1 Anatomy

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A. Eye

In addition to the eyeball (bulbus oculi, A), the visual organ consists of the protective structures of the eye (orbit, lids, conjunctiva, and lacrimal apparatus) and the movement apparatus consisting of the extrinsic ocular muscles and Tenon’s capsule. The optic nerve connects the sensory epithelium (the retina) with the brain. The eyeball is surrounded by the fatty tissue of the orbit.

B. Orbit

The frontal bone (orbital roof), zygomatic bone (lateral wall and floor), maxilla (floor), lacrimal bone and ethmoid bone (medial wall), and also the palatine bone and the sphenoid (blunt tip are involved in the structure of the orbit (B). The openings in the orbit are the optic canal (which contains the optic nerve), the superior and inferior orbital fissures, the infraorbital, ethmoidal, and zygomatico-orbital foramina, and the nasolacrimal canal.

C. Lids

The palpebral fissure is bounded by the upper and lower lids (palpebrae), the main structure of which is formed by a dense lid plate (tarsus). Exteriorly, the lids are covered by stratified keratinized squamous epithelium which becomes the palpebral conjunctiva at the lid margin. Lashes (ciliae) are found in 2 to 3 rows along the lid margin. The holocrine Zeis glands and the apocrine Moll glands end in the hair follicles. The excretory ducts of the larger Moll glands open close to the posterior lid margin. The eyebrow (supercilium) marks the upper border of the orbit. Blinking and closing of the lids is performed mainly by the orbicularis oculi muscle (innervated by the facial nerve). The levator palpebrae superioris (oculomotor nerve) and superior and inferior tarsal muscles (cervical sympathetic nerves) also open the lids. The sensory innervation of the upper lid is through branches of the first division of the trigeminal nerve (V1), that of the lower lid by branches of the second division (V2).

D. Conjunctiva

The conjunctiva covers the posterior surface of the upper and lower lids as the palpebral conjunctiva. It consists of two or more layers of isoprismatic – to highly prismatic epithelium. At

the upper and lower fornix it changes to the bulbar conjunctiva, which is slightly mobile where it lies over the sclera. The conjunctiva consists of stratified nonkeratinizing epithelium.

E. Lacrimal Apparatus (E)

The lacrimal gland (glandula lacrimalis) lies above the outer corner of the eye. It is a tubuloalveolar gland whose 6–12 excretory ducts end in the lateral upper conjunctival fornix. The secretory parasympathetic innervation follows the facial nerve and the sympathetic innervation is through the cervical sympathetic. The tear fluid is low in protein and of low viscosity. Through blinking, the tears reach the medial angle of the lids and the fluid drawn into the lacrimal punctum and into the lacrimal canaliculi (canaliculi lacrimales). These open into the lacrimal sac (saccus lacrimalis) and from there the tears flow out through the nasolacrimal duct into the lower nasal passage (E).

F. Motor Apparatus

The extrinsic ocular muscles (2 horizontal, 2 vertical, and 2 oblique) lie in the fat of the orbit and move the eyeball. The superior, inferior, medial, and lateral rectus muscles originate from the tendinous ring, which forms the tip of the muscle pyramid at the orbital apex, and pass over the equator of the eyeball. With the exception of the lateral rectus muscle, which is innervated by the abducent nerve (CN VI), and the superior oblique muscle, which is innervated by the trochlear nerve (CN IV), they are innervated by the oculomotor nerve (CN III). The inferior oblique muscle arises from the medial wall of the orbit. The superior oblique muscle passes from the tendinous ring initially to the medial wall of the orbit, where it changes its direction at the trochlea.

1 Anatomy

A. Blood Supply

The ophthalmic artery is a branch of the internal carotid artery and passes into the orbit with the optic nerve. It then runs forward with the superior oblique muscle and ends as the dorsal nasal artery and supratrochlear artery. Before thatitgives offthefollowingbranches:thecentral retinal artery, which travels to the retina in the optic nerve (A); short and long posterior ciliary arteries to the choroid and ciliary body; the lacrimal artery to the lacrimal gland; the supraorbital artery to the forehead; and the anterior and posterior ethmoidal arteries to the ethmoid air cells. The anterior ciliary arteries arise from the muscular branches to the extrinsic ocular muscles, which pass through the sclera to the ciliary body and iris. The superior ophthalmic vein collects the blood from the eyeball, upper orbit, lids and ethmoid air cells and drains into the cavernous sinus. The inferior ophthalmic vein arises on the floor of the orbit and flows either into the superior ophthalmic vein or into the pterygoid plexus.

B. Eyeball

The eyeball (B), bulbus oculi, has an almost spherical shape with an average diameter of 23mm. The eyeball is bounded anteriorly by the cornea. At the posterior pole, the optic nerve leaves the eye somewhat medial to the axis of the eye, and the fovea centralis-which is the site of most acute vision-is somewhat lateral to this. The circumference at the greatest transverse diameterof the eye is called the equator. The wall of the eye consists of three layers: the outer layer (tunica fibrosa) with sclera and cornea; the middle layer (tunica vasculosa) with choroid, ciliary body, and iris; and the inner layer (tunica interna) with the retina and the retinal pigment epithelium. Inside the eye a distinction is made between the anterior and posterior chambers of the eye and the vitreous space. The cornea, aqueous humor, lens, and vitreous body constitute the optic media of the eye. The lens, zonular fibers, and ciliary muscle are called the accommodation apparatus.

C. Sclera

becomes the substantia propria corneae (stroma).

D. Cornea

The cornea has a diameter of about 12mm in adults. The outside of the cornea consists of stratified nonkeratinized squamous epithelium, which changes to the epithelium of the bulbar conjunctiva at the corneal limbus. The inside is formed by a single layer of flat endothelial cells. Bowman’s membrane is situated between the epithelium and stroma and Descemet’s membrane between the endothelium and stroma (Da). The refractive power of the cornea is about 42 diopters (Db). The central thickness is approximately 500 m.

E. Lens

The lens, with a horizontal diameter of about 10mm, is situated in the posterior chamber of the eye. It is about 3–4 mm thick at the center. It is a biconvex lens, with the anterior surface less curved than the posterior surface. The lens shell, which surrounds the nucleus concentrically, lies beneath the lens capsule.

F. Vitreous Body

The vitreous body, which is 95% water, fills the vitreous space situated behind the lens. Its gelatinous consistency is due to the presence of hyaluronic acid, mucopolysaccharides, and collagen fibrils.

G. Choroid

The choroid occupies the major part of the middle layer of the eye. In addition to arteries and veins, it also carries approximately 15–20 ciliary nerves. It is separated from the retina by Bruch’s membrane, which is 2 m thick.

D. Cornea

a PAS stain, ca. 63

b Slit lamp photograph

Blood Supply and Eyeball

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1 Anatomy

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A. Ciliary Body

The ciliary body (Ba) extends from the ora serrata as far as the base of the iris and surrounds the iris like a ring. A distinction is made between the outer part, the orbiculus ciliaris with fine meridional folds where the zonular fibers arise, and the inner part, the corona ciliaris. The ciliary body is covered by a bilaminar epithelium, which is responsible for the production of aqueous humor. The anterior and posterior chambers together contain about 0.2–0.3 ml of aqueous humor, most of which drains out at the iridocornealangle.Partoftheciliarybodyisthe ciliary muscle, whose smooth muscle fibers are arranged meridionally, circularly, and radially (parasympathetic innervation via the oculomotor nerve predominates with some cervical sympathetic input). Contraction of the muscle leads to slackening of the zonular fibers and, through the associated increased curvature of the lens, to accommodation.

B. Iris and Pupil

The iris, like a diaphragm, forms the pupil. The iris has no epithelium on its anterior aspect, so that the iris stroma, which is arranged radially to the edge of the pupil, is exposed. The iris is thinnest at the margin of the pupil and allows the bilaminar pigmented epithelium on the back to be seen. The pupil is surrounded by the sphincter pupillae muscle (parasympathetic innervation via the oculomotor nerve), the innervation of which produces contraction of the pupil (miosis). At the margin of the pupil, the iris is widely connected with the ciliary body. The muscle fibers of the dilatator pupillae muscle (cervical sympathetic) run here, contraction of which leads to pupil dilatation (mydriasis). At the iridocorneal angle (Ba), the aqueous humor flows through gaps in the pectinate ligament of the iris (trabecular meshwork, Bb) into Schlemm’s canal.

C. Retina

The retina forms the inner layer of the eye. It is divided into a nonsensory part and an optic part, the boundary of which is formed by the ora serrata. The anterior part does not have any sensory epithelium and covers the ciliary body and iris as a bilaminar epithelium. The optic part consists of two layers, the outer layer (pigment layer) and the inner layer (cerebral layer),

which lie loosely on one another and are adherent only at the ora serrata and at the entrance of the optic nerve. The central retinal artery and vein unite at the entrance of the optic nerve (optic disc or papilla). The macula lutea (yellow spot) is lateral to this with the fovea centralis at its center, the site of maximum visual acuity (Ca). The pigment layer consists of a single layer of isoprismatic epithelium (retinal pigment epithelium). The inner retina includes the photoreceptor cells and nine further identifiable layers of the cerebral layer (Cb and c). They are primary sensory epithelial cells. About 120 million rods and 6–7 million cones are distinguished. There are only cones in the fovea centralis, with no other layers of the cerebral layer. The perikarya of the bipolar cells, which are the second neuron of the optic nerve, are located in the inner nuclear layer. They maintain synaptic contact with the sensory cells in the outer plexiform layer and with the multipolar ganglion cells of the ganglion layer (third neuron) in the inner plexiform layer, from where sensory impulses are conducted in unmyelinated nerve fibers to the optic disc. The horizontal and amacrine cells of the inner nuclear layer form the association apparatus of the retina through the parallel connections of several synapses.

D. Optic Nerve and Optic Tract

The optic nerve is about 45 mm in length, twothirds of which is inside the orbit. At the lamina cribrosa, ca. 1 million nerve fibers leave the eyeball and from this point are surrounded by a medullary sheath of oligodenroglia, dura mater and pia mater. After passing through the optic canal, it reaches the optic chiasm on the floor of the third ventricle after running about 10 mm in the middle cranial fossa. Here the nasal fibers of the retina cross to the opposite side. The optic nerve fibers run as the optic tract as far as the lateral geniculate body. The optic radiation (Gratiolet’s radiating fibers) runs from here through the posterior crus of the internal capsule to the primary optic visual cortex, the area striata, area 17.

Ciliary Body/Iris/Pupil/Retina/Optic Nerve

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