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MOVEMENTS OF THE EYES ments, they assist the medial recti in converging the visual axes, and by
reciprocal adjustment can elevate or depress the visual axes. As the
Movements of the eyes involve rotations around a centre of rotation eyeball is rotated laterally, the lines of traction of the superior and
within the globe. For practical purposes, this can be considered to lie inferior recti approach the plane of the anteroposterior ocular axis
13.5 mm behind the corneal apex. Normal eye movements are binocu- (see Fig. 41.10), and so their rotational effects about this and the
lar. Movements of the eyes in the same direction are termed versions, vertical ocular axis diminish. In abduction to approximately 23°, they
whilst those in opposite directions are termed vergences. Eye move- become almost purely an elevator and depressor, respectively, of the
ments are often accompanied by corresponding movements of the visual axis.
eyelids, particularly in upgaze, where the activity of levator palpebrae Superior oblique acts on the eye from the trochlea and, since the
superioris is closely coupled to that of superior rectus. The following attachment of inferior oblique is, for practical purposes, vertically below
section describes the ocular motor system in terms of the actions of this, both muscles approach the eyeball at the same angle, being
individual extraocular muscles, the diversity of eye movements and attached in approximately similar positions in the superior and inferior
their neural control. posterolateral ocular quadrants (see Fig. 41.10). Superior oblique ele-
vates the posterior aspect of the eyeball, and inferior oblique depresses
it, which means that the former rotates the visual axis downwards and
Actions of the extraocular muscles
the latter rotates it upwards, and both movements occur around the
horizontal axis. When the eye is in the primary position, the obliquity
Levator palpebrae superioris elevates the upper lid, and its antagonist of both muscles means that they pull in a direction posterior to the
is the palpebral part of orbicularis oculi. The degree of elevation, which, vertical axis and both therefore rotate the eye laterally around this axis.


apart from blinking, is maintained for long periods during waking With regard to the anteroposterior axis, in isolation, superior oblique
hours, is a compromise between ensuring an adequate exposure of the intorts the eye and inferior oblique extorts it. Like the superior and
cornea and controlling the amount of incident light. In bright sunshine, inferior recti, therefore, the two obliques have a common turning move-
the latter can be reduced by lowering the upper lid, so limiting glare. ment around the vertical axis but are opposed forces in respect of the
The role of the superior tarsal muscle is less clear. Its tonus is related to other two. Acting in concert, they could therefore assist the lateral rectus
sympathetic activity, and since ptosis is a consequence of impairment in abducting the visual axis, as in divergence of the eyes in transferring
of its sympathetic nerve supply, it may function as an accessory elevator attention from near to far. Again, like the superior and inferior recti,
of the upper eyelid. the directions of traction of the oblique muscles also vary with ocular
Six extraocular muscles rotate the eyeball in directions that depend position, such that they become more nearly a pure elevator and a
on the geometrical relation between their bony and global attachments depressor as the eye is adducted.
(Fig. 41.10), which are altered by the ocular movements themselves. Ocular rotations are, for the most part, under voluntary control,
For convenience, each muscle will be considered in isolation, but it whereas torsional movements cannot be initiated at will. When the
must be appreciated that any movement of the eyeball alters the tension head is tilted in a frontal plane, reflex torsions occur. Any small lapse
and/or length in all six muscles. It is useful to consider the four recti in the concerted adjustment of both eyes produces diplopia.
and two obliques as separate groups (remembering always that they act
in concert) because they form more obvious groupings as antagonists
Movements that shift or stabilize gaze
or synergists. The extrinsic ocular muscles collectively position the
eyeball in the orbital cavity and prevent its anteroposterior movements,
other than a slight retraction during blinks, because the recti exert a The role of eye movements is to bring the image of objects of visual


posterior traction while the obliques pull the eyeball to some degree interest on to the fovea of the retina and to hold the image steady in
anteriorly. They may be assisted by various ‘check ligaments’ (see order to achieve the highest level of visual acuity. Several types of eye
above). A simplified description of the actions of the extraocular movement are required to ensure that these conditions are met. More-
muscles is summarized in Figure 41.11. over, the movements of both eyes must be near perfectly matched to
Of the four recti, the medial and lateral exert comparatively straight- achieve the benefits of binocularity. Both voluntary and reflex move-
forward forces on the eyeball. Being approximately horizontal, when ments are involved and may be so classified. Alternatively, they may be
the visual axis is in its primary position, i.e. directed to the horizon, grouped into those movements that shift gaze as visual interest changes,
they rotate the eye medially (adduction) or laterally (abduction) about and those that stabilize gaze by maintaining a steady image on the
an imaginary vertical axis. They are antagonists. The visual axis can be retina. They have distinct characteristics, and are generated by different
swept through a horizontal arc by reciprocal adjustment of their lengths. neural mechanisms in response to different stimuli, but share a common
When, as is usual, both eyes are involved, the medial and lateral recti final motor pathway. Movements that shift or stabilize gaze include
of each eye either can adjust both visual axes in a conjugate movement saccades, vergence, pursuit and vestibular-generated reflexes (Leigh and
from point to point at infinity (their axes remaining parallel), or they Zee 2006).
can converge or diverge the axes to or from nearer or more distant Saccadic activity is almost omnipresent in human vision. Thus, both
objects of attention in the visual field. visual axes are endlessly and rapidly transferred to new points of inter-
The medial and lateral recti do not rotate the eye around its hori- est in any part of the visual field. Binocular gaze is frequently made to
zontal axis and so cannot elevate or depress the visual axes as gaze is travel routes of the most variable complexity in examining objects of
transferred from nearer to more distant objects or the reverse. This interest in the field, and both visual axes must be maintained with suf-


movement requires the superior and inferior recti (aided by the two ficient accuracy to avoid diplopia. In so-called ‘fixation’ of a focus of
oblique muscles). It is important to remember that the orbital axis does attention, whether uniocular or binocular, the visual axis is not ‘fixed’
not correspond with the visual axis in its primary position but diverges in a perfectly steady manner but undergoes minute, but measurable,
from it at an angle of approximately 23° (see Fig. 41.10A) (the value flicking (of a few minutes or even seconds of arc) across the true line
varies between individuals, and depends on the angle between the of fixation. These microsaccades are rapid and surprisingly complex.
orbital axes and the median plane). Thus, the simple rotation caused When interest changes to another feature of the visual scene, the eyes
by an isolated superior rectus, analysed with reference to the three execute a fast or saccadic movement to take up fixation. If the required
hypothetical ocular axes, appears more complex, being primarily eleva- rotation is small, the saccade is accurate, whereas small supplementary
tion (horizontal axis), and secondarily a less powerful medial rotation corrective saccades are needed if the shift is substantial. Saccades may
(vertical axis) and slight intorsion (anteroposterior axis) in which the also occur in response to other, i.e. non-visual, exteroceptive stimuli
midpoint of the upper rim of the cornea (often referred to as ‘12 (e.g. auditory, tactile or centrally evoked). They may be volitional or
o’clock’) is rotated medially. These actions, compounded as a single, reflex. As an example of the latter, in reading a line of print, the eyes
simple rotation, are easily appreciated when it is seen that the direction make three or four jerky saccades rather than following the line
of traction of superior rectus runs in a posteromedial direction from its smoothly; the line is usefully imaged only when the eye is stationary,
attachment in front, which is anterior to the equator and superior to which means that little of the line is seen by the centre of the fovea.
the cornea, to its bony attachment near the orbital apex (see Fig. 41.10). In general, reaction times and movements are measured in microsec-


Inferior rectus pulls in a similar direction to superior rectus but rotates onds, amplitude varies from seconds of arc to many degrees, with an
the visual axis downwards about the horizontal axis. It rotates the eye accuracy of 0.2° or better, and the velocity of a large saccade may reach
medially on a vertical axis but its action around the anteroposterior axis 500° sec−1. The speed of saccades is assured by an initial, slightly exces-
extorts the eye, i.e. rotates it so that the corneal ‘12 o’clock’ point turns sive, contraction of the appropriate muscles to overcome orbital viscous
laterally. The combined, equal contractions of the superior and inferior resistance. The necessary deceleration when the target is fixated is largely
recti therefore rotate the eyeball medially, since their effects around the dependent on the elasticity of the extraocular muscles and orbital soft
horizontal and anteroposterior axes are opposed. In binocular move- tissues, and not on antagonistic muscular activity.


Extraocular muscles
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A
c23°
Axis of orbit Anteroposterior axis
of eyeball
Medial
orbital
B walls C
Horizontal Superior Equator Superior
axis Anterior medial Equator Vertical axis
superior quadrant
Lateral Medial Medial Lateral
90° Posterior lateral
inferior quadrant
Inferior Inferior
Anterior quadrants Lateral orbital walls Posterior quadrants
D Horizontal axis E Trochlea Inferior oblique Equator
Anteroposterior
axis
Lateral rectus
Lateral
Superior rectus orbital wall
Medial rectus Inferior rectus Superior oblique
Apex of orbit
Optic nerve
Medial orbital walls
F G
Orbital roof
Superior rectus Superior oblique
Anteroposterior axis
Apex of orbit
Horizontal axis Optic nerve
Inferior rectus Inferior oblique
Orbital floor
H I
Intorsion Trochlea
Superior oblique
Superior rectus Anteroposterior axis Horizontal axis
Intorsion
M L M L
Medial rectus Lateral rectus
Inferior rectus
Extorsion
Inferior oblique
Extorsion Vertical axis
Fig. 41.10 The geometrical basis of ocular movements. A, The relationship between the orbital and ocular axes, with the eyes in the primary position,
where the visual axes are parallel. B and C, The ocular globe in anterior and posterior views to show conventional geometry. D, The orbits from above,
showing the medial and lateral recti and the superior rectus (left) and the inferior rectus (right), indicating turning moments primarily around the vertical
axis. E, The superior (left) and inferior (right) oblique muscles showing turning moments primarily around the vertical and also anteroposterior axes. F, A
lateral view to show the actions of the superior and inferior recti around the horizontal axis. G, A lateral view to show the action of the superior and
inferior oblique muscles around the anteroposterior axis. H, An anterior view to show the medial rotational movement of the superior and inferior recti
around the vertical axis. Conventionally, the 12 o’clock position indicated is said to be intorted (superior rectus) or extorted (inferior rectus), as indicated


by the small arrows on the cornea. I, An anterior view to show the torsional effects of the superior oblique (intorsion) and inferior oblique (extorsion)
around the anteroposterior axis, as indicated by the small arrows on the cornea.


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movements in locomotion destabilizes the retinal image, blurs vision
and may render locomotion intolerable.
The otoliths also respond to the pull of gravity, generating static
vestibulo-ocular reflexes associated with head tilt. When static otolith
orientation is changed, e.g. when the head is tilted upwards or down-
wards, the eyes counter-rotate to maintain fixation of the horizontal
meridian. Lateral tilt towards a shoulder generates a torsional counter-
Inferior oblique Superior rectus
rotation of the eyes, a movement that cannot be made voluntarily. The
torsional tilt reflex, equal and opposite in direction by the two eyes, is
fully compensatory over 40° or so in afoveate animals, but in humans
it is vestigial; it is fractionally compensatory and varies in extent between
Elevation
individuals. Because the foveal image is unaffected by torsional move-
ments, the subject is unaware of any visual penalty.
Abduction Lateral rectus Medial rectus Adduction
The optokinetic response is another visually mediated reflex that
stabilizes retinal imagery when a visual scene is rotated about a station-
ary subject. As the scene changes, the eyes follow and hold the retinal
Depression image steady until the eyes shift rapidly in the opposite direction to
another area of the visual scene. The full field of vision, rather than
small objects within it, is the stimulus, and the alternating slow and
Superior oblique Inferior rectus fast phases of movement that are generated describe optokinetic nys-
tagmus. This reflex functions in collaboration with the rotational
vestibulo-ocular reflex. In sustained rotations of the body, the vestibulo-
ocular reflex fades because of the mechanical arrangements of the semi-
circular canals. In darkness, the reflex, which is initially compensatory,
loses velocity, and after approximately 45 seconds the eyeballs become
stationary. With a visual input, the reflex is sustained by the optokinetic
response. Because the reflex is already initiated, the relative delay of
Fig. 41.11 A simplified summary of the actions of the extraocular visual input is overcome. The integration of the two systems is served
muscles. Clockwise and anticlockwise curved arrows represent intorsion


by an accessory visual system projection to the vestibular nuclei via the
and extorsion, respectively.
inferior olive and cerebellum. The usual method of evoking optokinetic
nystagmus in the laboratory or clinic is to present a horizontally moving
pattern of vertical black-on-white stripes while the head of the subject
Vergence is a relatively slow movement permitting maintenance of is held stationary.
single binocular vision of close objects. The eyes converge towards the
midline between the two eyes to achieve imagery of the object on both
Neural control of gaze
foveas. The view of the object at the two eyes is not quite the same and
the disparity is used to assess depth. In addition, the pupils constrict
and the eyes accommodate to achieve sharp, focused images. These Although the detailed anatomical substrates for the different types of
three activities constitute the near reflex. eye movement differ, they share common neural circuitry that lies
Pursuit eye movements are used to track a moving object of visual mainly in the pons and midbrain, for horizontal and vertical gaze
interest, maintaining the image approximately on the fovea. They are movements, respectively (Fig. 41.12). The common element for all
usually preceded by a saccade to capture the image but, unlike saccades, types of horizontal gaze movements is the abducens nucleus. It contains
they are slow and motivated by vision. If the angular shift required to motor neurones that innervate the ipsilateral lateral rectus and interneu-
track the moving object is large or the object is moving swiftly, the rones that project via the medial longitudinal fasciculus to the contra-
initial saccade is frequently inaccurate and one or more small corrective lateral oculomotor nucleus, which controls medial rectus. A lesion of
saccades are made before tracking begins. Because the stimulus is visual, the abducens nucleus leads to a total loss of ipsilateral horizontal con-
the pursuit system response is subject to a relatively long latency jugate gaze. A lesion of the medial longitudinal fasciculus produces
(approximately 100 msec); the limitation in performance this imposes slowed or absent adduction of the ipsilateral eye, usually associated
may be offset by a predictive capacity when object movement follows with jerky movements (nystagmus) of the abducting eye, a syndrome


a regular pattern, and the eye movements adjust in anticipation to speed called internuclear ophthalmoplegia (Leigh and Zee 2006). The gaze
and direction. motor command involves specialized areas of the reticular formation
The vestibular apparatus induces a variety of reflex eye movements of the brainstem, which receive a variety of supranuclear inputs. The
to compensate for the potentially disruptive effects on vision caused by main region for the generation of horizontal saccades is the paramedian
head and body movement (Ch. 38). Receptors in the semicircular pontine reticular formation, located on each side of the midline in the
canals respond to active or passive rotational (angular) accelerations of central paramedian part of the tegmentum, and extending from the
the head. When the body makes substantial rotational movements, a pontomedullary junction to the pontopeduncular junction. Each para-
vestibulo-ocular reflex generates a cycle of responses involving both the median pontine reticular formation contains excitatory neurones
shifting and stabilizing of gaze. Body rotation is matched by counter- (referred to as ‘burst’ cells) that discharge at high frequencies just prior
rotation of the eyes so that gaze direction is unaltered and clear vision to and during ipsilateral saccades. Excitatory burst cells make mono-
is maintained. Physical constraint limits the rotation to 30° or less and synaptic connections with the ipsilateral abducens nucleus. Pause neur-
is followed by a rapid saccadic movement of the eyes to another object ones, located in a midline caudal pontine nucleus, called the nucleus
in the visual scene and the cycle is repeated. Vision is therefore clear raphe interpositus, discharge tonically during fixation but stop firing
throughout most of the cycle while the image is stationary, but at the immediately prior to a saccade. They appear to exert an inhibitory influ-
cost of no useful vision during the brief periods of the saccades. The ence on the burst neurones and act as a switch to change from fixation
reflex is efficient and rapid; this speed could not be generated by to saccadic mode (Ramat et al 2007).
the visual system, which is slow relative to the short latency of vestibular The tonic activity of neurones in the nucleus prepositus hypoglossi
receptors. and medial vestibular nucleus is thought to provide an eye position


Other reflexes generated by the vestibular system, which induce signal to maintain the eccentric position of the eye against the visco-
compensatory eye movements to stabilize gaze, are activated during elastic forces in the orbit. These forces tend to move the eyeball back
brief head movements. When the head is sharply rotated in any direc- to looking straight ahead, i.e. the primary position, after a saccade.
tion, the eyeball rotates by an equal amount in the opposite direction Vestibular nuclei and the perihypoglossal complex project directly to
in response to the stimulation of semicircular canal cristae (angular the abducens nuclei. These projections probably also carry smooth
acceleration), and gaze is undisturbed. Brief rotational movements are pursuit signals, via the cerebellum.
commonly combined with translational movements (linear accelera- The final common pathway for vertical gaze movements is formed
tion) that are monitored by otolith organs. For example, a linear dis- by the oculomotor and trochlear nuclei. The rostral interstitial nucleus
placement occurs in walking as the head bobs vertically with each stride, of the medial longitudinal fasciculus contains excitatory burst neurones
and a rotational displacement occurs as the head rolls, invoking otolith that discharge in relation to up-and-down vertical saccadic movements
and canal responses, respectively, to stabilize the retinal image. Vestibu- and project to motor neurones involved in vertical gaze. The rostral
lar disease incurring the loss of the rapid, fine compensatory eye interstitial nucleus of the medial longitudinal fasciculus projects


Extraocular muscles
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Fig. 41.12 A summary of the control of eye movements.
The central drawing shows the supranuclear connections
SEF from the frontal eye field (FEF) and the posterior eye field
(PEF) to the superior colliculus (SC), rostral interstitial
nucleus of the medial longitudinal fasciculus (riMLF), and
the paramedian pontine reticular formation (PPRF). The
FEF and SC are involved in the production of saccades,
FEF
while the PEF is thought to be important in the
DLPFC production of pursuit movements. The diagram on the
PEF
left shows the brainstem pathways for horizontal gaze.
Axons from the PPRF travel to the ipsilateral abducens
riMLF nucleus innervating lateral rectus (LR). Abducens
internuclear axons cross the midline and travel in the
INC SC medial longitudinal fasciculus (MLF) to the neuronal pool
in the oculomotor nucleus (III) that innervates medial
III rectus (MR) of the contralateral eye. The diagram on the
IV right shows the brainstem pathways for vertical gaze.
Important structures include the riMLF, PPRF, the
PPRF
interstitial nucleus of Cajal (INC) and the posterior
VI commissure (PC). Other abbreviations: DLPFC,
LR MR dorsolateral prefrontal cortex; IV, trochlear nucleus; NPH,
PC nucleus propositus hypoglossi; SEF, supplementary eye
VN field; VI, abducens nucleus; VN, vestibular nucleus.
riMLF
INC
III
IV IV III MLF
PPRF MLF PPRF
VI VI
VN NPH VN NPH
through the posterior commissure to its equivalent on the other side of to voluntary saccades, especially of the memory-guided type. A posterior
the mesencephalon, as well as directly to the ipsilateral oculomotor and pathway originates in the posterior eye field and passes to the brainstem
trochlear nuclei (see Fig. 41.12). Neurones in and around the interstitial saccadic generators via the superior colliculus. This pathway is impor-


nucleus of Cajal, which lies slightly caudal to the rostral interstitial tant for triggering visually guided saccades.
nucleus of the medial longitudinal fasciculus, provide signals for verti- The smooth pursuit system has developed relatively independently
cal gaze holding. Vertical gaze palsies can affect upgaze, downgaze or of the saccadic oculomotor system to maintain foveation of a moving
both. Lesions within the posterior commissure predominantly give rise target, although there are inevitable interconnections between the two.
to disturbances in upgaze, associated with other signs of dorsal mid- The first task is to identify and code the velocity and direction of a
brain syndrome, e.g. pupillary abnormalities (light-near dissociation). moving target. This is carried out in the extrastriate visual area known
Discrete lesions placed more ventrally in the region of the rostral as the middle temporal visual area (also called visual area V5), which
interstitial nucleus of the medial longitudinal fasciculus may cause contains neurones sensitive to visual target motion. In humans, this lies
mixed up-and-down, or mainly downgaze disturbances (Leigh and Zee immediately posterior to the ascending limb of the inferior temporal
2006). sulcus at the occipitotemporal border. The middle temporal visual area
The cerebellum plays an important role in the control of eye move- sends this motion signal to the medial superior temporal visual area,
ments (p. 347, see Fig. 22.24). The vestibulocerebellum (flocculus and thought to lie superior and a little anterior to the middle temporal
nodule) is involved in gaze holding, smooth pursuit and the vestibulo- visual area within the inferior parietal lobe; damage to this area results
ocular reflex. The dorsal vermis and fastigial nucleus play a major role in an impairment of smooth pursuit of targets moving towards the
in programming accurate saccades and smooth pursuit. damaged hemisphere.
The cerebral hemispheres are extremely important for the program- Both the medial superior temporal visual area and the frontal eye
ming and coordination of both saccadic and pursuit conjugate eye field send direct projections to a group of nuclei that lie in the basal
movements (Ch. 25). There appear to be four main cortical areas in the part of the pons. In monkeys, the dorsolateral and lateral groups of


cerebral hemispheres involved in the generation of saccades. These are pontine nuclei receive direct cortical inputs related to smooth pursuit.
the frontal eye field, which is located at the intersection of the superior Lesions of similarly located nuclei in humans result in abnormal pur-
precentral sulcus with the superior frontal sulcus (Brodmann area 8) suit. These nuclei transfer the pursuit signal bilaterally to the posterior
(Amiez and Petrides 2009); the supplementary eye field, which lies in vermis, contralateral flocculus and fastigial nuclei of the cerebellum.
the dorsomedial frontal cortex (Brodmann area 6); the dorsolateral The pursuit signal ultimately passes from the cerebellum to the brain-
prefrontal cortex, which lies anterior to the frontal eye field in the stem, specifically to the medial vestibular nucleus and nucleus proposi-
second frontal gyrus (Brodmann area 46); and a posterior eye field, tus hypoglossi, and thence to the paramedian pontine reticular
which lies in the parietal lobe in the medial wall of the posterior half formation and possibly directly to the ocular motor nuclei. This cir-
of the intraparietal sulcus, including parts of the supramarginal and cuitry therefore involves a double decussation: firstly, at the level of the
angular gyri (Brodmann areas 39 and 40), and the adjacent lateral midpons (pontocerebellar neurones), and secondly, in the lower pons
intraparietal sulcus (Müri et al 1996). These areas all appear to be (vestibulo-abducens neurones).
interconnected and to send projections to the superior colliculus and The vestibulo-ocular reflex maintains coordination of vision during
the brainstem areas controlling saccades. movement of the head and results in a compensatory conjugate eye
Two parallel pathways are involved in the cortical generation of sac- movement that is equal but opposite to the movement of the head. This
cades. An anterior system originates in the frontal eye field and projects, essentially three-neurone arc consists of primary vestibular neurones
both directly and via the superior colliculus, to the brainstem saccadic that project to the vestibular nuclei, secondary neurones that project


generators. This pathway also passes indirectly via the basal ganglia to from these nuclei directly to the abducens and oculomotor nuclei, and
the superior colliculus. Projections from the frontal cortex influence tertiary neurones that innervate the extraocular muscles. Vestibular neu-
cells in the pars reticularis of the substantia nigra, via a relay in the rones responding to head rotation also respond to optokinetic stimuli,
caudate nucleus. An inhibitory pathway from the pars reticulata projects which means that the neural substrate is likely to include both the
directly to the superior colliculus. This may be a gating circuit related visual and vestibular systems.


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Muscular branches
Supraorbital
Muscular branches frequently spring from a common trunk to form
Lateral Supratrochlear superior and inferior groups, most of which accompany branches of the
palpebral oculomotor nerve. The inferior branch contains most of the anterior
ciliary arteries. Other muscular vessels branch from the lacrimal and
Dorsal nasal supraorbital arteries or from the trunk of the ophthalmic artery.
Medial Ciliary arteries
palpebral The ciliary arteries are distributed in long and short posterior, and in
anterior groups (see Fig. 41.13). Long posterior ciliary arteries, usually
two, pierce the sclera near the optic nerve, pass anteriorly along the
Anterior ethmoidal
spca horizontal meridian and join the major arterial circle of the iris (see
Fig. 42.8). About seven short posterior ciliary arteries pass close to the
optic nerve to reach the eyeball, where they divide into 15–20 branches.
Zygomatic lpca Anterior meningeal They pierce the sclera around the optic nerve to supply the choroid, and
anastomose with twigs of the central retinal artery at the optic disc (see
Medial posterior ciliary
Fig. 42.30). Anterior ciliary arteries arise from muscular branches of the
p
Posterior ethmoidal ophthalmic artery. They reach the eyeball on the tendons of the recti,
p
Central retinal m m form a circumcorneal subconjunctival vascular zone, and pierce the
m
sclera near the sclerocorneal junction to end in the major arterial circle
Recurrent meningeal of the iris.
Ophthalmic
Lacrimal p
lacrimal artery
Lateral posterior ciliary
The lacrimal artery is a large branch that usually leaves the ophthalmic
artery near its exit from the optic canal (see Fig. 41.13), although it
occasionally arises before the ophthalmic artery enters the orbit. It
accompanies the lacrimal nerve along the upper border of lateral rectus,
Internal carotid
supplies and traverses the lacrimal gland, and ends in the eyelids and


conjunctiva as the lateral palpebral arteries. The latter run medially in
Fig. 41.13 The orbital distribution of the ophthalmic artery. Abbreviations: the upper and lower lids and anastomose with the medial palpebral
lpca, long posterior ciliary artery; m, muscular arteries; p, pial arteries; arteries. The lacrimal artery gives off one or two zygomatic branches.
spca, short posterior ciliary arteries. One reaches the temporal fossa via the zygomaticotemporal foramen
and anastomoses with the deep temporal arteries. The other reaches the
cheek by the zygomaticofacial foramen and anastomoses with trans-
VASCULAR SUPPLY AND LYMPHATIC DRAINAGE verse facial and zygomatico-orbital arteries. A recurrent meningeal
branch, usually small, passes back via the lateral part of the superior
orbital fissure to anastomose with a middle meningeal branch. This
ARTERIES
branch is sometimes large and replaces the lacrimal artery, in which
case it becomes a more significant contributor to the orbital blood
The main vessel supplying orbital structures is the ophthalmic artery
supply.
(Fig. 41.13). Its terminal branches anastomose on the face and scalp
with those of the facial, maxillary and superficial temporal arteries, supraorbital artery
thereby establishing connections between the external and internal
The supraorbital artery leaves the ophthalmic artery where it crosses the
carotid arteries. The infraorbital branch of the maxillary artery, and
optic nerve (see Fig. 41.13), and ascends medial to superior rectus and
possibly the recurrent meningeal artery, also supply orbital structures.
levator palpebrae superioris. It accompanies the supraorbital nerve
between the periosteum and levator palpebrae superioris, passes
Ophthalmic artery
through the supraorbital foramen or notch, and divides into superficial
and deep branches. These supply the skin, muscles and frontal perios-
The ophthalmic artery leaves the internal carotid artery as it exits the teum, and anastomose with the supratrochlear artery and with the
cavernous sinus medial to the anterior clinoid process. It enters the orbit frontal branch of the superficial temporal artery and its contralateral


by the optic canal, inferolateral to the optic nerve and continues for- fellow (see Fig. 30.20). The supraorbital artery supplies superior rectus
wards for a short distance before turning medially by crossing (almost and levator palpebrae superioris, and sends a branch across the trochlea
always) over or (sometimes) under the optic nerve (see Fig. 41.13). The to the medial canthus. It often sends a branch to the diploë of the
main trunk of the artery continues along the medial wall of the orbit frontal bone at the supraorbital margin, and may also supply the muco-
between the superior oblique and lateral rectus, and divides into supra- periosteum in the frontal sinus.
trochlear (frontal) and dorsal nasal branches at the medial end of the
upper eyelid. Although the order of branches from the ophthalmic Posterior ethmoidal artery
artery is quite variable, a number may be identified consistently, includ- The posterior ethmoidal artery runs through the posterior ethmoidal
ing the central retinal artery, lacrimal artery, muscular branches, ciliary canal and supplies the posterior ethmoidal air sinuses. Entering the
arteries, supraorbital artery, anterior and posterior ethmoidal arteries, cranium, it gives off a meningeal branch to the dura mater, and nasal
meningeal branch, medial palpebral arteries, supratrochlear artery and branches that descend into the nasal cavity via the cribriform plate and
dorsal nasal artery (Hayreh and Dass 1962). Many of the branches of anastomose with branches of the sphenopalatine artery (see Fig. 33.9).
the ophthalmic artery accompany sensory nerves of the same name and
have a similar distribution. Variations in the origin and course of the Anterior ethmoidal artery
ophthalmic artery have been described (Huynh-Le et al 2005). The The anterior ethmoidal artery passes with its accompanying nerve
artery may arise within, near or significantly distal to the upper dural through the anterior ethmoidal canal to supply the ethmoidal and
ring, and the point of penetration of the dura lining the optic canal frontal air sinuses. Entering the cranium, it gives off a meningeal branch


may be posterior, below or anterior to the falciform ligament. The to the dura mater, and nasal branches that descend into the nasal cavity
ophthalmic artery may also arise from the clinoid or intracavernous with the anterior ethmoidal nerve (see Fig. 33.9). It runs in a groove
segment of the carotid artery. In the latter case, the ophthalmic artery on the deep surface of the nasal bone to supply the lateral nasal wall
may pass through either the superior orbital fissure or the optic canal. and septum. A terminal branch appears on the nose between the nasal
bone and the upper nasal cartilage (see Fig. 33.9).
Central retinal artery
The small central artery of the retina is the first branch of the ophthal- Meningeal branch
mic artery. It begins below the optic nerve and, for a short distance, lies A meningeal branch, usually small, passes back through the superior
in the dural sheath of the nerve. It enters the inferomedial surface of orbital fissure to the middle cranial fossa, where it anastomoses with
the nerve 6.4–15.2 mm behind the eye, and runs to the retina (accom- the middle and accessory meningeal arteries. It is sometimes large, in
panying the central retinal vein) along its axis. The further distribution which case it becomes a major contributor to the orbital blood
of the central retinal artery is described on page 704. supply.


innervation
677
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Medial palpebral arteries superior ophthalmic vein may also receive the inferior ophthalmic vein.
Above the trochlea, two medial palpebral arteries branch separately It traverses the superior orbital fissure, usually above the common
from the ophthalmic artery and join the lateral palpebral arteries to tendinous ring of the recti, and ends in the cavernous sinus.
complete the arcades of the upper and lower eyelids. The inferior ophthalmic vein begins in a network near the anterior
region of the orbital floor and medial wall. It runs backwards on inferior
supratrochlear artery rectus and across the inferior orbital fissure, and then either joins the
The supratrochlear artery is a terminal branch of the ophthalmic artery. superior ophthalmic vein or passes through the superior orbital fissure,
It leaves the orbit superomedially with the supratrochlear nerve, ascends within or below the common tendinous ring, to drain directly into the
on the forehead to supply the skin, muscles and pericranium, and cavernous sinus. The inferior ophthalmic vein receives tributaries from
anastomoses with the supraorbital artery and with its contralateral inferior rectus and inferior oblique, the nasolacrimal sac, the eyelids
fellow. and the two inferior vortex veins of the eyeball. It communicates with
the pterygoid venous plexus by a branch that passes through the inferior
dorsal nasal artery orbital fissure, and may also communicate with the facial vein across
The dorsal nasal artery is the other terminal branch of the ophthalmic the inferior margin of the orbit.
artery and emerges from the orbit between the trochlea and medial
palpebral ligament. It gives a branch to the upper part of the nasolac- Infraorbital vein
rimal sac and then divides into two branches. One branch joins the
terminal part of the facial artery, and the other runs along the dorsum The infraorbital vein runs with the infraorbital nerve and artery in the
of the nose, supplies its outer surface and anastomoses with its contra- floor of the orbit, and passes backwards through the inferior orbital
lateral fellow and the lateral nasal branch of the facial artery. fissure into the pterygoid venous plexus. It drains structures in the floor


of the orbit and communicates with the inferior ophthalmic vein; it
Infraorbital branch of the maxillary artery may also communicate with the facial vein on the face.
The infraorbital branch of the maxillary artery enters the orbit through
LYMPHATIC DRAINAGE
the inferior orbital fissure (see Fig. 32.20). Accompanying the infraor-
bital nerve, it passes along the infraorbital groove of the maxilla in the
floor of the orbit before entering the infraorbital canal, and comes out Lymphatic vessels other than those draining the conjunctiva have not
on to the face through the infraorbital foramen. While in the infraor- been identified.
bital groove, it gives off branches that supply inferior rectus and inferior
oblique, the nasolacrimal sac and, occasionally, the lacrimal gland.
INNERVATION
VEINS Somatic and autonomic motor and somatic sensory nerves are found
in the orbit (Figs 41.15–41.17). The oculomotor, trochlear and abdu-
The orbit is drained by the superior and inferior ophthalmic veins cens nerves supply the extraocular muscles. Parasympathetic fibres from
and the infraorbital vein (Fig. 41.14). The veins of the eyeball mainly the oculomotor nerve supply sphincter pupillae and the ciliary muscle
drain into the vortex veins; the retinal veins drain into the central retinal (ciliaris) via the ciliary ganglion, and those from the facial nerve inner-
vein. vate the lacrimal gland and choroid via the pterygopalatine ganglion.
Sympathetic fibres supply dilator pupillae. Both sympathetic and para-
Superior and inferior ophthalmic veins sympathetic nerves supply the arteries. The sensory nerves within the
orbit are the optic, ophthalmic and maxillary nerves (the maxillary
The superior and inferior ophthalmic veins link the facial and intracra- nerve and most of the ophthalmic branches only pass through the orbit
nial veins, and are devoid of valves. The superior ophthalmic vein forms en route to supply the face and jaws).
posteromedial to the upper eyelid from two tributaries that connect
anteriorly with the facial and supraorbital veins. It runs with the oph- Oculomotor nerve


thalmic artery, lying between the optic nerve and superior rectus, and
receives the corresponding tributaries, the two superior vortex veins of The oculomotor nerve is the third cranial nerve (see Fig. 41.15). It
the eyeball, and the central vein of the retina. The central retinal vein innervates levator palpebrae superioris and four of the extraocular
sometimes drains directly into the cavernous sinus, although it still muscles (superior, inferior and medial rectus and inferior oblique), and
gives a communicating branch to the superior ophthalmic vein. The also conveys parasympathetic fibres that relay in the ciliary ganglion.
The nerve emerges at the midbrain, on the medial side of the crus of
the cerebral peduncle, and passes along the lateral dural wall of the
Supraorbital Lacrimal Central cavernous sinus, dividing into superior and inferior divisions that
retinal
run beneath the trochlear and ophthalmic nerves. The two divisions
Superior enter the orbit through the superior orbital fissure, within the common
ophthalmic
Cavernous
Frontal
sinus
Supraorbital nerve
Supratrochlear
nerve
Oculomotor nerve,
Angular Trochlear nerve superior division
Lacrimal nerve
Infratrochlear nerve Zygomatic nerve
Zygomaticotemporal
nerve
Nasociliary nerve
Abducens nerve
Ciliary ganglion
Oculomotor nerve,
inferior division Zygomaticofacial
nerve
Inferior Infraorbital nerve
Vortex ophthalmic
Pterygoid
plexus Fig. 41.15 The left orbit viewed from in front, showing motor and sensory
Facial
nerves and the origins of the extraocular muscles. Note that in this figure
the trochlear nerve is shown inserting on the inferior aspect of the
superior oblique, whereas it more commonly inserts on the superior
Deep facial
surface of the muscle. (With permission from Dutton JJ 1994 Atlas of
Fig. 41.14 The principal veins of the orbit. Clinical and Surgical Orbital Anatomy. Philadelphia: Saunders.)


ORBiT And ACCEssORy visuAl APPARATus
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nOiTCEs
tendinous ring of the recti, separated by the nasociliary branch of the Abducens nerve
ophthalmic nerve.
The superior division of the oculomotor nerve passes above the optic
The abducens nerve is the sixth cranial nerve and exclusively innervates
nerve to enter the inferior (ocular) surface of superior rectus. It supplies
lateral rectus. It emerges from the brainstem between the pons and the
this muscle and gives off a branch that runs to innervate levator palpe-
medulla oblongata, and usually runs through the inferior venous com-
brae superioris. The inferior division of the oculomotor nerve divides
partment of the petroclival venous confluence in a bow-shaped canal,
into medial, central and lateral branches. The medial branch passes
Dorello’s canal. It then bends sharply across the upper border of the
beneath the optic nerve to enter the lateral (ocular) surface of medial
petrous part of the temporal bone to enter the cavernous sinus, where
rectus; the central branch runs downwards and forwards to enter the
it lies lateral to the internal carotid artery (unlike the oculomotor, tro-
superior (ocular) surface of inferior rectus; the lateral branch travels
chlear, ophthalmic and maxillary nerves, which merely invaginate the
forwards on the lateral side of inferior rectus to enter the orbital surface
lateral dural wall of the sinus). The abducens nerve enters the orbit
of inferior oblique and also communicates with the ciliary ganglion to
through the superior orbital fissure, within the common tendinous ring,
distribute parasympathetic fibres to sphincter pupillae and the ciliary
at first below, and then between, the two divisions of the oculomotor
muscle.
nerve and lateral to the nasociliary nerve (see Fig. 41.15). It passes
forwards to enter the medial (ocular) surface of lateral rectus.
Trochlear nerve
Optic nerve
The trochlear nerve is the fourth cranial nerve and innervates superior
oblique exclusively. It is the only cranial nerve to emerge from the


The optic nerve is the second cranial nerve. It arises from the optic
dorsal surface of the brainstem, passing from the midbrain on to the
chiasma on the floor of the diencephalon and enters the orbit through
lateral surface of the crus of the cerebral peduncle. It runs through
the optic canal, accompanied by the ophthalmic artery. It changes its
the lateral dural wall of the cavernous sinus and then crosses the oculo-
shape from being flattened at the chiasma to rounded as it passes
motor nerve to enter the orbit through the superior orbital fissure,
through the optic canal. In the orbit, the optic nerve passes forwards,
outside the common tendinous ring, above levator palpebrae superioris
laterally and downwards, and pierces the sclera at the lamina cribrosa,
and medial to the frontal and lacrimal nerves (see Fig. 41.16). The
slightly medial to the posterior pole. It has a somewhat tortuous course
trochlear nerve travels a short distance to enter the superior (orbital)
within the orbit to allow for movements of the eyeball, and is sur-
surface of superior oblique.
rounded by extensions of the three layers of the meninges.
The optic nerve has important relationships with other orbital struc-
tures (see Fig. 41.4). As it leaves the optic canal, it lies superomedial to
the ophthalmic artery, and is separated from lateral rectus by the oculo-
motor, nasociliary and abducens nerves, and sometimes by the ophthal-
mic veins. It is closely related to the origins of the four recti, whereas
more anteriorly, where the muscles diverge, it is separated from them
by a substantial amount of orbital fat. Just beyond the optic canal, the
ophthalmic artery and the nasociliary nerve cross the optic nerve to
Infratrochlear
Lacrimal gland nerve reach the medial wall of the orbit. The central artery of the retina enters
Supraorbital nerve the substance of the optic nerve about halfway along its length. Near
Supratrochlear nerve Anterior ethmoidal the back of the eyeball, the optic nerve becomes surrounded by the long
nerve
and short ciliary nerves and vessels.
Frontal nerve Ophthalmic artery
Communication from Ophthalmic nerve


zygomaticotemporal Nasociliary nerve
nerve
The ophthalmic division of the trigeminal nerve arises from the trigemi-
Lacrimal nerve
Trochlear nerve nal ganglion in the middle cranial fossa. It passes forwards along the
lateral dural wall of the cavernous sinus, giving off three main branches
Maxillary division in
Optic nerve – the lacrimal, frontal and nasociliary nerves – just before it reaches the
foramen rotundum
Internal carotid superior orbital fissure (see Figs 41.16–41.17). These branches sub-
Mandibular division artery sequently travel through the orbit to supply targets that are primarily
in foramen ovale in the upper part of the face (see Fig. 30.22).
Trigeminal ganglion
Oculomotor nerve lacrimal nerve
Trochlear nerve
Sensory root of The lacrimal nerve enters the orbit through the superior orbital fissure,
trigeminal nerve Motor root of outside the common tendinous ring and lateral to the frontal and
trigeminal nerve trochlear nerves. It passes forwards along the lateral wall of the orbit
Fig. 41.16 The nerves of the left orbit, superior aspect. on the superior border of lateral rectus, and travels through the lacrimal
Long ciliary nerve Ciliary ganglion Fig. 41.17 The nerves of the left orbit and the
Nasociliary nerve ciliary ganglion, lateral aspect.
Anterior ethmoidal nerve
Frontal nerve
Levator palpebrae superioris Lacrimal nerve
Internal carotid artery
Superior rectus
Optic nerve
Lacrimal nerve Oculomotor nerve
Communication between lacrimal Trochlear nerve
and zygomaticotemporal nerve
Inferior oblique Mandibular nerve
Ophthalmic nerve
Infraorbital
nerve
Maxillary nerve
Lateral rectus Abducens nerve
Inferior rectus Short ciliary nerves


innervation
679
14
RETPAHC
gland and the orbital septum to supply the conjunctiva and skin cover- Sclera Long ciliary nerve
Nasociliary
ing the lateral part of the upper eyelid. The nerve also communicates Ciliary body
nerve
with the zygomatic branch of the maxillary nerve, which may carry
some postganglionic parasympathetic fibres from the pterygopalatine
ganglion to the lacrimal gland (see orbital branches from pterygopala-
tine ganglion, below). Sensory root
Iris
Frontal nerve Cornea
Sympathetic root
The frontal nerve is the largest branch of the ophthalmic nerve. It enters
the orbit through the superior orbital fissure outside the common Short ciliary nerves Ciliary ganglion Motor (parasympathetic) root
tendinous ring, and lies between the lacrimal nerve laterally and the
Fig. 41.18 The ciliary ganglion, with its roots and branches of distribution.
trochlear nerve medially. It passes forwards towards the rim of the orbit
Key: red, sympathetic fibres; green, parasympathetic fibres; blue, sensory
on levator palpebrae superioris; about halfway along this course, it
fibres. Alternative pathways are given for the sympathetic fibres.
divides into the supraorbital and supratrochlear nerves.
Supraorbital nerve
nerve. It is sometimes joined by a filament from the internal carotid
The supraorbital nerve, the larger of the two terminal branches of the
sympathetic plexus or from the superior ramus of the oculomotor nerve
frontal nerve, continues forwards along levator palpebrae superioris
as it enters the posterosuperior angle of the ganglion.
until it leaves the orbit through the supraorbital notch or foramen. It
emerges on to the forehead and supplies the mucous membrane that
Maxillary nerve
lines the frontal sinus, the conjunctiva and skin covering the upper
eyelid, and the skin over the forehead and scalp. The postganglionic
sympathetic fibres that innervate the sweat glands of the suprao rbital Most of the branches of the maxillary division of the trigeminal nerve


area probably travel in the supraorbital nerve, having entered the oph- arise in the pterygopalatine fossa. They include the zygomatic and
thalmic nerve via its communication with the abducens nerve within infraorbital nerves, which pass into the orbit through the inferior
the cavernous sinus. orbital fissure (see Figs 32.30, 41.17).
Supratrochlear nerve Zygomatic nerve
The supratrochlear nerve runs medially above the trochlea for the The zygomatic nerve lies close to the base of the lateral wall of the orbit.
tendon of superior oblique. It gives a descending branch to the infra- It divides soon after entering the orbit into two branches, the zygo-
trochlear nerve and ascends on to the forehead through the frontal maticotemporal and the zygomaticofacial nerves, which run within the
notch to supply the conjunctiva and skin covering the upper eyelid and orbit for only a short distance before passing on to the face through the
the forehead. lateral wall of the orbit. Either they may enter separate canals within
the zygomatic bone or the zygomatic nerve itself may enter the bone
nasociliary nerve before dividing. For a description of the distribution of the zygomatico-
temporal and zygomaticofacial nerves on the face, see page 502.
The nasociliary nerve is intermediate in size between the frontal and
lacrimal nerves, and is more deeply placed in the orbit, which it enters
infraorbital nerve
through the common tendinous ring, lying between the two rami of
The infraorbital nerve initially lies in the infraorbital groove on the floor
the oculomotor nerve. It crosses the optic nerve with the ophthalmic
of the orbit (see Fig. 32.30B). As it approaches the rim of the orbit, it
artery and runs obliquely below superior rectus and superior oblique
runs into the infraorbital canal, through which it passes to emerge on
to reach the medial orbital wall, where it gives off the anterior and
to the face at the infraorbital foramen. It supplies the skin of the lower
posterior ethmoidal nerves. The nasociliary nerve also has long ciliary
eyelid and possibly the conjunctiva. For a description of the subsequent


and infratrochlear branches and a connection with the ciliary
distribution of the infraorbital nerve on the face, see page 502.
ganglion.
Ciliary ganglion
Anterior ethmoidal nerve
The anterior ethmoidal nerve passes through the anterior ethmoidal
The ciliary ganglion is a parasympathetic ganglion concerned with the
foramen and canal, and enters the cranial cavity. It runs forwards in a
innervation of certain intraocular muscles. It is a small, flat, reddish-
groove on the upper surface of the cribriform plate beneath the dura
grey swelling, 1–2 mm in diameter, connected to the nasociliary nerve,
mater, and descends through a slit lateral to the crista galli into the
and located near the apex of the orbit in loose fat approximately 1 cm
nasal cavity, where it occupies a groove on the internal surface of the
in front of the medial end of the superior orbital fissure. It lies between
nasal bone and gives off the medial and lateral internal nasal branches.
the optic nerve and lateral rectus, usually lateral to the ophthalmic
For a description of the subsequent distribution of the anterior ethmoi-
artery. Its neurones, which are multipolar, are larger than those found
dal nerve, see page 564.
in typical autonomic ganglia; a very small number of more typical
neurones are also present.
Posterior ethmoidal nerve
Its connections or roots (motor, sensory and sympathetic) enter or
The posterior ethmoidal nerve leaves the orbit by the posterior
leave the ganglion posteriorly (see Fig. 41.18). Eight to ten delicate fila-
ethmoidal foramen and supplies the ethmoidal and sphenoidal
ments, termed the short ciliary nerves, emerge anteriorly from the gan-
sinuses.
glion, arranged in two or three bundles, the lower being larger. They
run forwards sinuously with the ciliary arteries, above and below the
Infratrochlear nerve
optic nerve, and divide into 15–20 branches that pierce the sclera
The infratrochlear nerve leaves the orbit below the trochlea and supplies around the optic nerve and run in small grooves on the internal scleral


the skin of the eyelids, the conjunctiva, lacrimal sac, lacrimal caruncle surface. They convey parasympathetic, sympathetic and sensory fibres
and the side of the nose above the medial canthus. between the eyeball and the ciliary ganglion; only the parasympathetic
fibres synapse in the ganglion.
Long ciliary nerves The parasympathetic root, derived from the branch of the oculo-
Two or three long ciliary nerves branch from the nasociliary nerve as it motor nerve to the inferior oblique, consists of preganglionic fibres
crosses the optic nerve. They accompany the short ciliary nerves, pierce from the Edinger–Westphal nucleus, which relay in the ganglion. Post-
the sclera near the attachment of the optic nerve, and run forwards ganglionic fibres travel in the short ciliary nerves to the sphincter pupil-
between the sclera and choroid. They supply the ciliary body, iris and lae and ciliary muscle. More than 95% of these fibres supply the ciliary
cornea, and contain postganglionic sympathetic fibres for the dilator muscle, which is much the larger muscle in volume.
pupillae from neurones in the superior cervical ganglion. An alternative The sympathetic root contains fibres from the plexus around the
pathway for the supply of the dilator pupillae is via the sympathetic internal carotid artery within the cavernous sinus. These postganglionic
root associated with the ciliary ganglion (Fig. 41.18). fibres, derived from the superior cervical ganglion, form a fine branch
that enters the orbit through the superior orbital fissure inside the
Ramus communicans to the ciliary ganglion common tendinous ring. The fibres may pass directly to the ganglion,
The ramus communicans to the ciliary ganglion usually branches from or may join the nasociliary nerve and travel to the ganglion in its
the nasociliary nerve as the latter enters the orbit lateral to the optic sensory root; either way, they traverse the ganglion without synapsing


ORBiT And ACCEssORy visuAl APPARATus
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nOiTCEs
to emerge into the short ciliary nerves. They are distributed to the blood the eyelashes and is termed the ciliary part of the eyelid. The margin
vessels of the eyeball. Sympathetic fibres innervating dilator pupillae medial to the papilla lacks eyelashes and forms the lacrimal part of the
may sometimes travel via the short ciliary nerves (rather than the more eyelid.
usual route via the ophthalmic, nasociliary and long ciliary nerves). When looking straight ahead, the upper eyelid overlaps the upper
The sensory fibres that pass through the ciliary ganglion are derived part of the cornea by 2–3 mm, whereas the lower lid margin lies just
from the nasociliary nerve. They enter the short ciliary nerves and carry below the corneoscleral junction (limbus). When the eyelids are closed,
sensation from the cornea, the ciliary body and the iris. the upper lid moves down to cover the whole of the cornea. Malposition
of the lower eyelid is common, particularly in the elderly. Ectropion
Orbital branches of the describes the rolling out of the lower eyelid so that it is no longer in
pterygopalatine ganglion contact with the cornea, leading to epiphora (watering). Entropion
describes the inversion of the eyelid with corresponding inturning of
the eyelashes (trichiasis), which contact the cornea and cause irritation.
Several rami orbitales arise dorsally from the pterygopalatine ganglion
Each eyelid margin is 2–3 mm thick from front to back. The anterior
and enter the orbit through the inferior orbital fissure. Branches leave
two-thirds is skin and the posterior third is conjunctival mucosa. A
the orbit through the posterior ethmoidal air sinus. There is strong
narrow ‘grey line’ lies anterior to the mucocutaneous junction. This
experimental evidence from studies of animals, including monkeys,
corresponds to the location of the ciliary (marginal) part of orbicularis
that postganglionic parasympathetic branches pass directly to the lac-
oculi and is an important surgical landmark, since an incision at this
rimal gland, ophthalmic artery and choroid.


point allows the eyelid to be split into anterior and posterior lamellae
along a relatively bloodless plane. The eyelashes lie in front of the grey
EYELIDS, CONJUNCTIVA AND LACRIMAL SYSTEM line, and the circular openings of the tarsal glands (Meibomian glands)
lie behind it. The tarsal glands are often visible through the palpebral
conjunctiva, when the eyelids are everted, as a series of parallel, faint
The exposed ocular surface is protected by retractable eyelids and by a
yellow lines arranged perpendicular to the lid margins.
tear film produced mainly by the lacrimal gland, with contributions
Eyelashes are short, thick, curved hairs, arranged in double or triple
from glands within the eyelid and conjunctiva (a transparent mucous
rows. The upper lashes, which are longer and more numerous, curve
membrane that covers the inner surface of the eyelid and the exposed
upwards, while those in the lower lid curve down, so that upper and
surface of the sclera) (see Fig. 42.1).
lower lashes do not interlace when the lids are closed.
The eyelids pass to adjacent facial skin without obvious demarca-
EYELIDS tion, although their limits are clearly defined in pathological conditions
such as oedema. Various skin folds or furrows are of topographical
interest. A prominent superior palpebral furrow or fold lies approxi-
The eyelids (palpebrae) are two folds of modified skin that cover the
mately opposite the upper margin of the tarsal plate and is deeply
anterior surface of the eye (Fig. 41.19). By their reflex closure, achieved
recessed when the lids are open. Asians have a skin flap, the epicanthus,
by contraction of orbicularis oculi, they protect the eye from injury and
which begins laterally in the superior palpebral fold and progresses
shield the eyes from excessive light. Periodic blinking maintains a thin
medially to cover the medial canthus. Infant Caucasians frequently have
film of tears over the cornea that prevents desiccation; movement of the
a transient epicanthus, which sometimes persists in the adult. A less
eyelids during blinking helps ensure the even distribution of the tear
prominent inferior palpebral furrow occupies a similar position in the


film and facilitates tear outflow through the nasolacrimal drainage
skin of the lower lid and deepens on downward gaze. A nasojugal
system.
furrow extends obliquely from the medial lower margin of the bony
The upper eyelid is larger and more mobile than the lower eyelid and
orbit to the cheek, and a malar furrow may be seen laterally along the
contains an elevator muscle, levator palpebrae superioris (see above). A
inferior orbital rim in middle age, but only infrequently and faintly in
transverse opening, the palpebral fissure, lies between the free margins
the young.
of the lids, which join at their extremities (termed the medial and lateral
canthus). The lateral canthus is relatively featureless. The medial canthus Structure
is approximately 2 mm lower than the lateral canthus; this distance is
From its anterior surface inwards, each eyelid consists of skin, subcuta-
increased in some Asiatic groups. It is separated from the eyeball by a
neous connective tissue, fibres of the palpebral part of orbicularis oculi
small triangular space, the lacrimal lake (lacus lacrimalis), in which a
(see Fig. 30.18A), submuscular connective tissue, the tarsal plate (tarsus)
small, reddish body called the lacrimal caruncle is situated. The caruncle
with its tarsal glands and orbital septum, and palpebral conjunctiva
represents an area of modified skin containing some fine hairs and is
(Fig. 41.20). The upper lid also contains the aponeurosis of levator
mounted on the plica semilunaris, a fold of conjunctiva that is believed
palpebrae superioris.
by some to be a vestige of the nictitating membrane of other animals.
The skin is extremely thin and is continuous at the palpebral
A small elevation, the lacrimal papilla, is located on each palpebral
margins with the conjunctiva. The subcutaneous connective tissue is
margin approximately one-sixth of the way along from the medial
very delicate, seldom contains any adipose tissue, and lacks elastic
canthus of the eye. There is a small aperture, the punctum lacrimale, in
fibres.


the centre of the papilla that forms the opening to the lacrimal drainage
The palpebral part of orbicularis oculi is subdivided anatomically
system. The margin of the eyelid lateral to the lacrimal papilla bears
into ciliary, pretarsal and preseptal parts. The palpebral fibre bundles
are thin and pale, and lie parallel with the palpebral margins. Deep to
them is the submuscular connective tissue, a loose fibrous layer that is
continuous in the upper lid with the subaponeurotic layer of the scalp;
6 effusions of blood or pus at this level can therefore pass down from the
scalp into the upper eyelid. The main nerves lie in the submuscular
1 layer, which means that local anaesthetics should be injected deep to
7 orbicularis oculi.
2
Tarsal plates
8 The two tarsal plates (Fig. 41.21A) are thin, elongated, crescent-shaped
3 plates of firm, dense fibrous tissue approximately 2.5 cm long. There is
one in each eyelid to provide support and determine eyelid form. Each
9
4 is convex and conforms to the configuration of the anterior surface of
the eye. The free ciliary border is straight and adjacent to the eyelash
5 10
follicles. The orbital border is convex and attached to the orbital septum.
The superior tarsus, the larger of the two, is semi-oval, approximately
11
10 mm in height centrally. Its inferior edge is parallel to, and approxi-
mately 2 mm from, the lid margin. The smaller inferior tarsus is nar-
Fig. 41.19 The eyelids and anterior aspect of the eyeball. Key: 1, pupil; 2, rower and approximately 4 mm in vertical height.
plica semilunaris; 3, lacrimal caruncle; 4, medial canthus; 5, conjunctiva; The tarsal plates are connected to the margins of the orbit by the
6, upper eyelid; 7, eyelashes; 8, lateral canthus; 9, lid margin; 10, iris; 11, orbital septum and by the medial and lateral palpebral (canthal) liga-


lower eyelid. (With permission from Berkovitz BKB, Moxham BJ 2002 ments (see Fig. 41.21A). The medial palpebral ligament passes from the
Head and Neck Anatomy. London: Martin Dunitz.) medial ends of the two tarsal plates to the anterior lacrimal crest and


Orbit and accessory visual apparatus
680.e1
14
RETPAHC
The palpebral fissure undergoes complex changes during infancy
(Paiva et al 2001). At birth, the upper eyelid is at its lowest position and
the margin of the lower eyelid is close to the centre of the pupil.
Between the ages of 3 and 6 months, the upper eyelid reaches its
maximum height and then declines in a linear fashion. The distance
between the centre of the pupil and the margin of the lower eyelid
increases linearly until the age of 18 months, when it stabilizes in posi-
tion. The most common pattern of the lower eyelid crease is a single
crease at birth and a double crease by the age of about 3 years. Ageing
mainly affects the size of the horizontal eyelid fissure, which lengthens
by about 10% between the ages of 12 and 25, and shortens by almost
the same amount between middle age and old age. The peak level of
growth in the horizontal dimension of the palpebral fissure is reached
between the ages of 17 and 19 years, in the vertical dimension between
10 and 13 years, and in the intercanthal distance between 14 and 16
years in Asian children (Park et al 2008).


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Fig. 41.20 The upper eyelid and anterior segment
Orbicularis oculi (orbital part) of the eye, sagittal section.
Orbital adipose tissue
Eyebrow (hair)
Levator palpebrae superioris
Accessory lacrimal gland (of Krause)
Orbicularis oculi (preseptal part)
Orbital septum
Superior tarsal muscle (of Müller)
Aponeurosis of levator palpebrae superioris
Peripheral arterial arcade Conjunctival lymphoid follicles
Small hairs of skin
Bulbar conjunctiva
Subcutaneous and submuscular Conjunctival crypts (of Henle)
connective tissue layers
Orbicularis oculi (pretarsal part)
Superior tarsus
Tarsal gland
Marginal arterial arcade
Ciliary sweat gland (of Moll)
Sebaceous gland (of Zeis)
Cilia Orbicularis Tarsal glands Mucocutaneous
(eyelashes) oculi orifices border (occlusal
(ciliary part) surface of eyelid)
A B
Levator
palpebrae
Transverse ligament superior
(of Whitnall)
Tarsal plates Fascial slips to
orbicularis oculi
Levator
aponeurosis
Medial Lateral horn
palpebral Medial horn
ligament
Lateral palpebral
ligament
Suspensory Fibrous extension
ligament to inferior tarsus
(of Lockwood)
Fig. 41.21 A, The tarsal plates and their attachments. B, The anterior insertion of levator palpebrae superioris.
the frontal process of the maxilla. It splits at its insertion into the tarsal therefore also pulls the lower lid downwards. The lower lid is capable of
plates to surround the lacrimal canaliculi, and lies in front of the naso- depressing by 4–5 mm, although it is not equipped with a striated
lacrimal sac and the orbital septum. The lateral palpebral ligament is muscle counterpart to the levator of the upper lid.
relatively poorly developed. It passes from the lateral ends of the tarsal
plates to a small tubercle on the zygomatic bone within the orbital Palpebral glands


margin and is more deeply situated than the medial palpebral ligament. Tarsal (Meibomian) glands are modified sebaceous glands embedded
It lies beneath the orbital septum and the lateral palpebral raphe of in the tarsi. They are yellow and arranged in approximately 25 parallel
orbicularis oculi. rows perpendicular to the eyelid margin in the upper lid, and slightly
The deepest fibres of the aponeurosis of levator palpebrae superioris fewer in the lower lid. They occupy the full tarsal height and are there-
are attached to the anterior surface of the superior tarsus. The superior fore longer centrally where the tarsi are higher. Each gland consists of
and inferior tarsal plates are also associated with a thin lamina of a straight tube with many lateral secretory diverticula, and opens by a
smooth muscle forming the superior and inferior tarsal muscles, respec- minute orifice on the free palpebral margin. It is enclosed by a basement
tively. Opposite the equator of the eye, the superior tarsal muscle passes membrane, and is lined at its orifice by stratified epithelium and else-
from the inferior face of levator palpebrae superioris to a fibrous exten- where by a single layer of polyhedral cells. The sebaceous secretion of
sion that projects to the upper margin of the superior tarsus (see Fig. the tarsal glands spreads over the margins of the eyelids, and so an oily
41.20). The muscle is innervated by the sympathetic nervous system and, layer is drawn over the tear film as the palpebral fissure opens after a
on contraction, elevates the eyelid. Although it may be regarded as sup- blink, reducing evaporation and contributing to tear film stability. The
plementing the action of the levator muscle, its full role is not clear. The presence of the oily, hydrophobic secretions of tarsal glands along the
mild ptosis that is a characteristic feature of Horner’s syndrome is the margins of the eyelids also inhibits the spillage of tears on to the face.
result of an interruption to the sympathetic supply to the superior tarsal Obstruction of the tarsal gland ducts by lipid and cellular debris may


muscle. A corresponding but less prominent inferior tarsal muscle in the result in lipogranulomatous inflammation and the clinical manifest-
lower eyelid unites the inferior border, and possibly also the anterior ations of an internal hordeolum or chalazion.
surface, of the inferior tarsus to the capsulopalpebral fascia, which is the Small sebaceous glands (of Zeis) and sweat glands (of Moll) are
anterior expansion of the fused fascial sheath of inferior rectus and in- associated with the eyelashes (see Fig. 41.20) and are prone to infection,
ferior oblique. Contraction of inferior rectus during downward gaze producing an external hordeolum (stye).


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Vascular supply and lymphatic drainage Conjunctival goblet cells contribute the mucin component of the
preocular tear film and play a central role in the defence of the ocular
The arterial supply to the eyelids is derived principally from the medial
surface against microbial infection. The conjunctiva is conventionally
and lateral palpebral branches of the ophthalmic artery. Their branches
divided into five regions: marginal, tarsal, orbital, bulbar and limbal
course laterally along the tarsal edges to form superior and inferior
(see Fig. 41.22). The marginal, tarsal and orbital regions are collectively
arcades (two in the upper eyelid and one in the lower). The eyelids are
referred to as the palpebral conjunctiva.
also supplied by branches of the infraorbital, facial, transverse facial
The marginal zone extends from a line immediately posterior to the
and superficial temporal arteries.
openings of the tarsal glands and passes around the eyelid margin to
The veins that drain the eyelids are larger and more numerous than
continue on the inner surface of the lid as far as the subtarsal groove
the arteries; they pass either superficially to veins on the face and fore-
(a shallow sulcus that marks the marginal border of the tarsus). The
head, or deeply to the ophthalmic veins within the orbit.
tarsal conjunctiva is highly vascular and is firmly attached to the under-
The lymph vessels that drain the eyelids and conjunctiva commence
lying tarsal plate. The orbital zone extends as far as the fornices, which
in a superficial plexus beneath the skin, and in a deep plexus in front
mark the line of reflection of the conjunctiva from the lids on to the
of and behind the tarsi. These plexuses communicate with one another,
eyeball. The conjunctiva is more loosely attached to underlying tissues
and medial and lateral sets of vessels arise from them. The lateral lymph
over the orbital zone and so folds readily. Elevations of the conjunctival
vessels drain the whole thickness of the lateral part of the upper
surface in the form of papillae and lymphoid follicles (part of the
and lower lids, and pass laterally from the lateral canthus to end in


mucosa-associated lymphoid tissue, MALT) are commonly observed in
the superficial and deep parotid lymph nodes. The lymph vessels of the
this region (Knop and Knop 2002).
medial set drain the skin over the medial part of the upper eyelid, the
Ducts of the lacrimal gland open into the lateral part of the superior
whole thickness of the medial half of the lower lid, and the caruncle;
fornix. The bulbar conjunctiva is loosely connected to the eyeball over
they follow the course of the facial vein to end in the submandibular
the exposed sclera, is thin and transparent, and readily permits the visu-
group of lymph nodes.
alization of conjunctival and episcleral blood vessels. The loose attach-
Innervation ment of the conjunctiva to the fascial sheath of the globe (Tenon’s
The sensory innervation of the eyelids comes from both the ophthalmic capsule) in this region means that the conjunctiva is freely movable
and maxillary divisions of the trigeminal nerve. The upper eyelid is here; as the bulbar conjunctiva approaches the cornea, its surface
supplied mainly by the supraorbital branch of the frontal nerve. Addi- becomes smoother and its attachment to the sclera increases. The
tional contributions come from the lacrimal nerve, the supratrochlear limbal conjunctiva extends approximately 1–1.5 mm around the
branch of the frontal nerve, and the infratrochlear branch of the naso- cornea and contains a dense network of capillaries.
ciliary nerve. The nerve supply to the lower eyelid is derived principally
Structure
from the infraorbital branch of the maxillary nerve, with small contri-
butions from the lacrimal and infratrochlear nerves. The conjunctiva is composed of an epithelial layer and an underlying
fibrous layer or substantia propria. The form of the epithelium and
thickness of the substantia propria vary with location. At the margin
CONJUNCTIVA of the lids, the epithelium is non-keratinized stratified squamous and
10–12 cells thick. The epithelium of the tarsal conjunctiva thins to
The conjunctiva is a thin, transparent mucous membrane that extends two or three layers and consists of columnar and flat surface cells.


from the eyelid margins anteriorly, providing a lining to the lids, before Near the fornices, the cells are taller, and a trilaminar conjunctival
turning sharply on itself to form the fornices; from here, it is reflected epithelium covers much of the bulbar conjunctiva. It thickens closer
on to the globe to cover the sclera up to its junction with the cornea. to the corneoscleral junction and then changes to stratified squamous
The conjunctiva thus forms a sac that opens anteriorly through the epithelium typical of the cornea. A proportion of limbal conjunctival
palpebral fissure (Fig. 41.22). At the free palpebral margins, the con- epithelial cells serve as stem cells for the corneal epithelium; this
junctiva is continuous with the skin of the eyelids, the lining epithelium region of the conjunctiva is therefore essential for maintaining corneal
of the ducts of the tarsal glands, and with the lacrimal canaliculi and integrity.
lacrimal sac (see below). The continuity between the conjunctiva and Mucous-secreting goblet cells are scattered within the conjunctival
the nasolacrimal duct and nasal mucosa is important in the spread of epithelium. They show a marked regional variation in density, being
infection. most frequent in conjunctival regions normally covered by the eyelids,
and reduced in number within exposed interpalpebral locations
(Doughty 2012).
The substantia propria, or fibrous layer, is thickest at the fornix and
thinnest over the tarsi where conjunctival attachment is firmest. With
the exception of the tarsal conjunctiva, the substantia propria adjacent
to the epithelium is mainly loose. It merges with the fibrous fascia bulbi
and episclera in the limbal and bulbar regions, and is loosely attached
Fornix to the sheaths of the recti. At the fornix, it is continuous with orbital
fascial tissues and has loose attachments to the overlying tendon of
levator palpebrae superioris and the superior tarsal muscle fascia su-
Orbital periorly, an arrangement that provides support for the fornix during
Bulbar eyeball rotation.
Vascular supply and lymphatic drainage


The arteries of the palpebral conjunctiva are derived from the vascular
Limbal
arcades of the eyelids. Many of the small vessels of the eyelid drain to
the conjunctival veins, which, in turn, pass back to the orbital and facial
veins. The substantia propria of the tarsal and orbital conjunctiva is
richly vascular. A dense capillary network is arranged in a single subepi-
Tarsal
thelial lamina, a distribution that meets the requirement of the cornea
during sleep, when oxygen is available to the cornea only through this
vascular system. The bulbar conjunctiva receives blood from the palpe-
bral arcades and from the anterior ciliary arteries that reach the con-
junctiva on the orbital surface of the recti. The veins drain by the same
Marginal route and join the ophthalmic veins.
Lymphatics drain into pre-auricular and submandibular lymph
nodes.
Innervation
The conjunctiva is innervated by sensory nerves from the same ophthal-
mic and maxillary branches of the trigeminal nerve that serve the eyelid.
Autonomic nerve fibres are abundant in the conjunctiva, particularly in
Fig. 41.22 The regions of the conjunctiva, sagittal section. bulbar and limbal regions, and are probably vasomotor in function.


Eyelids, conjunctiva and lacrimal system
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LACRIMAL SYSTEM roles in protecting the ocular surface from microbial infection (e.g.
lysozyme, lactoferrin, immunoglobulin A (IgA)) and also maintaining
The lacrimal system consists of structures responsible for the produc- ocular surface epithelial integrity.
tion of tears (principally, the main lacrimal gland with a contribution The lacrimal gland consists of orbital and palpebral parts that are
from accessory lacrimal glands) and the lacrimal drainage pathway that continuous posterolaterally around the concave lateral edge of the
collects the tear fluid and conveys it into the nasal cavity (paired lac- aponeurosis of levator palpebrae superioris. The orbital part, about the
rimal canaliculi, lacrimal sac and nasolacrimal duct) (Fig. 41.23). size and shape of an almond, lodges in a shallow fossa on the medial
The total tear volume is approximately 7 µl. It is distributed within aspect of the zygomatic process of the frontal bone, just within the
the precorneal tear film (1–2 µl), and along the upper and lower mar- orbital margin. It lies above levator palpebrae superioris and, laterally,
ginal tear strips (5–6 µl), which are wedge-shaped menisci that run along above lateral rectus. Its lower surface is connected to the sheath of
the posterior border of the lid margins and join together at the canthi. levator palpebrae superioris and its upper surface is connected to the
orbital periosteum. Its anterior border is in contact with the orbital
Lacrimal gland septum and its posterior border attached to the orbital fat. The palpe-
bral part, about one-third the size of the orbital part, is subdivided into
two or three lobules and extends below the aponeurosis of levator
The lacrimal gland is the primary producer of the aqueous component
palpebrae superioris into the lateral part of the upper lid, where it is
of the tear layer. Its secretion is a watery fluid with an electrolyte content
attached to the superior conjunctival fornix. It is visible through the
similar to that of plasma and containing several proteins that play key


conjunctiva when the lid is everted. The lacrimal gland may be congeni-
tally absent (Keith and Boldt 1986).
The main ducts of the lacrimal gland, up to 12 in number, discharge
Lacrimal gland into the conjunctival sac at the superior lateral fornix. Those from the
Superior and inferior canaliculi Upper meniscus orbital part penetrate the aponeurosis of levator palpebrae superioris
to join those from the palpebral part. Excision of the palpebral part
is therefore functionally equivalent to total removal of the gland
(Maitchouk et al 2000).
Lacrimal sac Many small accessory lacrimal glands (glands of Krause and
Wolfring) occur in or near the fornix. They are more numerous in the
upper eyelid, and their presence may explain why the ocular surface
does not dry up after extirpation of the main lacrimal gland.
Microstructure
The lacrimal gland is lobulated and tubulo-acinar in form. Its secretory
units are acini similar to those found in the salivary glands (Fig. 41.24)
Lower meniscus (Ruskell 1975). Acini consist of secretory cells that discharge their
product into a central lumen continuous with an intercalated duct
Superior and inferior puncta
formed from a single layer of epithelial cells that lack secretory granules.
Nasolacrimal duct Myoepithelial cells extend processes around the perimeter of acini and
ducts; their contraction imparts a mechanical force on the acini and
Fig. 41.23 A representation of tear dynamics on the ocular surface. Tears ducts, which promotes the expulsion of tears from the gland. The inter-
are produced by the lacrimal gland and flow within the upper and lower stices of the gland are composed of loose connective tissue that contains
meniscus. Tears drain sequentially through the puncta, canaliculi, lacrimal numerous immune cells, mainly B-lymphocytes and plasma cells (par-
sac and nasolacrimal duct. ticularly IgA-secreting cells).
Acinus
Myoepithelial cell
Secretory acinar cell
Duct
Terminal tubule
Nerve fibre
Basal lamina
Process of Parasympathetic
myoepithelial hypolemmal nerve


cell terminal
Fig. 41.24 The organization of the secretory units in the lacrimal gland.


ORBiT And ACCEssORy visuAl APPARATus
684
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nOiTCEs
Vascular supply
The lacrimal gland receives its arterial blood supply from the lacrimal
branch of the ophthalmic artery. It may also receive blood from the in- Lacrimal gland
fraorbital artery. Venous drainage is into the superior ophthalmic vein.
Innervation Accessory
lacrimal
The lacrimal gland is innervated by secretomotor postganglionic para- glands
sympathetic fibres from the pterygopalatine ganglion (Ch. 32). They
reach the gland either via zygomatic and lacrimal branches of the maxil-
lary nerve, or by passing directly from the ganglion (Ruskell 2004).
Sympathetic fibres that issue from the superior cervical ganglion also
supply the lacrimal gland. These fibres may be involved in the regula-
tion of blood flow and the modulation of gland secretion (Dartt 2009). Protein,
Tear film ions, H2O
Conjunctival
lacrimation reflex epithelium
Protein,
The lacrimation reflex is stimulated by irritation of the conjunctiva and Eyelid ions, H2O
cornea, which elicits a large increase in tear volume (Situ and Simpson
2010). The afferent limb of the reflex involves branches of the ophthal-
mic nerve, with an additional contribution from the infraorbital nerve Tarsal gland
Corneal
if the conjunctiva of the lower eyelid is involved. Impulses enter the epithelium
brain and spread by interneurones to activate parasympathetic neur- Lipid Conjunctival
ones in the superior salivatory nucleus and sympathetic neurones in the epithelium
upper thoracic spinal cord. The efferent pathway to the lacrimal gland
involves the greater petrosal nerve, which carries preganglionic para-
sympathetic secretomotor fibres, and the deep petrosal nerve, which Tarsal gland
conveys postganglionic sympathetic fibres; the parasympathetic fibres
relay in the pterygopalatine ganglion, and the sympathetic fibres pass Mucus
through the ganglion without synapsing.
Lacrimation may also occur in response to emotional triggers Goblet cells


without any irritation of ocular structures, when it may be accompanied
Fig. 41.25 Orbital glands that contribute the various components of the
by alterations in the mimetic facial muscles, vocalizations and sobbing
preocular tear film. The aqueous component (consisting primarily of
(Gračanin et al 2014).
proteins, ions and water) is produced by the main and accessory lacrimal
glands. Tarsal glands in the eyelids produce the lipid layer of the tear film,
Preocular tear film
and the mucous component is derived from conjunctival goblet cells.
The tear film is a complex fluid that covers the exposed parts of the ocular
surface framed by the eyelid margins. Classically, the tear film has been
The mucosa lining the canaliculi has a non-keratinized stratified
regarded as a trilaminar structure, with a superficial lipid layer (secreted
squamous epithelium lying on a basement membrane, outside which
by the Meibomian glands) that overlies an aqueous phase (derived from
is a lamina propria rich in elastic fibres (the canaliculi are therefore
the main and accessory lacrimal glands) and an inner mucinous layer
easily dilated when probed). Striated muscle fibres of orbicularis oculi
(produced mainly by conjunctival goblet cells) (Fig. 41.25). The tear
interweave on each side of the canaliculus in a manner suggesting a
film performs a number of important functions. By smoothing out
sphincter-like spiralling, and supporting the claim that lumen size is
irregularities of the corneal epithelium, it creates an even surface of good
regulated on blinking, possibly facilitating tear drainage.
optical quality that is reformed with each blink. The air–tear interface
The lacrimal sac is the closed upper end of the nasolacrimal duct. It
forms the principal refractive surface of the optical system of the eye.
is approximately 12 mm long and lies in a fossa in the lacrimal bone
Since the cornea is avascular, it is dependent on the tear film for its
in the anterior part of the medial wall of the orbit (see Fig. 41.2). The
oxygen provision. When the eye is open, the tear film is in a state of


sac is bounded in front by the anterior lacrimal crest of the maxilla and
equilibrium with the oxygen in the atmosphere, and gaseous exchange
behind by the posterior lacrimal crest of the lacrimal bone. Its closed
takes place across the tear–epithelial interface. The constant turnover of
upper end is laterally flattened, its lower part is rounded and merges
the tear film also provides a mechanism for the removal of metabolic
into the duct, and the lacrimal canaliculi open into its lateral wall near
waste products. Tears play a major role in the defence of the eye against
its upper end.
microbial colonization; the washing action of the tear fluid reduces the
A layer of lacrimal fascia, continuous with the orbital periosteum,
likelihood of microbial adhesion to the ocular surface, and the tears
passes between the lacrimal crest of the maxilla and the lacrimal bone.
contain a host of protective antimicrobial proteins.
It forms a roof and lateral wall to the lacrimal fossa and separates the
lacrimal sac from the medial palpebral ligament in front and the lac-
Lacrimal drainage pathway
rimal part of orbicularis oculi behind. A plexus of minute veins lies
between the fascia and the sac. The upper half of the lacrimal fossa is
There is a constant turnover of tears; production is matched by elimin- related medially to the anterior ethmoidal sinuses, and the lower half
ation. Some tears are lost by evaporation or absorption across the to the anterior part of the middle meatus. The lacrimal sac has a fibro-
conjunctiva, but the majority are eliminated via the nasolacrimal drain- elastic wall and is lined internally by mucosa that is continuous with
age system (see Fig. 41.23). Tears collect at the medial canthal angle, the conjunctiva through the lacrimal canaliculi, and with the nasal
where they drain into the puncta of the upper and lower lids, which mucosa through the nasolacrimal duct (Paulsen 2003).
are directed towards the surface of the eye to receive tear fluid. From The nasolacrimal duct is approximately 18 mm long, and descends


each punctum, tears drain into lacrimal canaliculi. There is one canal- from the lacrimal sac to open anteriorly in the inferior meatus of the
iculus, approximately 10 mm long, in each lid. Each canaliculus first nose at an expanded orifice. A fold of mucosa (plica lacrimalis) forms
passes vertically from its punctum for about 2 mm and widens to form an imperfect valve just above its opening (ostium lacrimalis). The duct
an ampulla, before passing medially towards the lacrimal sac. The runs down an osseous canal formed by the maxilla, lacrimal bone and
superior canaliculus is smaller and shorter than the inferior. The canal- inferior nasal concha. It is narrowest in the middle and is directed
iculi almost always unite to form a common canaliculus before reaching downwards, backwards and a little laterally. The mucosa of the lacrimal
the lacrimal sac. Congenital absence of the lacrimal puncta, as well as sac and the nasolacrimal duct has a bilaminar columnar epithelium,
supernumerary lacrimal puncta and canaliculi, have been described which is ciliated in places. A rich plexus of veins forms erectile tissue
(Satchi and McNab 2010). around the duct; engorgement of these veins may obstruct the duct.


685
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Key references
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formed by the lacrimal gland, conjunctiva and lacrimal drainage system.
Adv Exp Med Biol 506B:635–44.
A review of the structural organization of eye-associated lymphoid tissues
and their role in the immune defence of the eye.


CHAPTER
42
Eye
The outer surface of the eye is composed of parts of two spheres with choroid lying posteriorly, a thicker ciliary body and an anterior iris
different radii. The anterior segment, part of the smaller sphere, is that is displaced from the outer coat and terminates at the pupillary
formed by the transparent cornea and accounts for approximately 7% aperture. The internal surface of the choroid is covered by the photo-
of the ocular surface. The posterior segment of the eyeball is part of the sensory retina, which terminates anteriorly at the ora serrata; the latter
larger sphere formed by the opaque sclera (Fig. 42.1). also marks the junction between the ciliary body and choroid. The
Internal to the sclera is a vascular, pigmented layer consisting of vasculature of the choroid supplies nutrients to the avascular outer
three continuous parts that collectively make up the uveal tract: a thin retina.
Aqueous fluid
in anterior chamber
Corneal
surface
Iris
Eyelash
Posterior chamber
Follicle and
sebaceous gland
Limbus
Orbicularis oculi Scleral venous
sinus
Tarsal gland (of Schlemm)
Tarsal plate Ciliary body
and muscles
Conjunctiva
Zonular ligament
Lens
Ora serrata
Pigmented Anterior surface of
epithelium vitreous body
of retina
Hyaloid canal
Neural layer
Extraocular muscle
of retina
Retinal blood vessels
Nerves and vessels
of choroid
Suprachoroid
lamina
Sclera
Central retinal
Choroid artery and vein
Long ciliary
Vitreous body artery and nerve
Fovea centralis in centre Short ciliary
of macula lutea artery and nerve
Area of optic disc Dura
Arachnoid
Pia
Optic nerve fibres
Optic nerve
686 Fig. 42.1 The left eye and part of the lower eyelid in horizontal section, viewed from above. Parts have been cut away to show internal structures.