Once the eyeball moves, the position of the insertions relative to the origin changes and thus the muscle action changes. Hence, the careful coordination of all six muscles together is important and they all pull as a group of mus-cles. There must be conjugate movement of both eyes and hence 12 muscles must be coordinated through con-stant visual feedback to allow fi ne adjustments and pre-vent diplopia. Two principles contribute to regulation of eye movement:
● Hering’s law of equal innervation states that during any
conjugate eye movement, equal and simultaneous inner-vation fl ows to the yoke muscle (contralateral synergist).
● Sherrington’s law of reciprocal innervation (inhibition)
states that increased innervation to an extraocular mus-cle (e.g. right medial rectus) is accompanied by reciprocal decrease in innervation to its antagonist (i.e. right lateral rectus). This applies to both versions and vergences.
The eyelids
The eyelids protect the eyes from injury and excessive light, spread tears over the surface of the eyes, and help with the exit of tears through the tear ducts.
A horizontal furrow (the superior palpebral sulcus) divides the eyelid into an orbital part and a tarsal part. The sulcus is created because of the insertion of aponeurotic fi bres from the levator palpebrae superioris.
There is a similar sulcus in the lower lid, the inferior palpe-bral sulcus. This is created by adhesions between the skin and the orbicularis oculi. With age, two other sulci are also formed, the naso-jugal sulcus and the lateral (or malar) sulcus.
The eyelids meet at the medial and lateral canthal angles or canthi, and the opening between the two lids is called the palpebral fi ssure. In Orientals, the medial angle is over-lapped by a vertical skin fold, the epicanthus. The lateral angle is in direct contact with the surface of the eye, where-as the medial angle lies around 6 mm medial to the part of the lid that remains in contact with the globe.
Medially, the two eyelids are separated by the lacus lacri-malis, in the middle of which lies the caruncula lacrimalis. A semi-lunar fold, the plica semilunaris, lies medial to the caruncula ( Fig. 1.26 ).
Each eyelid margin is around 2 mm thick and 30 mm long.
About 5 mm from the medial angle there is a mound, the papilla lacrimalis, at the summit of which is the punctum lac-rimale (0.4–0.8 mm diameter). The punctum leads into the lacrimal canal, which is discussed further in the section on the lacrimal system (p. 39). The papilla lacrimalis projects into the lacus lacrimalis and this apparatus serves to drain tears from the surface of the eyes.
Table 1.4 Primary, secondary, and tertiary action of extraocular muscles
Muscle Primary action Secondary action Tertiary action
Medial rectus Adduction–rotation around the y -axis
None None
Lateral rectus Abduction–rotation around the y -axis
None None
Superior rectus Elevation around the x -axis Adduction around the y -axis Intortion around the z -axis Inferior rectus Depression around the x -axis Adduction around the y -axis Extortion around the z -axis Superior oblique Depression around the x -axis Abduction around the y -axis Intorsion around the z -axis Inferior oblique Elevation around the x -axis Abduction around the y -axis Extortion around the z -axis All three axes are mutually perpendicular: the z -axis represents the anteroposterior axis and equates to the optical axis, the x -axis represents the lateral-medial axis, and the y -axis represents the superoinferior axis.
Table 1.5 Primary action in adduction and abduction
Muscle Primary action when eye
is in adduction
Primary action when eye is in abduction
Superior rectus Intortion Elevation
Inferior rectus Extortion Depression
Superior oblique Maximal depression Maximal intortion
Inferior oblique Maximal elevation Maximal extortion
37
Orbit and adnexa
From superfi cial to deep, the eyelid layers are as follows:
● skin
● subcutaneous tissue
● orbicularis oculi—striated muscle fi bres
● orbital septum/tarsal plate
● conjunctiva.
The skin of the eyelids is extremely thin and folds easily. It becomes continuous with the conjunctiva at the site of the orifi ces of the meibomian glands. Eyelashes are present in
double or triple rows on the lid margin from the lateral angle to the papilla lacrimalis. These are more numerous on the upper lid (150) than the lower lid (75), are commonly darker than scalp hair, and do not lose pigmentation with age. They do not possess erector pili muscles and the seba-ceous glands of Zeiss and the ciliary glands of Moll open into the follicles of the lash or directly onto the lid margin.
The subcutaneous tissue is loose and mostly comprises elastic tissue and very little fat.
The orbicularis oculi muscle ( Fig. 1.27 ) is a striated muscle that extends to the temporal region and cheek (orbital part) and onto the eyelids (palpebral part). The latter consists of thin bundles of fi bres that arise from the medial palpebral ligament (MPL). These fi bres are attached to the superfi cial and deep surface of the ligament but not to the inferior surface. They pass laterally and concentri-cally, and laterally they meet at the lateral palpebral raphe.
The lacrimal part of the orbicularis oculi lies behind the lacrimal gland. Beneath the orbicularis oculi lies a layer of connective tissue that contains the blood vessels and nerves of the eyelids.
Medially, the attachments of orbicularis oculi are com-plex. The pretarsal muscles attach medially by a deep and superfi cial head. The superfi cial head forms the medial can-thal tendon. The deep head has fi bres which begin at the medial end of the tarsal plates and insert to the posterior lacrimal crest, behind the lacrimal sac. The preseptal orbicu-laris oculi muscles also insert medially by a superfi cial and deep head. The superfi cial head joins with the medial can-thal tendon. The deep head inserts into the fascia overlying the lacrimal sac as well as into the medial orbital wall above and below the deep head of the pretarsal attachment of the orbicularis oculi.
Fig. 1.26 Right eye with lids everted to show conjunctival sac, lacus lacrimalis, and the caruncula lacrimalis.
Reproduced from G.J. Romanes, Cunningham’s Manual of Practical Anatomy, Head and Neck and Brain , Volume Three, Fifteenth Edition, Figure 15, Page 13, 1986 with permission from Oxford University Press.
Palpebral part
of orbicularis Preseptal Pretarsal
Orbital part of orbicularis muscle
Terminal branches of the zygomatic division
of the facial nerve Fig. 1.27 Orbicularis oculi muscle and the terminal branches of the facial nerve.
This fi gure was published in Colour Atlas of Ophthalmic Plastic Surgery , Third Edition, AG Tyers and JRO Collin, Figure 1.6, p. 8, Copyright Elsevier 2007.
38
Chapter 1 Anatomy
The lateral palpebral ligament attaches the lateral ends of the tarsi to the marginal (Whitnall’s) tubercle on the orbital margin, as formed by the zygomatic bone. It lies under the lateral palpebral raphe.
The orbicularis oculi is supplied by temporal and zygo-matic branches of the facial nerve, which enter the deep surface of the nerve from laterally. The orbital part muscle is mostly under voluntary control and pulls on the surround-ing tissues like a purse strsurround-ing to draw the lids towards the medial angle of the orbit. The palpebral part is under both voluntary and involuntary control. It draws up the lower lid and draws down the upper lid. The blink refl ex (involuntary) is initiated by drying of the cornea. The lacrimal portion draws the eyelids medially and contributes to the pumping mechanism of tear drainage as well as positioning the punc-ta lacrimalis.
The orbital septum is a membranous sheet (composed of seven layers) that is attached to the orbital margin, where it is continuous with the periosteum. It acts to separate the eyelids from the contents of the orbit.
The tarsal plates consist of fi brous tissue that acts as a skeleton for the eyelids. The tarsal plate of the upper lid is around 10 mm in height in the centre. Attached at its upper end is the orbital septum and the smooth muscle fi bres of the levator palpebrae superioris. The tarsal plate of the lower lid measures around 5 mm in height centrally. The orbital septum is attached at its lower end.
The tarsal plate is actually 70% composed of glandular structure because of the presence of meibomian glands
within them. There are 20–25 meibomian glands arranged in a single row in each upper and lower lid. These are modifi ed sebaceous glands, made of a long central canal surrounded by 10–15 acini. They secrete an oily sub-stance that prevents overfl ow of tears, reduces evapora-tion of tears, and allows the closure of the eyelids to be airtight.
The superior tarsal muscle (Müller’s muscle) is continu-ous superiorly with the levator palpebrae superioris and inferiorly with the tarsal plate of the upper lid. This muscle assists the striated muscle of the upper lid in elevating the upper lid. The inferior tarsal muscle is attached to the lower margin of the tarsal plate of the lower eyelid and is continu-ous with the fascial sheath of the inferior rectus muscle.
Both muscles have a sympathetic nerve supply.
The conjunctiva is described on page 42. Importantly with relation to the adnexa, the fornices of the conjunctivae (the points at which it is refl ected back on itself ) have attachments to the aponeurosis of the levator palpebrae superioris (superior fornix) and the aponeurosis of the infe-rior rectus muscle (infeinfe-rior fornix).
The normal position of the upper lid is maintained by the levator palpebrae superioris. This is a striated muscle that is only present in the upper lid. It has an aponeurotic tendon that begins posterior to the orbital septum.
The orbital septum is attached to the anterior surface of the aponeurotic fi bres as a thickened band about 8 mm below Whitnall’s ligament and 3–4 mm above the tarsus.
This inserts medially into the trochlea and laterally into the
Preaponeurotic fat pad
Whitnall’s ligament Levator muscle Common sheath Superior rectus muscle Arcus marginale
Accessory lacrimal glands Müller’s muscle
Muscle of Riolan Orbital septum
Orbicularis muscle
Retro-orbicular fascia Levator aponeurosis Postaponeurotic space Levator aponeurotic insertion into orbicularis Levator aponeurotic insertion into tarsus
Fig. 1.28 Cross-section of the upper eyelid.
This fi gure was published in Colour Atlas of Ophthalmic Plastic Surgery , Third Edition, AG Tyers and JRO Collin, Figure 1.16, p. 18, Copyright Elsevier 2007.
39
Orbit and adnexa
capsule of the lacrimal gland and orbital wall. It acts as a fulcrum for the levator palpebrae.
The preaponeurotic fat pad, an important surgical land-mark, lies between the posterior surface of the septum and the levator aponeurosis.
The levator aponeurosis passes anteriorly and inferiorly and has four attachments ( Fig. 1.28 ):
The nerve supply to the levator palpebrae superioris is by the superior branch of the oculomotor nerve.
The medial and lateral extents of the levator are in the form of two ‘horns’ into the region of the canthal tendons.
The lacrimal gland is closely associated with the posterior edge of the lateral horn.
Müller’s muscle is a smooth muscle with sympathetic innervations that arises from the underside of the levator muscle. It is 15–20 mm wide and descends between the levator aponeurosis and conjunctiva for 15–20 mm to insert into the superior border of the tarsal plate.
Arterial supply of the eyelids comes from the lateral and medial palpebral arteries:
● The lateral palpebral artery branches from the lacrimal
artery, itself a branch of the ophthalmic artery.
● The medial palpebral arteries (superior and inferior)
arise from the ophthalmic artery below the trochlea of the superior oblique muscle, pass behind the lacrimal sac, and enter the eyelids.
● Each medial palpebral artery divides into two branches
that pass laterally, forming two arches in each upper and lower lid. The arches anastomose with the lat-eral palpebral arteries as well as with branches of the superfi cial temporal, transverse facial, and infraorbital arteries.
Lymphatic vessels from the lateral two-thirds of the upper and lower lids drain to the superfi cial parotid nodes.
Those from the medial angle drain to the submandibular nodes.
Sensation to the upper lids is from the ophthalmic divi-sion of the trigeminal nerve. The following branches are involved: infratrochlear, supratrochlear, supraorbital, and lacrimal nerves.
Sensation to the lower lid is supplied by the infratrochlear branch of the ophthalmic division of the trigeminal at the medial angle. The rest of the lower lid is supplied by branch-es of the infraorbital nerve, the terminal portion of the max-illary branch of the trigeminal nerve.