward and lid folds develop.
60 mm (4 months)
Hyaloid vessels atrophy, iris sphincter, dilator and ciliary muscles develop.
230-265 mm (8th month)
Fetal nucleus of lens is complete, all layers of retina nearly developed and macula starts differentiation.
265-300 mm (9th month)
Except macula, retina is fully developed, infantile nucleus of lens begins to appear, pupillary membrane and hyaloid vessels disappear. Medulation of optic nerve reaches lamina cribrosa.
 
Eye at birth
  • Anteroposterior diameter of the eyeball is about 16.5 mm (70% of adult size which is attained by 7-8 years).
  • Corneal diameter is about 10 mm. Adult size (11.7 mm) is attained by 2 years of age.
  • Anterior chamber is shallow and angle is narrow.
  • Pupil is small and does not dilate fully.
  • Lens is spherical at birth. Infantile nucleus is present.
  • Retina. Apart from macular area the retina is fully differentiated. Macula differentiates 4–6 months after birth.
  • Myelination of optic nerve fibres has reached the lamina cribrosa.
  • Refractive status. Newborn is usually hypermetropic by +2 to +3 D.
  • Orbit is more divergent (50°) as compared to adult (45°).
  • Lacrimal gland is still underdeveloped and tears are not secreted.
 
Postnatal period
  • Fixation starts developing in first month and is completed in 6 months.
  • Macula is fully developed by 4–6 months.
  • Fusional reflexes, stereopsis and accommodation are well developed by 4–6 months.
  • Cornea attains normal adult diameter by 2 years of age.
  • Lens grows throughout life.