Ординатура / Офтальмология / Английские материалы / Basic Sciences in Ophthalmology_Velayutham_2009

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The Eyeball

The eyeballs, which are responsible for vision the most precious sensation, is well protected by the bony orbit. The eyeballs are situated in the anterior part of the orbit closer to the roof and lateral wall than the floor and the medial wall. When a line is drawn from the superior orbital margin down to the inferior margin it will just touch or will be very close to the cornea. But on the lateral side the eyes are exposed. Hence, any injury to the eye is more common from the lateral side causing rupture globe on the upper medial part of the eye. Beside the orbit, the lids also give protection to the eyeballs.

The eyeball is not a true sphere. The anterior part, which is formed by the cornea is more curved with a radius of curvature of 8 mm or even less. The posterior part formed by the sclera is a bigger sphere of 12 mm radius. Since the cornea is more curved the anteroposterior diameter is around 24 mm while the horizontal diameter is about 23.5 mm. The vertical diameter is still less (23 mm).

The eyeball has three layers: The outer tough sclera with the cornea anteriorly, the vascular choroid forms the middle layer and the neurosensory retina forms the inner most layer.

Now let us see the different parts of the eye individually (Fig. 1.1).

Fig. 1.1: Parts of an eyeball

Anatomy of the Eyelids and Adnexa

The eyelids are highly mobile folds of skin, which are very essential for the protection of eyes. The lids protect the eyes from injuries as well as control the amount of light entering the eyes. Only when the eyes are closed the visual cortex can take rest.

The lids spread the tear fluid uniformly over the eyeball by blinking there by keeping the surface of the eye moist. The excessive secretion of tears is also pumped out of the palpebral fissure by the lids. Correct eyelid position and function are needed for spreading the tear fluid uniformly over the conjunctiva and the cornea.

Embryology

The lids develop from the mesenchymal condensations above and below the optic cup (the frontonasal and maxillary processes). The skin develops from the surface ectoderm. Differentiation begins in the 4th to 5th week of gestation. It starts at the lateral canthus and then elongates medially. By the 8th week both the lids are seen. The palpebral fissure is initially round. The mesenchyme from the condensations forms the tarsus and the orbital septum while that from the second visceral arch form the nerve fibres and blood vessels. By the 10th week the lids begin to fuse from the edges. By the 12th week the lids are fused together protecting the developing cornea from the amniotic fluid. The cilia also develop at about this time while the hair follicles develop a little later. The meibomian glands and the glands of Zeis and Moll develop from the epithelial cells. Separation of eyelid margins start at the anterior margin around the fifth month. This is completed by 7th or 8th month. The palpebral muscles develop from the mesoderm.

Superficial Anatomy (Fig. 1.2)

The contour of the eyelids is different in different races but usually the lateral canthus is 2 mm higher than the medial. The highest point of the upper eyelid is just medial to the centre of the lid. The lid covers about 2 mm of the upper part of the cornea. The lower eyelid lies just below the cornea and rises just a little when the eyes are closed. The upper eyelid extends up to the eyebrow while there is no clear-cut demarcation from the lower lid. Medially the nasojugal sulcus and laterally the malar sulcus are present in the skin. At the sulci the skin is adherent to the periosteum thereby limiting any effusion.

The Canthi

The lateral canthus has an acute angle and there is a groove, which passes downward from the lateral canthus which lies 1 cm from the fronto-zygomatic suture.

Fig. 1.2: Superficial anatomy

The lower margin of the medial canthus is horizontal while the upper margin is directed slightly upwards. A small area called the lacus lacrimalis separates the medial canthus from the globe. This gap is filled by a yellowish lacrimal caruncle and a reddish semilunar fold called the plica semilunaris. It is the rudimentary form of nictitating membrane, and it contains sympathetic muscle fibres. As the caruncle is modified skin, sebacious glands and hair follicles will be present in the caruncle. Closely opposed to the plica is an elevation called the lacrimal papilla near the medial end of the lids in which the lacrimal puncta are situated. This demarcates the ciliary portion of the lid from the lacrimal portion.

The palpebral fissure measures 30 mm horizontally and 15 mm vertically. When the eyes are shut the upper lid moves down and covers the eyeball but when the eyes are partially closed the part of the cornea just below the centre is exposed. This is the reason for exposure keratitis and injuries occurring in this area.

The Structure of the Lids (Fig. 1.3)

The margin of the lid is 2 mm broad. The anterior border is rounded and has two or three rows of eyelashes. The upper eyelid has more number of lashes, which turn upwards while lower lashes turn downwards. It takes about ten weeks for the lashes to grow and replace lost ones.

The posterior margin is sharp and is closely opposed to the eyeball. Between the orifices of the meibomian glands and the eyelashes is the grey line, which separates the anterior part of the lid from the posterior.

Fig. 1.3: Structure of the lids

The anterior most structure is the skin followed by a layer of subcutaneous areolar tissue, the orbicularis oculi, submuscular areolar tissue, the tarsal plate, a layer of nonstriated muscle and finally the conjunctiva.

The skin is very delicate and because of this during old age it develops many folds and even overhangs the lateral part of the upper lid. A furrow is formed in the upper lid by the attachment of the levator palpebrae superioris. The nasal portion of the lids is smoother and contains less number of hairs. The hairs on the lids are smoother and have small sebaceous glands. The sweat glands are also of small size. Besides this light pigmented cells are present in the eyelid skin which can wander and change the color of the skin in the same individual. The mucocutaneous junction lies behind the openings of the meibomian glands.

The subcutaneous areolar tissue has loose connective tissue and this can easily accommodate large amount of fluids and blood. At the medial and lateral ends the skin is adherent to the ligaments underneath.

Muscular Layer

The fibers of orbicularis palpebrarum supplied by the facial nerve are circularly arranged around the palpebral aperture. The fibers overlap as in a dove’s tail. The ciliary part of the muscle occupies the whole of the lid margin and is called the muscle of Riolan.

The submuscular areolar tissue occupies the area between the orbicularis and the tarsal plate. The main nerves to the eyelids lie in this space. Hence,

lacrimal crest behind the Horner’s muscle, lacrimal sac and medial palpebral ligament. It lies in front of the medial check ligament. It then gets attached to the anterior lacrimal crest at the level of the lacrimal tubercle. Inferiorly this the septum is attached to the orbital margin. The inferior medial palpebral artery runs in between the caruncle and the Horner’s muscle.

The septum is pierced by the following structures:

a.the lacrimal vessels and nerves

b.the supraorbital vessels and nerves

c. the supratrochlear nerve and artery

d.the infratrochlear nerve

e.the anastomosis between the angular and ophthalmic veins

f.the superior and inferior palpebral arteries

g.The levator palpebrae superioris in the upper lid and in the lower by the prolongation of the inferior rectus.

The nonstriated muscle fibers lie deep to the septum orbitale in both the

lids. The fibers of these muscles arise from the levator in the upper lid and prolongation of the inferior rectus in the lower.

THE GLANDS OF THE LIDS

The Meibomian Glands

These are otherwise called the tarsal glands. These are sebaceous glands without hair follicles. They are arranged vertically in a parallel row 25 in the upper lid and 20 in the lower lid. They secrete the oily layer of the tear film. It is an acinar gland arranged around a central tubule.

Ciliary glands of Moll: Spiral tubules of sweat glands lie close to the bulbs of the cilia. This is different from the glomerulus type present in the sweat glands. They open separately or into the glands of Zeis.

The sebaceous glands of Zeis: Two modified sebaceous glands are present for each eyelash. Unlike other hair follicles there is no erector pilorum muscle.

Blood supply: The medial and lateral branches of the ophthalmic artery and lacrimal arteries supply the lids. The two palpebral arteries join together to form the tarsal arches in the submuscular plane close to the lid margin. In the upper lid there is another arch formed by the medial palpebral artery in front of the upper margin of the tarsal plate.

The lids are supplied by blood from two sources. The external carotid through the facial, superficial temporal and infraorbital branches and the internal carotid through branches of ophthalmic artery such as dorsal, nasal, frontal, supraorbital and lacrimal arteries.

The veins form a dense plexus in the fornices and drain into the veins of the forehead and temple or into the ophthalmic vein.

Lymphatics

The lymphatics in front of the tarsus drain into the skin whereas those of the structures behind drain into the conjunctiva. The lateral lymphatics drain into the preauricular and parotid nodes whereas those from the medial drain into the submandibular lymph glands.

Nerve Supply

The medial part of the eyelid is supplied by the supra-trochlear and infratrochlear nerves while the lateral side is supplied by the lacrimal branches of the ophthalmic division of the trigeminal nerve. The lower lid is supplied by the infraorbital nerve.

Development the cartilages, fat and connective tissue develop from the frontonasal and maxillary processes formed by the neural crest cells.

MUSCLES PRESENT IN THE LID

Orbicularis Oculi (Fig. 1.4)

This muscle forms an elliptical sheet surrounding the palpebral fissure and spreads a little over the cheek, forehead and also temporally.

Orbicularis is the sphincter muscle of the eyelids. The palpebral part closes the lids without effort and is used for blinking to prevent drying of the cornea. It also holds the lid in contact with the globe. The orbital part is used to close the lids tightly. The palpebral portion is opposed by the levator palpebrae while the orbital portion is opposed by the frontalis. Eyelid closure during sleep involves active tonus of orbicularis oculi and inhibition of levator palpebrae superioris.

Fig. 1.4: Orbicularis oculi

The palpebral portion is divided into pretarsal and preseptal fibers. The junction between the two, forms furrows in the upper and lower lid. The fibers meet laterally in the lateral palpebral raphe. It is adherent to the dermis at the medial and lateral canthi.

The orbital portion originates from the medial side of the orbit medial to the supraorbital notch, from the maxillary process of the frontal bone, frontal process of the maxilla, medial palpebral ligament and lower orbital margin medial to the infraorbital foramen. The fibers sweep across the orbital margin some of which forms a complete ring.

The Horner’s Muscle

Pars lacrimalis or tensor tarsi arises from the upper part of the posterior lacrimal crest and lacrimal fascia behind the lacrimal sac. It divides into two and surrounds the canaliculi and becomes continuous with the pretarsal fibers and the muscle of Riolan, which lies in the eyelid margin.

The upper zygomatic branch and the temporal branch of the facial nerve supply the upper part of the muscle. The lower zygomatic branch supplies the lower part.

The Corrugator Supercilii

Originates from the medial end of the superciliary ridge, passes laterally and inserts into the skin of the eyebrow about its middle. It pulls the eyebrows towards the nose producing a furrow in the forehead skin when a person frowns. It also forms a protection to the eye from bright sun light by forming a projection. The superior zygomatic branch of the facial nerve supplies it.

Occipitofrontalis

The occipital part of the muscle arises from the nuchal line of the occipital bone and its extension into the mastoid bone and passes into the epicranial aponeurosis. The frontalis part arises from the epicranial aponeurosis midway between the coronal suture and the orbital margin and is inserted into the skin of the eyebrow. It raises the eyebrows and draws the scalp forwards producing wrinkles on the forehead. It is the muscle of attention.

This muscle supplied by the posterior auricular and temporal branches of the facial nerve.

Musculus Procerus

It occupies the bridge of the nose and is attached to the nasal bones and lateral nasal cartilages and also to the skin. It pulls the skin down producing the transverse ridges giving it the name the muscle of aggression or menace. The superior buccal branch of the facial nerve supplies it.