Chapter 46: The eye
The eye (L., oculus; Gk, ophthalmos) (fig. 46-1A and B) lies in the cavity of the orbit and measures approximately 24 mm in diameter. The midpoints of the two pupils lie about 60 mm apart.
If the eye is too short in relation to the lens, near objects are focused behind the retina (hypermetropia: farsightedness or longsightedness). In contrast, if the eye is too long in relation to the lens, distant objects are focused in front of the retina (myopia: nearsightedness or shortsightedness) (fig. 46-1C).
The development of the eye is summarized in figure 46-2. The retina, which may be regarded as an extension of the wall of the brain, develops from neural ectoderm, whereas the lens and the anterior epithelium of the cornea are derived from somatic ectoderm. Neural crest and mesoderm also participate in ocular development.
The eyeball (globe or bulb) has three concentric coverings (figs. 46-3 and 46-4): (1) an external, fibrous tunic comprising the cornea and sclera; (2) a middle, vascular tunic comprising the iris, ciliary body, and choroid; and (3) an internal, nervous tunic, or retina.
The cornea is the anterior, transparent part of the eye, and it forms about one-sixth of the circumference of the fibrous coat. Most of the refraction by the eye takes place not in the lens but at the surface of the cornea. The cornea is continuous with the conjunctiva and the junctional region is known as the limbus. The cornea is supplied by the ophthalmic nerve (from the fifth cranial nerve) by means of its ciliary branches. The eyelids close on stimulation of the cornea (corneal reflex, fig. 46-8A). The cornea is avascular and consists of five layers histologically: a largely collagenous substantia propria enclosed by anterior and posterior epithelia and limiting laminae.
When the cornea does not conform to a sphere but is more curved in one axis than in another, the condition is termed astigmatism.
Irritation of the eye, e.g., from a foreign body, causes hyperemia of the conjunctiva, which may also result from infection or allergic conditions (conjunctivitis). The posterior conjunctival arteries (from the palpebral arcades, fig. 46-10) become dilated and give a brick-red color to the conjunctiva.
Inflammation of the cornea (keratitis) or of the iris and ciliary body (iridocyclitis) causes dilation of the anterior ciliary arteries (from muscular branches of the ophthalmic, fig. 46-10), resulting in a rose-pink band of "ciliary injection". These vessels, unlike those of the conjunctiva, do not move when the conjunctiva is moved.
The sclera is the posterior, opaque part of the external tunic. Its anterior part can be seen through the conjunctiva as "the white of the eye". The sclera consists of fibrous tissue, and it receives the tendons of the muscles of the eyeball. Posteriorly, the fibers of the optic nerve pierce the sclera through a weak plate termed the lamina cribrosa (fig. 46-3).
External to the sclera, the eyeball is enveloped by a thin fascial sheath (so-called Tenon's capsule) that extends from the optic nerve to the sclerocorneal junction (fig. 46-3). The sheath separates the globe from the orbital fat and acts as a socket in which the eye moves as in a ball-and-socket joint. It blends with the sheaths of the muscles of the globe.
Hormonal disturbances (especially hyperthyroidism) may result in swelling of the orbital fat and extra-ocular muscles, causing protrusion of the eyes (exophthalmos).
An important, circular canal termed the scleral venous sinus (known to ophthalmologists as the canal of Schlemm) is situated at the sclerocorneal junction, anterior to a projection termed the scleral spur (fig. 46-5). The aqueous humor, formed by the ciliary processes, filters through intercellular channels leading from the anterior chamber to the venous sinus and drains by means of aqueous veins into scleral plexuses. The iridocorneal angle (between the iris and the cornea), also known as the angle of the anterior chamber or as the filtration angle, is very important physiologically (for the circulation of aqueous humor) and pathologically (in glaucoma).
The middle tunic, frequently termed the uvea, comprises the choroid, the ciliary body, and the iris, from posterior to anterior.
(a) The choroid is a vascular, highly pigmented coat that lines most of the sclera.
(b) The ciliary body connects the choroid with the iris. The part near the choroid is a smooth ciliary ring (pars plana), whereas that near the iris is a ridged crown (pars plicata). The ciliary body contains the ciliary muscle and the ciliary processes, and is lined by the ciliary part of the retina.
The ciliary muscle comprises two main sets of smooth-muscle fibers (fig. 46-5): (1) longitudinal fibers connect the sclera (anterior) to the choroid (posterior), and (2) oblique fibers enter the base of the ciliary processes. The ciliary muscle is supplied by parasympathetic fibers by way of the ciliary nerves (fig. 46-7). On contraction, the ciliary body moves anteriorward. This decreases the tension on the fibers of the ciliary zonule so that the central part of the lens assumes a more globular, curved shapeto permit the eye to focus on near objects, a process known as accommodation (fig. 46-8C and D).
The iris is a circular, pigmented diaphragm that lies anteiro to the lens in a more or less coronal plane (fig. 46-4). It is anchored peripherally to the ciliary body, whereas its central border is free and bounds the aperture known as the pupil. The iris divides the space between the cornea and the lens into two chambers (fig. 46-5). The anterior chamber is bounded largely by the cornea and iris. It communicates through the pupil with the posterior chamber, which is bounded by the iris, ciliary processes and zonule, and lens. Both chambers are filled with aqueous humor.
The anterior surface of the iris presents a fringe known as the collarette. The pattern of radial striations in the iris is unique from one individual to another and, like fingerprints, can be used for identification. The stroma of the iris normally contains melanin pigment, and the amount, which is low in blue eyes, is considerable in brown irides. A congenital, radial defect of the iris is termed a coloboma.
The sphincter pupillae is situated in the posterior part of the iris, near the pupil, and consists of smooth muscle. The sphincter pupillae is supplied by parasympathetic fibers by way of the short ciliary nerves, and its contraction results in constriction of the pupil (miosis) (fig. 46-7). The iris contracts reflexly when light reaches the retina (the light reflex) and when focusing on a near object (part of the accommodation reaction) (fig. 46-8B-D). A drop of an atropine-like drug placed on the eye annuls the action of the ciliary muscle and the sphincter pupillae, both of which are under parasympathetic control. The resultant dilatation of the pupil (caused by overaction of the dilator) is of use in the examination of the eye.
The dilator pupillae consists of smooth muscle anterior to the
pigmented epithelium on the posterior aspect of the iris, which
iridial part of the retina. The dilator pupillae is supplied by
and its contraction results in dilatation of the pupil (mydriasis)
This sympathetic innervation arises as preganglionic nerve fibers
leaving the spinal cord in the upper 4 thoracic ventral roots. White
rami communicans transmit the sympathetics to the gangliated chain and
the preganglionic fibers enter and ascend the cervical sympathetic
chain. These sympathetic preganglionic fibers synapse in the superior
cervical ganglion. Postganglionic sympathetic nerve fibers originating
from this ganglion join the carotid artery and comprise a dense plexus
of nerves surrounding the branches of this artery. The nerve fibers
follow the internal carotid and ophthalmic arteries to reach the eye.
Damage to sympathetic nerve fibers anywhere along this pathway can
result in Horner syndrome, with a small (meiotic) pupil and slight
drooping of the upper eyelid due to paralysis of the superior tarsal
The autonomic innervation of the eye (fig. 46-7) may be summarized in the following manner.
Parasympathetic (synapses in ciliary ganglion) - sphincter pupillae, ciliary muscle.
Sympathetic (synapses in superior cervical ganglion) - dilator pupillae, orbitalis (smooth muscle of inferior orbital fissure), superior tarsal muscle (smooth muscle in eyelid), blood vessels of choroid and retina.
The retina contains special receptors on which is projected an inverted image of objects seen. Because of the partial crossing of nerve fibers at the optic chiasma, the retina of each eye is connected with both right and left visual areas of the forebrain. The retina is shaped like a sphere that has had its anterior segment removed, leaving an irregular margin termed the ora serrata (fig. 46-3). The sensory elements of the retina end at the ora, but a pigmented continuation lines the ciliary body and the posterior part of the iris as the ciliary and iridial parts of the retina (fig. 46-5). In other words, the ciliary body and the posterior iris are lined by retinal epithelium (a double layer), which, however, is insensitive to light.
Basically, the retina comprises two main strata: (1) an external, pigmented stratum derived from the external lamina of the embryonic optic cup and (2) an internal, transparent, nervous stratum derived from the inverted lamina of the optic cup (fig. 46-2). A separation of the nervous from the pigmented stratum may occur along a plane that represents the residual cavity of the embryonic optic vesicle. This is commonly called detachment of the retina, and it may arise from an accumulation of fluid caused by a hole or a tear in the retina. Methods for repair include the use of a cryoprobe or a laser to produce an adhesive scar between these layers, preventing further separation.
The macula is a small, yellowish area of the retina on the temporal side of the optic disc (fig. 46-9A). It contains a pit, the fovea centralis, which in turn presents a depression, the foveola. The foveola contains only cone photoreceptor cells and functions in detailed color vision, when an object is looked at directly.
The entering optic nerve fibers form the optic disc. This is the "blind spot", insensitive to light because photoreceptor cells are absent there. It is situated nasal to the posterior pole of the eye and to the fovea centralis. Normally the optic disc is flat and does not form a papilla, but, near its center, where vessels enter and leave, a variable depression, the "physiological cup", is present.
The optic nerve is surrounded by meningeal sheaths and the subarachnoid space (fig. 46-3), so that an abnormal rise in intracranial pressure (e.g., caused by an intracranial tumor or hemorrhage) also places pressure on the optic nerve. This may result in a hydrostatic phenomenon that can be detected by ophthalmoscopy as a blurring of the margins of the optic disc ("choked disc" or papilledema) (fig. 46-9C) and loss of the physiological cup. Compression of the central vein of the retina, which courses inside of the optic nerve, may be a factor in the production of this swelling of the optic nerve head.
The retina is nourished externally by the choroid and internally by the central artery of the retina, a branch of the ophthalmic artery. The central artery travels in the optic nerve and divides at the optic disc. The branches of the central artery of the retina are endarteries, so that an occlusion causes loss of vision in the corresponding part of the visual field.
The fundus oculi is the posterior part of the interior of the eye as seen on ophthalmoscopy (fig. 46-9).
The refractive apparatus of the eye are collectively termed the dioptric media and consist of the cornea (which contributes most of the optical power), aqueous humor, lens, and vitreous body.
The aqueous humor, formed by the ciliary processes, circulates through the posterior chamber, pupil, anterior chamber, iridocorneal angle, trabecular meshwork, and scleral venous sinus, thereby reaching the ciliary veins. The intra-ocular pressure depends chiefly on the ease of drainage of the aqueous humor. The scleral venous sinus (known to ophthalmologists as the canal of Schlemm) is an annular, endothelial channel at the sclerocorneal junction.
Glaucoma is a disorder generally (although not always) characterized by increased intra-ocular pressure. In the angle-closure (narrow-angle) type the iris blocks either the trabecular meshwork or the pupil, thereby hindering drainage of aqueous humor to the scleral venous sinus. In the open-angle type no grossly visible obstruction is seen, but abnormalities within the trabecular meshwork, for example, may be present. As a result of pressure, excavation (cupping) of the optic disc may occur, as well as a diminution of the visual field. In one type of operation, a small segment of the iris is excised (peripheral iridectomy), thereby re-establishing adequate humoral communication between the posterior and anterior chambers.
The lens, biconvex and 1 cm in diameter, is covered by a capsule and consists of cellular lens fibers. The lens capsule is anchored to the ciliary body by its suspensory ligaments, or ciliary zonule (figs. 46-4 and 46-5). When distant objects are being looked at, the ciliary muscle is relaxed and elastic fibers in the choroid pull on the ciliary body, which, in turn, keeps the zonular fibers and also the lens capsule under tension. This pul results in flattening of the lens (fig. 46-8D).
The lens, in addition to becoming increasingly yellow with age, also becomes harder and less elastic, as a result of which the power of accommodation is lessened (presbyopia) and convex spectacles may be required for reading.
An opacity of the lens is termed a cataract. It is commonly age-related and it may interfere with vision. The lens can be removed by either intracapsular extraction (removal of the entire lens and its capsule) or extracapsular extraction (retaining the posterior part of the capsule and the zonule to support a plastic lens implanted in the posterior chamber). "Couching" for cataract, i.e., a mere displacement of the lens by a needle introduced into the eye, is one of the oldest of surgical operations (it was performed in Roman times).
The vitreous body is a transparent, gelatinous mass that fills the eyeball posterior to the lens. The movement of specks in the vitreous body is sometimes seen as muscae volitantes (L., flitting flies), or "floaters".
Sensory fibers from the cornea and uvea reach the nasociliary nerve (of the ophthalmic nerve) by way of the short and long ciliary nerves. The eye receives its blood supply (fig. 46-10) from the ophthalmic artery by way of the central artery of the retina, short and long posterior ciliary arteries, and the anterior ciliary arteries (from muscular branches of the ophthalmic artery). Most of the veins from the eye accompany the arteries and drain into the cavernous sinus by way of the ophthalmic veins.
Adler's Physiology of the Eye. Clinical Applications, 8th ed., ed. by R. A. Moses, Mosby, St. Louis, 1987. A good text on functional aspects.
Waring, G. O. and O'Rahilly, R. A laboratory exercise in the study of the gross structure of the eye. elin. Anat. 9: 46-49, 1996. An informative dissection of the bovine eye based on procedures used in ophthalmic surgery.
Duke-Elder, S., and Wybar, K. c., The Anatomy of the Visual System, vol. 2 of System of Ophthalmology. Ed. by S. Duke-Elder, Mosby, St. Louis, 1961. An excellent work of reference for the orbit and the eye.
Wolff's Anatomy of the Eye and Orbit, 8th ed., revised by A. J. Bron, R. C. Tripathi, and B. J. Tripathi. Chapman & Hall, London, 1997. An attractive, wellillustrated text.
Remington, L. Clinical Anatomy of the Visual System, 1st ed. Butterworth-Heinemann, Boston, 1998. A recent, well-illustrated textbook including descriptions of pupillary and visual pathways.
46-1. Where is most of the optical power of the eye concentrated?
46-2. Is the cornea covered by conjunctiva?
46-3. Where does the retina terminate anteriorly?
46-4. Into which channel does the aqueous humor in the anterior chamber drain?
46-5. When "drops" are used in the examination of the eye, which muscles are removed from action?
46-6. Where do the posterior and anterior chambers communicate?
46-7. Which disease is generally characterized by increased intra-ocular pressure?
46-8. In obstruction of the aqueous pathway, what effect would excision of a portion of the iris have?
46-9. Which muscle and nerve are involved in accommodation?
46-10. What is the autonomic innervation of the muscles of the eye?
46-11. Where does detachment of the retina occur?
46-12. On which side of the optic disc is the macula?
46-13. What is the name for an opacity of the lens?
Figure 46-1. Optical comparison of (A) the eye and (B) a miniature camera. Each has two refractive components: (1) an anterior (cornea and aqueous humor) and (2) a posterior (the lens of the eye), separated by an iris diaphragm. The lens systems have a focal length of 2 cm and 5 cm, respectively, and an aperture range of f2.5 to f11 and f2 to f22, respectively. The visual image, however, is not imprinted as on a film, but is coded and transmitted more as in television. C shows common refractive errors and their correction.
Figure 46-2. Summary of the early development of the eye. A, an embryo of 4 weeks showing the optic vesicle. B, a section through the head including the diencephalon (Di) and both optic vesicles. C, the optic vesicle presents retinal and lens discs. D, the retinal and lens discs have become indented to form the optic cup and the lens pit respectively. E, the lens pit has become closed from the surface to form the lens vesicle. F, the optic stalk (future optic nerve) and cup, showing the retinal fissure and the lens. G, section of the eye at the end of the embryonic period (8 postfertilization weeks). The pigmented and inverted strata of the retina are evident. The rim of the optic cup will give rise to the ciliary and iridial parts of the retina. H, the postnatal retina comprises the pigmented stratum (layer 1) and the nervous stratum (layers 2-10 of the retina). The broad, white arrow shows the direction of the light, which reaches as far as the pigmented stratum. The neural pathway, which proceeds in the reverse order, includes (1) photo receptors [cones and rods], (2) bipolar cells, and (3) ganglion cells, which give rise to the fibers of the optic nerve. Modified from R. O'Rahilly and F. Muller, Human Embryology and Teratology, Wiley-Liss, New York, second edition, 1996.
Figure 46-3. Schematic horizontal section of right eye showing axes, fovea centralis (receiving the visual axis), optic disc, central vein of retina, lamina cribrosa sclerae (interrupted lines) and fascial sheath (in blue).
Figure 46-4. Meridional section of the eye showing the three tunics. The trabecular meshwork lies between the iridocorneal angle and the scleral venous sinus. The ciliary region and the iridocorneal angle are shown in greater detail in figure 46-5. The structure of the eyelids has been illustrated already in figure 46-10.
Figure 46-5. Meridional section of the ciliary region, showing the iridocorneal angle. The vertical schemes at the left are to clarify the terminology and layers of the retina and ciliary body. The inset sketch shows (by arrows) the formation (in the ciliary processes) and circulation of the aqueous humor through the posterior (p) and anterior (a) chambers, and drainage through the trabecular meshwork to the sinus venosus sclerae and thence to the ciliary veins.
Figure 46-6. Photographs of the eye in vivo. From Medical Radiography and Photography, 1946, 28: 123, courtesy of Mr. H. L. Gibson, Eastman Kodak Company.
Figure 46-7. The anterior half of the right eye seen from posterior. The ciliary ring is that portion of the ciliary body situated between the ora serrata and the ciliary processes. Based on Wolff.
Figure 46-8. Reflex pathways. A, the corneal reflex. Touching the cornea lightly results in bilateral blinking. The afferent limb is the ophthalmic nerve. The efferent limb is the facial nerve, which supplies the orbicularis oculi. B, the light reflex. The stimulus provided by light results in miosis (pupillary constriction). The afferent pathway is the optic nerve and tract, from the retina to the pretectal region of the midbrain. The efferent pathway is in the oculomotor nerve: parasympathetic fibers from the accessory oculomotor nucleus, synapsing in the ciliary ganglion, and supplying the sphincter pupillae. Because of contralateral connections, exposure of only one eye to light causes constriction of both pupils (consensual light reflex). E-W, accessory oculomotor nucleus of Edinger-Westphal. C, the accommodation reaction. The afferent pathway extends from the retina to the visual cortex, and then to the pretectal region. The efferent pathway, similar to that of the light reflex, ends in the ciliary muscle (accommodation for focusing on near objects), sphincter pupillae (miosis), and medial rectus (convergence of the eyes). D, the change in the lens (thickening) that occurs during accommodation. The pull of the ciliary muscle relaxes the zonular fibers and allows the lens to become more convex. Small pupils that react poorly to light, although the accommodation reaction is present, may be found in neurosyphilis. The origin of this Argyll Robertson Pupil is unclear, although a lesion in the pretectum has been suggested.
Figure 46-9. Right fundus oculi in vivo. The optic disc is on the right side of each photograph (i.e., medially). A, the normal appearance. The whitish component of the optic disc is produced by the lamina cribrosa. The lateral border of the disc is sharper than the medial. The retinal vessels radiate from the disc. The arteries may show a light streak along their middle; the veins are darker and wider. The central vein is lateral to the central artery at the disc. The macula, situated lateral (on the left side of the photograph) to the optic disc, appears as a dark oval area that contains the fovea and foveola. B, "cupping"of the optic disc found in glaucoma. C. the "choked disc" of papilledema. This edema at the disc is a hydrostatic phenomenon that can result from any rise in intracranial pressure.
Figure 46-10. The blood supply of the eye. The short posterior ciliary arteries give rise to numerous capillaries (the choriocapillaris, not shown here) that supply the external part of the retina. The internal part is supplied by the branches of the central artery of the retina, which do not anastomose with each other. At the anterioraspect of the eye, the conjunctival vessels become dilated in conjunctivitis, whereas the anterior ciliary vessels become dilated in inflammation of the cornea, iris, or ciliary body. m, marginal arcade, and p, peripheral arcade of eyelid. S.V., scleral venous sinus, the canal described by Schlemm. It transmits aqueous humor to the ciliary veins.