Ординатура / Офтальмология / Английские материалы / Textbook of Visual Science and Clinical Optometry_Bhattacharya_2009
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Anatomy of the Eyeball 11
Fig. 1-5: Structures of the angle of the anterior chamber and the anterior chamber
through these openings tortuous canals (spaces of Fontana) exist between the anterior chamber and the Schlemm’s canal.
JUXTACANALICULAR TISSUE
It lies between the deeper part of the trabecular meshwork and the Schlemm’s canal.
SCHLEMM’S CANAL
It is a circular venous sinus and plays significant role in the drainage of the aqueous humour. It is lined by a continuous layer of endothelial cells joined by junctions which are not truly tight. Only 1% of aqueous drains through these tight junctions. Villi from the cytoplasm of the endothelial cells projects into both juxtacanalicular tissue and the Schlemm’s canal. Macropinocytic vesicles and micropinocytic vesicles presents in the cytoplasm act as the major outflow pathway.
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COLLECTOR CHANNELS
Twenty-five to thirty-five collector channels drain aqueous from the outer wall of the Schlemm’s canal to the anterior ciliary veins via the intrascleral and episcleral plexuses. Aqueous vein also drains aqueous directly into the anterior ciliary veins.
POSTERIOR CHAMBER
•It is also a space filled with aqueous humour
•Aqueous humour is secreted here by the ciliary processes
•Volume of posterior chamber — 0.06 cc.
•It is bounded in front by the posterior surface of the iris and anterior surface of the lens and zonules of Zinn from behind. It is bounded laterally by the ciliary processes of the ciliary body.
UVEAL TRACT
This is the intermediate vascular coat of the eyeball consisting of the three following parts; Iris, Ciliary Body and Choroid.
IRIS
It is the most anterior part lying in front of the crystalline lens and behind the cornea. It is circular in shape with a central opening called pupil (like a diaphragm of a camera). It is peripherally attached to the middle the anterior surface of the ciliary body. Anterior surface of the iris is divided by a ridge called collarette (thickest part) into smaller pupillary zone and larger ciliary zone (Fig. 1-6). The collarette is formed by roughly circular series of ridges and minor arterial circle of iris. Major arterial circle which supplies blood to the iris is located in the ciliary body adjacent to the root of the iris (Fig. 1-7). The peculiarity of the iris vessels is that, they usually do not bleed when the iris is cut. This is due to the fact that they are enclosed by thick collagen bundles.
Histology
It consist of 4 layers:
a.Anterior endothelium—It is continuous with the corneal endothelium. Iris crypts of Fuchs are pit-like depressions (Fig. 1-6)
Anatomy of the Eyeball 13
Fig. 1-6: Anterior surface of the iris
Fig. 1-7: Location of arterial circles and unstriated muscles of the iris
caused by the focal absence of anterior endothelium. These crypts are present near collarette and the root of the iris. Iris crypts located peripherally are difficult to visualise without gonioscopy.
b.Stroma— The sphincter pupillae muscle, vessels and nerves of the iris and pigment cells are embedded in the connective tissue of the stroma. Sphincter pupillae muscle is, unstriated and
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involuntary, 1 mm wide, present near the posterior surface in
a circular fashion (Figs 1-5 and 1-7).
c.Posterior membrane—It consists of a thin layer of unstriated and involuntary muscle fibres, called dilator pupillae. Dilator pupillae is arranged radially near the root of the iris (Fig. 1-7). Dilator pupillae fibres originate from the ciliary body and fuse with the sphincter pupillae muscle (Fig. 1-5). Contraction of the dilator pupillae draws the pupillary margin towards the ciliary body to cause dilation of the pupil.
d.Posterior epithelium—It consists of two layers of pigmented epithelium,bothofwhichhaveoriginateddevelopmentallyfrom the retina. They are continuous with each other at the pupillary margin. Anterior layer of epithelium contains flat spindle cells whereas, the posterior layer contains large cubical cells.
Function of the Iris
The central opening of the iris, i.e. pupil regulates the entry of light into the eye through the actions of the dilator pupillae and the sphincter pupillae muscles.
Blood Supply
It is from the minor and major arterial circle of the iris.
Nerves Supply
a.Sphincter pupillae—Oculomotor (IIIrd cranial) nerve
b.Dilator pupillae—Nerves from the cervical sympathetic
c.Sensory—Nasociliary nerve [a branch of 1st division of the trigeminal (Vth cranial) nerve].
CILIARY BODY
It is the intermediate part of the uveal tract. It extends from ora serrata to the root of the iris, where it is attached to the scleral spur. It is a circular band width of which is 5.9 mm nasally and 6.7 mm temporarily. It is divided into two anatomical parts:
a.Pars plicata: Anteriorly about 70 ridges (ciliary processes) are arranged in a radiating manner. The region of the ciliary
Anatomy of the Eyeball 15
processes is the most vascular area of the eye. The ciliary processes are actively involved in the secretion of the aqueous humour. Width of the pars plicata is 2 mm.
b.Pars plana: This part is smooth and extends upto ora serrata. During the operations of vitrectomy and lensectomy, ports are made in this area due to relative avascularity and it’s location away from the crystalline lens.
It is roughly triangular in sagittal section with the base facing anteriorly. The external side of the triangle is against the sclera and here lies ciliary muscle which is unstriated and involuntary. Ciliary muscles form the chief mass of the ciliary body. It has three parts and a common ring shaped origin mostly from the scleral spur and partly from the trabecular meshwork (TM). The parts of the ciliary muscle are as follows:
a.Meridional (or Brucke’s muscle): It runs anteroposteriorly to be inserted into the suprachoroid and forms the main mass of the ciliary body (Fig. 1-5).
b.Radial: They are embedded in the meridional fibres and inserted into the root of the iris close to the dilator pupillae muscle.
c.Circular (or Müller’s muscle): They remain in the anterior and inner portion of the ciliary body and run parallel to the limbus to form a concentric ring (Fig. 1-5).
Ciliary muscle is innervated by the parasympathetic fibres derived from the oculomotor (IIIrd cranial) nerve and sympathetic fibres through the short ciliary nerves. Stimulation of the parasympathetic nerve causes contraction of the ciliary muscle resulting in shortening of it’s length. Thus, the whole muscle moves forward and inward. Consequently the zonule of Zinn (or suspensory ligament of lens), which suspends the lens, relaxes. This results in release of tension on the capsule of the lens, allowing it to become more convex. This is the basis of accommodation. Contraction of this muscle also opens up trabecular meshwork openings and facilitate aqueous humour outflow.
Inner surface of the ciliary body is covered by layers of epithelium, and are continuous with the similar layers of the iris. The outer layer consisting of flattened cells is continuous with the anterior pigment epithelium of iris and is also pigmented. The inner layer which consists of cubical cells, is continuous with the
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posterior pigment epithelium of iris. However, this layer is nonpigmented.
Functions of the Ciliary Body
a.Production of aqueous humour
b.It is involved in accommodation:
c.Increase outflow of aqueous through the trabecular meshwork:
d.Inner nonpigmented layer of the epithelium secretes hyaluronic acid, the essential component of the vitreous humour.
Blood Supply
It is supplied by two long posterior ciliary arteries and seven anterior ciliary arteries via major arterial circle of iris.
Nerve Supply
i.Ciliary muscle— It is supplied by the oculomotor (IIIrd cranial) nerve and the sympathetic nerve.
ii.Sensory— Nasociliary branch of the trigeminal (Vth cranial) nerve.
Anatomy of the Eyeball 17
CHOROID
It is a highly vascular thin tunic located between the sclera and the retina and extends from ora serrata to optic nerve. It consists of four layers from outside inwards (Fig. 1-8):
a.Suprachoroid (Lamina fusca)
b.Layer of blood vessels
i.Outer larger vessel layer (Haller’s layer)
ii.Inner smaller vessel layer (Sattler’s layer).
c.Choriocapillaries— It is a layer of capillary plexus of fenestrated vessels and it nourishes outer half of the retina.
d.Membrane of Bruch—It is avascular, separating choriocapillaries from the pigment epithelium of the retina. It consists of outer lamina elastica and inner lamina vitrea, i.e. basement membrane of the pigment epithelium of the retina. It is an important constituent of the blood retinal barrier.
Fig. 1-8: Layers of choroid
Functions of the Choroid
a.It provides blood supply and nutrition to the retinal pigment epithelium (RPE) and outer half of the sensory retina.
b.It regulates ocular temperature.
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Arterial Supply
it is supplied by the following group of arteries:
i.Short posterior ciliary arteries (20 in number)
ii.Long posterior ciliary arteries (2 in number)
iii.Anterior ciliary arteries (7 in number).
Venous Drainage
Venous blood from the iris, ciliary body and choroids is collected by a series of intermediate small veins, which drain into vortex veins (usually 4 in number). The vortex veins are located behind the equator of the eyeball. The vortex veins drain into cavernous sinus through superior and inferior ophthalmic veins.
The walls of the choriocapillaries are fenestrated which allow relatively free movement of fluids and solids between the choroids and the adjacent retinal pigment epithelium (RPE) via the Bruch’s membrane. The Bruch’s membrane offers no resistance to the fluid traffic.
RETINA
•It is the light receptive inner neural coat of the eyeball consisting of outer retinal pigment epithelium (RPE) and inner sensory retina with a potential space called subretinal space between them.
•It lies between the choroid and the vitreous.
•It extends from the optic disc to the ora serrata.
•Point of importance must be noted that the two pigment epithelium layers of the iris may be traced back upto retina. The anterior pigment epithelium of the iris continues as the outer pigment epithelium of the ciliary body and later forms the retinal pigment epithelium (RPE). The posterior pigment epithelium layer of the iris similarly continues to become the inner nonpigmented epithelium of the ciliary body. This again continues to form the inner nonpigmented sensory layer of the retina (Fig. 1-9).
•Surface area of the retina— 266 mm2
Anatomy of the Eyeball 19
•Thickness at the ora serrata – 125 μ Thickest at the macula – 350 μ
Thinnest at the centre of the fovea – 90 μ
Fig. 1-9: Continuation of the retina to epithelial layers of the ciliary body and iris
Fig. 1-10: Layers of the retina
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LAYERS OF RETINA
Retina consists of 10 distinct layers (Fig. 1-10) from outside inwards of the following layers:
a.Retinal pigment epithelium (RPE): It is a single layer of flattened, mostly hexagonal cells which is firmly adherent to lamina vitrea of the choroid. On direct ophthalmoscopy the retina exhibits fine mottled appearance due to the following facts:
i.The RPE cells are not equally pigmented.
ii.Pigments in each RPE cells are distributed at the periphery
of the cells and the central nuclear area remains relatively pigment free.
The taller and narrower pigment cells at the macula confer darker colour to this region. RPE cells transport substances to the photoreceptor cells which are needed for metabolism. Free exchange of products of metabolism occur between the RPE cells and the photoreceptor cells.
b.Layer of the rods and cones: Rods and cones with their nucleus and processes form the sensory receptor. They are arranged on the external limiting membrane like a palisade. The rods contain visual purple called rhodopsin which combines vitamin A with protein. Rhodopsin is responsible for vision in dim light (scotopic vision) and peripheral vision. The cones are responsible for acuity of vision, vision in strong light (photopic vision) and colour vision.
No. of rods— 125 million No. of cones— 7 million Each photoreceptor, i.e. rod and cone consist of 3 parts;
i.Outer and inner segment connected by a tubular connection called cilium in the layer of the rods and cones.
ii.Cell body and nucleus in the outer nuclear layer.
iii.Cell processes, i.e. axons that extend into the outer plexiform
layer.
Rod outer segment, cylindrical in shape, contains dense vertical stack of numerous lamellar discs. The inner segment of the photoreceptors consist of outer ellipsoid (containing large number of mitochondrias) and an inner myoid portion (containing endoplasmic reticulum). The cone outer segment is conical in shape. New rod discs are produced in the inner