بحث انجليزى طبى عن العيون - كتبته بنفسىEnglish--Eye physiology

[mohamed_ameer]

بحث انجليزى طبى عن العيون - كتبته بنفسىEnglish--Eye physiology

PHYSIOLOGY OF THE EYE--- ENGLISH LANGUAGE

علم وظائف لبعيون


جفون العين

The eyelids

The eyelids form one of the most important elements in the protective system of the eye. This protective function is mediated by the screening and sensing action of the cilia and is mediated by the secretions of the glands of the eyelids, and the movements of the lids.



The skin of the eyelids is thinner than that of any other part of the body. This thin skin folds easily and permits rapid opening and closing of the palpebral fissure. The lids are covered with fine lanugo hairs, which may escape notice on casual inspection.. these fine hairs may be responsible for corneal irritation if the skin of the lids comes into contact with the eyeball. In each lid there is a wide fibrous tarsal plate that follows the curvature of the eyeball and is firmly attached to the medial and lateral palpebral ligaments and to the orbital septum. The orbicularis muscle and the skin lie anterior to the tarsal plate. The tarsus is covered by conjunctiva posteriorly.



The CILIA AND EYEBROWS

The first line of defense in the protective function of the lids is made up of the cilia and the eyebrows. The cilia , or eyelashes are hairs situated on the margin of the lids. These are disposed in two rows, totaling about 100 to 150 cilia in the upper lid and half that number in the lower lid. In the upper lid the cilia are curved outward and upward and in the lower lid, outward and downward. Each cilium is a short, stout, cylindric hair growing from a typical hair follicle. Each follicle is surrounded by a nerve plexus with a very low threshold of excitation . .. Touching a cilium is sufficient to excite the nerve plexus of the follicle and produce a reflex blink….

The base of each cilium is surrounded by sebaceous glands (the glands of Zeis) , which open into the hair follicle by short, wide ducts…Infection of these glands results in the common stye , or hordeolum….Excessive and altered secretion of the glands produces marginal blepharitis.. It is not surprising that the lid margins and the cilia can be involved in many of the same disease processes that affect the scalp and its hair—for example , seborrhea…. The pigmentation of the cilia is often deeper than that of the scalp hair throughout adult life…occasionally cilia turn gray or white with advancing age, and in some disease conditions the newly formed lashes fail to become pigmented and stay white…This is known as poliosis. .. the average life of each cilium is from 3 to 5 months, after which it falls out and a new one grows in to take its place…if a cilium is pulled out, the new one replacing it reaches full size in about 2 months…if the cilia are cut short, as is often done preceding operations on the eye, growth is so rapid that the lashes appear nearly normal in a few weeks…

Not infrequently a loose cilium will find its way into the upper or lower lacrimal punctum, and the protruding end scratches the cornea,causing considerable pain…. When a patient complains of a foreign body in the eye and none is found, it is always wise to look for such cilia—they may be easily missed…occasionally a cilium becomes twisted and turned back toward the skin of the lid, which it punctures, and then it grows into the skin…cilia have often been reported in the anterior chamber following penetrating injuries…









The eyebrows are elevated by the frontails muscle, depressed by the orbicularis muscle in forced lid closure, and drawn together in the act of frowning by the corrugator supercilii,, they are not moved in the ordinary act of blinking… the eyebrows can be voluntarily elevated without the globes turning upward, but extreme upward gaze carries with it an elevation of the brows…extreme upward gaze therefore involve three synergistic actions : upward rotation of the globes, elevation of the lids, and elevation of the eyebrows….in nuclear or infranuclear seventh (facial) never paralysis the eyebrow on the paralyzed side is lower than that on the unaffected side; in the presence of unilateral ptosis the eyebrow on the involved side is frequently elevated as the frontalis muscle contracts in an effort to keep the upper lid raised….



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(lacrimal gland, palpebral portion)

(lacrimal gland,orbital portion)

(levator palpebrae superioris)

(Mueller's muscle)

(superior tarsus)

(lateral palpebral ligament)

(septum orbitale)

(inferior tarsus)

(medial palpebral ligament)



(orbital portion of lid)

(tarsal portion of lid )

(lateral canthus)

(tarsal portion of lid)

(inferior palpebral furrow)

(orbital portion of lid)

(superior palpebral furrow)

(caruncle)

(medial canthus)



SECRETIONS OF THE EYELIDS

The secretions of the glands of the eyelids form the second complex of their protective system ..Chief among these is the oily layer secreted by the meibomian glands, which are large sebaceous glands located in the tarsal plates; there are approximately 30 of these glands in each tarsus… they are oriented perpendicular to the lid margins, with the ostia of the glands lying in a single row on the lid margin.posterior to the two rows of cilia….when the eyelid is everted, these glands may appear as light parallel strips beneath the tarsal conjunctiva…Mild pressure over the glands causes sebum to exude from the openings on the lid margins…the oily layer secreted by these glands forms the most superficial element of the precorneal tear film and functions to prevent evaporation of tear fluid as well as to prevent spillage of tears at the lid margins…









The eyelids also contain accessory lacrimal gland tissue, the glands of Krause and Wolfring.







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(lacrimal caruncle)

(lacrimal papillae)

(semilunar fold)

(lacrimal lake)

(lacrimal punctum)

(front edge of lid )

(rear edge of lid)

(gray line)

(orifices of tarsal glands)



NORMAL MOVEMENTS OF THE EYELIDS

The third and most important element in the protective action of the eyelids is the motor system, which depends on the levator palperbrae superioris, the orbicularis oculi, and the smooth muscles of Muller in the upper and lower eyelids… the movements of the eyelids, opening and closing may be voluntary or reflex…



ELEVATION

When the eyelids are opened, the upper lid is raised approximately 10 mm against gravity and it is drawn back under the orbital rim at the fold of the lid…this action takes place by contraction of the levator palpebrae superioris, which is innervated by the third cranial nerve… the levator tendon inserts into a broad area of the skin, from the free edge of the lid to the top of the tarsal plate…a few fibers insert into the anterior surface of the tarsal plate…in their passage to the skin, tendinous fibers of the levator interdigitate with bundles of fibers of the orbicularis muscle…..the fold of the upper eyelid is formed by retraction of the skin when the lid is raised…in oriental people this fold is absent because the terminal fibers of the levator palpebrae do not run forward to the skin, but rather insert only on the anterior surface of the tarsal plate……elevation of the upper lid is assisted by smooth muscle fibers that arise from the under surface of the levator and insert in the upper margin of the tarsus…these fibers, supplied by nerves of the sympathetic system, are called Muller's muscle of the eyelid…..the lower lid has an inferior palpebral muscle of Muller, which arises from a fascial expansion of the inferior rectus and breaks into two parts, one inserting in the conjunctiva of the lower fornix and the other entering the lower lid to insert in the tarsal plate….contraction of these fibers exerts a direct pull between the tarsus and the undersurface of the globe…













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(Muller's muscle)

(orbicularis muscle)

(peripheral arcade)

(descending branch)

(Meibomian gland in tarsal plate)

(ascending branch)

(marginal arcade)

(levator palpebrae superioris)

(superior rectus)

(anterior ciliary artery)

(circulus major )

(conjunctiva)

(circulus minor)



The levator is the chief muscle producing elevation of the upper lid; its activity is usually associated with contraction of the superior rectus…in extreme upward gaze the frontalis muscle aids the levator in further elevating the lid… when the gaze is directed upward, the lid follows the upward movement of the globe…the levator and superior rectus muscles are innervated by branches of the same nerve and are connected by a common fascial sheath…these relations account for the frequent association of congenital ptosis and paralysis of the superior rectus, a fact especially important to remember when surgery for ptosis is contemplated…there is excellent evidence derived from clinical observation that the levator muscles of the two upper lids behave as yoke muscles in that they act as a team or pair and that Hering's law of equal innervation of yoke muscles applies… when the levator on one side is weak, as in unilateral myasthenia gravis or unilateral congenital ptosis, the lid on the unaffected side may be retracted in an unconscious attempt to elevate the ptotic lid …if the eye with ptosis is covered, relieving the necessity of effort to elevate the lid, retraction on the normal side disappears, showing that there has been abnormally great innervation stimulus to both levators…on the other hand , lid retraction of local origin may result in pseudoptosis of the sound eye; when the eye with retraction is covered, relieving the unconscious attempt to lower the affected lid, the pseudoptosis of the sound eye disappears…..although the upper lid follows the globe on voluntary upward gaze, in the reflex act of blinking the globe and lid move in opposite directions…. The globe turns upward as the eyelids close and downward as they open again….the upward movement of the eyeball during the act of blinking rotates the anteroposterior axis approximately 15 degrees above the horizontal….. in operations on the superior rectus the fascial connections of this muscle with the levator must be kept….thus if the insertion of the superior rectus is recessed on the globe, the levator will be carried with it, so the upper lid will be raised and the fissure will be widened….on the other hand, if the superior rectus is advanced or resected, the levator will be pulled forward and the eyelid will droop , creating ptosis…the synergism between the superior rectus, frontalis, and levator is sometimes employed in operations for the relief of ptosis….it might be supposed that the eyelid would be kept from drooping by the action of the sympathetically innervated smooth muscle when the levator is paralyzed…this does not occur,however because the fibers of Muller's muscle have their origin from the undersurface of the levator…when the latter is paralyzed, the smooth muscle fibers have no firm origin from which to pull and their contraction becomes ineffective; the ptosis is therefore usually complete…if the sympathetic nerve supply is paralyzed, only a slight droop of the lid results, as seen in Horner's syndrome…degeneration or interruption of the sympathetic nerves results in hypersensitivity to adrenergic agents, so the lid elevates when 1:1000 epinephrine is instilled into the conjunctival sac…this dosage does not affect normally innervated smooth muscle…pharmacologic paralysis of Muller's muscle by topical application of guanethidine is used in the treatment of lid retraction…..not only does the upper lid follow upward rotation of the eyeball, but it also follows downward movements by relaxation of tonus in the levator muscle. The studies of Bjork very clearly demonstrate this point…down ward motion of the lid is not caused solely by gravity; it also occurs when a person is placed on a tilted table with his head lower than his feet… the mechanism is not clear..





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(simultaneous electromyographic recording from levator muscle (upper trace) and orbiculars (lower trace) during repeated voluntary blinking )



Closure

The eyelids are closed by the action of the orbicularis oculi, supplied by the seventh cranial facial nerve ..the function of the orbicularis is reviewed in detail by Hoyt and Loeffler…Anatomically there are two main portions of the orbicularis muscle; the palpebral and the orbital…the palpebral portion of the muscle overlying the tarsus and the orbital septum, is used in the acts of blinking and voluntary winking…when the eyelids are forced shut, such as occurs in blepharospasm , the orbital portion of the muscle is brought into play along with the muscles of the eyebrow… the two portions of the orbicularis are differentiated physiologically by their chronaxie…the chronaxie of the palpebral portion is about half that of the orbital portion; this agrees well with the general rule that muscles designed for rapid movement have a lower chronaxie than those of slower but more forcible action… three distinct types of closure of the eyelid are effected or affected by different combinations of fiber bundles of the orbicularis working together with the muscles that control the eyebrows…these are blinking, voluntary winking and blepharospasm…







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(right temporalis)

(right zygomaticus)

(right buccalis)

(right marginalis mandibulae)

(six patterns of branching of facial nerve)











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Search on internet or look for meanings of these words that are connected with physiology of the eyes

(blinking , reflex blinking, spontaneous blinking, voluntary winking, blepharospasm, movements of the eyelids, the palpebral fissure, the eyelid during sleep)





Blinking

There are two types of blinking :- blinking of reflex origin and spontaneous blinking, probably of central origin……

Reflex blinking

Many different stimuli cause reflex blinking. Strong lights ; the sudden approach of an object toward the eyes ; loud noises ; touching of the cornea , conjunctiva , or lashes; and other stimuli call forth rapid reflex closure of both eyes…it is possible , therefore, to divide reflex blinking into at least three different subtypes- tactile, optic, and auditory – depending on the nature of the stimulus… the cornea reflex is an example of a tactile stimulus… any object touching the unanesthetized cornea will produce a reflex blink… the dazzle reflex is produced by shining a bright light into the eye. ( in some persons this elicits a fit of sneezing as an additional reflex…it is not uncommon for a patient to complain that he sneezes when he first goes outdoors into bright sunlight, and occasionally a patient sneezes when an ophthalmoscope light is turned on his eye )….. the so – called menace reflex is produced by an unexpected or threatening object coming suddenly into the near field of vision……. In the tactile corneal reflex the afferent path way is the fifth cranial nerve and the efferent pathway the seventh cranial nerve…. The reflex persists in the thalamic animal….in humans connections with the cortex are shown by the pain felt on touching the cornea and the strong spasm of the eyelids that occurs when such pain is produced…. The reflex may be lost if there is a cortical lesion in the rolandic area….. the corneal reflex is characteristically lost or impaired on the side of a tumor of the cerebellopontile angle…. This is a diagnostic sign of great importance because the corneal reflex is lost before impairment of the other branches of the fifth nerve is clinically detectable….. when testing for corneal sensitivity or blink reflex, it is important to remember that the cornea is easily traumatized …a wisp of cotton may be introduced from the side so close to the eye that the patient is hardly aware of its presence and does not blink in response to the menace reflex…..contact lens wearers frequently experience abolition or diminution of the tactile corneal reflex… if it were not for this decreased corneal response, failures in adapting to contact lenses would be much more frequent than they are….. the afferent pathway of menace and dazzle reflex blinking is the optic nerve…the dazzle reflex appears to subcortical…. Association fibers to the facial nucleus form the efferent path… the dazzle reflex may be lost in certain mensencephalic lesions that give no other external signs…the menace reflex is cortical and requires not only the presence of the occipital lobe, but also of its connections with the rolandic area…because of the cortical involvement , this reflex may be lost when the corneal tactile and dazzle reflexes persist….









A fourth type of blink is in response to stretching or striking structures near the orbit… the orbicularis reflex blink has two components… a fast component occurs as a response to proprioceptor afferents in the fifth and seventh nerves, and a slow, nociceptive component is afferent in the fifth nerve…



Spontaneous blinking :- it is the common form of blinking that occurs in a normal human being at frequent intervals during the walking hours and without any obvious stimuli… each person seems to have his own own individual rate of blinking, which is maintained as long as the external environment is not changed… the rate may be altered by changed in the surroundings or by the mental state of the subject; if there is any cause for excitement, the blink rate is usually increased considerably…. Spontaneous blinking does not occur or is very infrequent during the first few months of life ; yet the delicate infant cornea does not suffer from dryness…blinking occurs in all vertebrates possessing eyelids and living in air…its rate varies considerably in the animal kingdom…the lion blinks at a rate of less than 1 blink/min, whereas some species of monkeys have a rate of 45 blinks/min… the blink rate continues in persons who are blind and thus does not depend on retinal stimulation… it has been suggested that blinking provides rest for the ocular muscles in that blinking allows a momentary upturning of the eyes, which is regarded as a position of rest… this is analogous to the position of the eyes during sleep….Blinking has also been regarded as a conditional reflex because it is a frequently repeated response to indifferent sensory stimuli….

Motion pictures of blinking show that the lower lid remains almost stationary during the act… the upper lid begins to close as a shade would drop… the movement is completed by a narrowing of the palpebral fissures in a zipper like action from the later canthus toward the medial canthus….this aids the displacement of the tear film toward the lacrimal puncta…most persons blink about 15 times/min…the duration of a full blink is approximately 0.3 to 0.4 second… the average period between blinks is about 2.8 seconds in men and just under 4 seconds in women….during the period of blinking ,, vision must be in abeyance; therefore if blinking is occurring frequently enough, it might have some practical bearing on occupations in which constant perception is a necessity, such as piloting high-speed aircraft….. spontaneous blinking does not produce a discontinuity of visual sensation, despite the fact that vision is obviously interrupted during the blink…in experimental situations, obscuration of vision of 0.003 second or 1/100 of a blink duration is barely detectable by the subject at daylight levels of illumination ; a darkening of the entire visual field for 0.03 second by means other than spontaneous blinking is easily noticed… the continuity of visual sensation during the spontaneous blink is similar to the continuity of visual sensation during saccadic (rapid) eye movements, when vision is also suspended to a large degree….. by means of electromyographic studies, it has been shown that there are three main functional groups of motor units in the orbicularis oculi : 1- those responding in blinking and in corneal reflex, 2- those responding in blinking and in sustained activity and 3—those responding only in sustained activity… there is considerable anatomic overlap among the distributions of these three groups of motor units… those in the first group lie mainly in the pretarsal region and those in the second group lie mainly in the preseptal region, whereas those in the third group extend from the preseptal region to the orbital region… during blinking the units in the first group discharge brief bursts of impulses at very high impulse frequency-up to 182/sec in a single unit……







This is of the order of the highest frequencies of discharge observed by Reid in the extraocular muscle montoneurons of the cat under reflex drive… the extraocular muscles have a very brief contraction time; this may also be true of the white fibers of the orbicularis, which are mainly in the pretarsal portion…this extremely rapid movement is capable of intermittence at very high frequencies, since voluntary blinking in humans has been recorded at a maximum frequency of 390/min…. the maintenance of the narrowed palpebral fissure after a blink depends on the activity of units in other parts of the muscle and on reciprocal innervation (relaxation of the levator )….. the first change to occur in a blink is a relaxation of the levator rather than contraction of the pretarsal part of the orbicularis… the preliminary relaxation of the antagonist is not a recognized feature of muscular contractions generally, but it may have significance in this unusual movement in that it allows the orbicularis to contract from the start against reduced resistance; this tends to shorten the time during which vision is disturbed….



Voluntary winking :-

A wink is a forced closure of one eye.. voluntary winking is produced by stimultaneous contraction of the palpebral and orbital portions of the orbicularis muscle… many persons can not wink until they are taught, and some never learn to close one eye at a time… occasionally a subject may learn to wink with one eye but not with the other… it has been noted that more people have difficulty in voluntary winking with the right eye than with the left… this has been attributed to the predominance of right-handedness, the theory being that there is greater development of the uncrossed fibers in the path of the facial nerve from the cortical centers to the subcortical centers on the left side in right handed persons…the act can be repeated at frequent intervals, but even at a maximum rate there must be a minimum period between winks of about 0.3 second….



Blepharospasm

Squeezing the eyelids together entails contraction of both portions of the orbicularis as well as of the muscles of the brow… since this invariably raises the intraocular pressure, it is dangerous to have a patient squeeze the eyelids shut during any surgical procedure in which the globe is opened, such as in a cataract extraction… even the speculum best designed for keeping the lids apart during eye surgery will not prevent the pressure of squeezing the eyelids from being transmitted to the globe, thereby making it possible to forcibly expel the vitreous.. for this reason the branches of the seventh cranial nerve supplying the orbicularis muscle are usually temporarily paralyzed by injections of anesthetic solution prior to opening of the globe… the branches of the nerve may be injected as they course into the eyelids, or the nerve trunk may be injected instead… the trunk of the temporal and zygomatic branches is found by following the zygoma to a point just in front of and slightly below the external auditory meatus, where the anesthetic may be injected….













.blepharospasm is a frequent accompaniment of inflammatory disease of the anterior segment of the globe and makes examination of the eye difficult…many patients find it extremely hard to keep both eyes open when one is being examined.. particularly if that eye is light sensitive, as most inflamed eyes are… they tend to keep the uninvolved eye closed, with the result that both eyes turn upward (Bell's phenomenon).. the patient should be told to keep both eyes open at all time and, if possible to fix his attention on some object in the room…. If a patient is afforded a definite fixation point, he will find it easier to keep the unexamined eye open than if he is allowed to look off into space…fibrillary twitching of the eyelids is a common complaint of some persons… it may be related to fatigue, thyrotoxicosis, or mental tension, but refractive error is frequently the cause… when marked, it is called myokymia of the eyelids… the focus of irritation is thought to be in the nerve fibers of the orbicularis muscle….. Weakness of the orbicularis is seen in myotonic dystrophy and other diseases that affect facial musculature…in myasthenia gravis the orbicularis may fatigue during the day, so ectropion and epiphora appear by afternoon….



Pathways for movements of the eyelids:-

In the frontal cortex close to the oculogyric centers there is an area where stimulation produces raising of one eyelid or both… this eyelid elevation is generally greater on the side opposite the stimulated side… stimulation of the region of the motor cortex close to the representation for thumb results in closure of the eyelid, usually bilaterally, but greater on the side opposite the stimulation…in monkeys lesions of this general region result in inability to close the eyes… the fact that closure of the eyes is not obtained from the same portion that causes movements of the face, is understandable when it is recalled that the extrinsic eye muscles are supplied by the third, fourth, and sixth cranial nerves; the orbicularis muscle is supplied by the seventh cranial nerve, which also supplies the muscles of the face…. The pathway from the frontal cortex to the brain stem nuclei that control movements of the eyelids is not known…lesions in the region of the posterior commissure frequently produce an association of paralysis of upward gaze and changes in motility of the eyelids- either ptosis or retraction… on the other hand, the paths for conjugate lateral or vertical movements of the eyes and those for the eyelids must be separate, since lesions in the pons or tectum may separately abolish movements pf either the eyelids or the eyes…in lower animals the centers for lid movement may continue to function in response to visual stimuli after removal of the occipital lobes… this has been demonstrated in dogs and monkeys…….



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(subdivisions of frontal eye field and area yielding closure of eyes in monkey)

(outer aspect of brain of chimpanzee, showing position of motor centers..electric stimulation at parts that cause coordinate movements of corresponding muscle groups.)

(diagram of corticobulbar fibers serving facial nuclei)











Associated movements of the eyelids:-

Associated movements of the upper lid and the superior rectus muscle have been mentioned previously… whenever the gaze is directed upward from the horizontal, the upper lid follows the movements of the globe… there is also an association between the movements of the eyelids and the globes when the eyes are closed…in the majority of the persons the eyes are turned sharply upward when the eyelids are forcibly closed… this is a protective action that brings the cornea up under the covering eyelid and away from impending danger… it is called Bell's phenomenon …Bell's phenomenon is not present in 10% of otherwise healthy persons, and therefore its absence is not necessarily a sign of disease… although the pathway for Bell's phenomenon is not known, it is known that the pathway is different from that for voluntary upward gaze, since Bell's phenomenon may be present in supranuclear paralysis of upward gaze…. Other movements of the eyelids and the simultaneous contraction of the ocular or facial muscles form associated movements under pathologic conditions… an interesting phenomenon of the eyelid known as the pseudo-Graefe phenomenon occasionally occurs following recovery from paralysis of the third cranial nerve…..in straight – ahead gaze there is slight ptosis on the side of the previous palsy of the cranial nerve… the ptosis persists when this eye is abducted, but disappears and is even replaced by excessive widening of the fissure when the eye is adducted … the cause of the pseudo-Graefe phenomenon is misdirection of regenerating nerve fibers intended for the medial rectus muscle into the third nerve bundle going to the levator muscle… the ptosis in the primary and abducted positions represents a residual weakness of the levator…in adduction, on the other hand, the medial rectus is innervated and the misdirected fibers that have now reached the levator succeed in elevating the lid normally or even excessively…… another abnormal phenomenon of the eyelid is that known as jaw winking, or the Marcus Gunn phenomenon … it consists of opening and shutting of the eye on one side during the act of chewing… the spontaneous movements of the eyelid are best produced by requesting the subject to move the lower jaw horizontally across the upper jaw… apparently the pterygoid muscle, which is involved in this motion, is abnormally linked neurologically in some fashion with the levator muscle on that side… thus when the pterygoids are innervated , the surplus innervation flows to the levator and causes the lid retraction… although it has frequently been accepted that this association movement is caused by the misdirection of fibers intended for the pterygoid into the levator, it is difficult to see how such misdirection could occur … it may be significant that the cortical locus for elevation of the eyelid lies very close to the locus for jaw movements, and the fact that the condition is congenital may indicate that the defect is in the cortex and not further down in the pathways for innervation of these two areas….























The palpebral fissure :-

The normal adult palpebral fissure is usually 27 to 30 mm long and when the eyes are in primary position 8 to 11 mm wide.. the maximum width is medial to the center of the palpebral fissure, making the fissure slightly almond shaped… the maximum excursion of the upper lid without frontalis action widens the fissure to about 15 mm.. frontalis action adds another 2 to 3 mm of upward movement… the lower lid moves about 5mm from downward to upward gaze… no difference in these measurements was found between men and women or between blacks and whites… although orientals were not included in the series, they show characteristic almond or slit-shaped palpebral fissure…. In children the fissures are not so long, but are relatively wider, and in infants the fissures may be nearly circular…. Normally the edge of the upper lid covers the upper limbus from 10 to 2 o'clock… the edge of the lower lid is more variable in position, but usually lies less than 1mm below the lower limbus… if the upper limbus is completely exposed, the fissure is wider than normal… if this exposure is bilaterally symmetric, it may have little significance, the eyeballs being equally prominent …unequal palpebral fissures generally have pathologic significance…. Since the position of the eyeball in the orbit determines the width of the fissure to a large degree, the more prominent globe will produce a wider fissure…hence anything that causes one eyeball to protrude will lead to a wider fissure on that side… similarly, if one eye is abnormally recessed in the orbit or is smaller in size, the fissure on that side will be narrower…. The width of the fissure may reflect the psychologic state of the individual ; for example the wide- open eyes of surprise and fright are often seen in chronic anxiety states.. the intensity of light incident on the eyes may account for narrowing of the palpebral fissures…..the width of the fissures also depend on the combined tonus of the muscles- that is the levator palpebrae superioris and the sympathetic-innervated Muller's muscle, which raise the upper eyelids, and the orbicularis muscle, which closes the eyelids… when a person is fatigued, the levator muscle loses its tonus, the fissure becomes narrower, and the lids feel heavy because the levator muscles have to be activated by a willed effort…one thus feels the annoying sensation of the upper lids drooping as one tries to stay awake… people remark that they are so tired that they can hardly keep their eyes open….widening of the palpebral fissures caused by retraction of the upper eyelids is responsible for the greater part of the apparent exophthalmos seen in thyrotoxicosis… the distribution curves of exophthalmometer measurements in normal persons and in a group of patients of the same age with diffuse toxic goiter are not dissimilar.. normal persons measure from 12 to 21 mm, with a mean of 16 mm… those with toxic goiter measure from 12 to 24 mm, with a mean of 18 mm… However, only 5% of normal persons have measurements greater than 19 mm, whereas in 32 % of patients with toxic goiter the measurements are greater than this…nevertheless, the principal element in the prominent appearance of the eye of patients with diffuse toxic goiter is the lid retraction… the amount of exophthalmos should be determined only byexophthalmometer measurements and should not be estimated by the width of the palpebral fissure….















Widening of the palpebral fissure, known as Dalrymple's sign , may be caused by either excessive tonus of the sympathetically innervated Muller's muscle or excessive tonus of the levator palpebrae.. most of the evidence supports the former… although there are no accompanying papillary signs of increased sympathetic tonus… the fact that both the upper and lower lids are retracted in thyrotoxicosis supports the opinion that the sympathetically innervated Muller's muscles are primarily concerned… additional evidence is found in the fact that the lid retraction in endocrine exophthalmos can be reduced by instillation of topical sympatholytic agents such as guanethidine.. Von Graefe's sign or " lid-lag"- inability of the upper lid to follow the movements of the globe in downward gaze –is also caused by retraction of the upper eyelid , resulting from the sympathicotonia… when marked signs of thyrotoxicosis are present, the widening of the palpebral fissure and slowness of the eyelid to close cause a characteristic appearance of apprehension or fright …in type of disease in which there may be little evidence of active thyrotoxicosis , edema and lymphocytic infiltration of the ocular muscles and the orbital tissues create a true protrusion of the globe… the exophthalmometer measurements in this type of exophthalmos are typically greater than in toxic thyroid disease….. retraction of the eyelid (tucked lid. Collier's sign), if thyroid disease has been excluded, may be caused by an increased tonus of the levator resulting from a lesion of the posterior commissure… a lesion lower in the brain stem results in ptosis rather than lid retraction…. The eyelids owe their form chiefly to the tarsal plates, which are held against the globe by the overlying portion of the orbicularis oculi, a thin, striated muscle.. the palpebral portion of the orbicularis is strongly attached to the medial palpebral ligament, which moors it to the nasal orbital margin… the domed form of the normal tarsus depends on pressure of the lid against the globe… a prosthesis must be fitted after enucleation of the eye ; otherwise the lids bridging the empty eyesocket will slowly contract… changes in position of the margins of the eyelids in respect to their apposition to the globe are important…. The margins of the eyelids normally remain closely applied to the globe as the eyes move in various directions or as the lids open and close… numerous pathologic states cause the eyelid margins to lose normal contact with the globe and to turn either in toward the globe (entropion) or away from the globe (ectropion)… ectropion of the lower lid is a common consequence of seventh nerve paralysis (for example, Bell's palsy), in which, in the absence of orbicularis support, even the firm tissues of young people are insufficient to keep the lower lid against the globe…in senile ectropion the lower lid may sag outward as the orbicularis loses tone… tears spilling over the lid margin macerate the thin skin, and the ensuing inflammation enhances the ectropion..one surgical procedure for senile ectropion is tightening of the band of orbicularis fibers overlying the tarsus… in some persons the pretarsal fascia holding the orbicularis over the tarsus becomes loose with age.. the orbicularis can then slide toward the lid margin and leave the orbital edge of the tarsus unsupported.. concomitantly, diminution of orbital fat, which is common in older people, allows the globe to recede to the orbit, and the laxity of the lids is compounded….

















Lid closure then presses the palpebral margin against the globe, while allowing the unsupported tarsal edge to tilt forward (entropion)…digital traction on the skin may reposit the lid, but it inverts again at the next blink… the lower lid, in which the tarsal plate is less developed, is more prone to senile entropion than is the upper lid… entropion may be initiated by blepharospasm secondary to inflammation or photophobia… the irritation of inturned lashes against the globe causes further orbicularis spasm and perpetuates the condition….the orbicularis muscle at the very margin of the lid consists of fine striated muscle fibers… this anatomically distinct portion of the orbicularis is known as the muscle of Riolan …Because of its position at the lid margin, the muscle of Riolan has been indicted by some authors as a particular culprit in the development of spastic entropion….. A surgical procedure for entropion utilizes the principle of shortening the orbicularis fibers at the orbital margin of the tarsus and securing them to the tarsus to prevent sliding toward the lid margin.. Spastic entropion seldom occurs in the young despite long –continued blepharospasm…Trauma and disease often lead to malposition of the lids… contracting scars can evert or invert the lid to produce cicatricial ectropion or entropion… entropion and resultant trichiasis are characteristic of such disease as late trachoma and ocular pemphigus……..



The eyelids during sleep

During sleep, closure of the eyelids is not a simple relaxation of the muscles that keep the eyes open ; rather, it is a tonic stimulation of the orbicularis together with an inhibition of the levator- the converse of what occurs during waking hours… with closure of the eyelids at the onset of sleep, the receptors for the dazzle and menace blinking reflexes cease to act, but the corneal reflex is not eliminated until the deeper stages of sleep have been reached…some otherwise normal persons do not completely close their eyes during sleep, and in these persons the lower cornea may become eroded from exposure…closure of the eyelids that accompanies the onset of sleep is an inherited automatic act that takes place when cortical inhibition reaches a certain degree… it serves the purposes of protecting the eye from injury and of allowing the ocular muscles to rest… voluntary closure of the eyelids in preparation for sleep has been called the ritual of going to sleep… when sleep is desired under difficult circumstances, shutting the eyes is one of the few available acts of this ritual… it is not always successful in its purpose, but at least it does reduce visual stimuli that would otherwise prevent or at least delay the onset of unconsciousness….



مصادر المراجع و الابحاث

References

1- Anantanarayana..Note on the mechanism of eyelid closure in blinking..Proc. All India opthalmol. Soc . 10:154, 1949



2-Bjork A:- Electromyographic studies on coordination of antagonistic muscles in cases of abducens and facial palsy. BR.J. Ophthalmol. 38:605. 1954



3-Fox, S.A: The palpebral fissure, Am. J. Ophthalmol. 62:73. 1966



4-Martino.G: The conditioned reflex of blinking, J. Neurophysiol. 2:173. 1939

[mohamed_ameer]

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الجهاز الدمعى

The lacrimal apparatus

The optical integrity and normal function of the eye depend on an adequate supply of fluid covering its surface.. this moist layer serves 1) an optical function by maintaining an optically uniform corneal surface, 2) a mechanical function by flushing cellular debris and foreign matter from the cornea and conjunctival sac by lubricating the surface , 3) a corneal nutritional function and 4) an antibacterial function…….

The maintenance of such moist layer depends on a secretory mechanism and excretory, or elimination, mechanism… the secretory component includes the lacrimal gland and accessory lacrimal gland tissue, as well as the sebaceous glands of the eyelids and the goblet cells and other mucin-secreting elements of the conjunctiva…the elimination of the lacrimal secretions depends on a movement of tears across the eye, aided by the act of blinking, and a drainage system consisting of the lacrimal puncta, canaliculi, sac, and nasolacrimal duct… the integrity of the cornea as the most important refractive element of the eye depends on the normal functioning of each of these two mechanisms, as well as on a proper balance between them…thus a disturbing excess of tear fluid may result from obstruction in the drainage system or alternatively , from an excessive secretion of tears despite a normal drainage mechanism..conversely, normal comfort and normal hydration of the cornea may exist in the face of some relative impairment of the outflow mechanism, if this is accompanied by a diminution of lacrimal secretion… such a comfortable balance of these two components is seen in older patients who have relative insufficiency of the lacrimal puncta caused by mild eversion of the lacrimal papillae, but who at the same time exhibit a decrease in lacrimal secretory activity, which is normal with advancing years….

The preservation of this balance between tear formation and tear outflow must be borne in mind in therapeutic programs… the importance of such a balance is illustrated by a 65 year- old woman who presented typical findings of keratoconjunctivitis sicca- a dry eye- associated with other evidences of Sjorgren's syndrome…in addition, she had a chronic dacryocystitis by removal of the tear sac has long been superseded by surgical procedures directed at restoration of the lacrimal drainage, in this instance the decision to remove the infected tear sac was predicated on the need to conserve, rather than eliminate, the already inadequate supply of tears….

[mohamed_ameer]

The tear film

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Tear film

(superficial lipid layer- 0.1 mm consisting mainly of waxy and cholesterol esters and some polar lipids)

(aqueous layer- 7mm containing in dissolved from inorganic salts, glucose, urea, surface-active biopolymers, proteins, and glycoproteins…)



Microvilli

(Mucus layer 0.02-0.05 mm hydrated layer of mucoproteins rich in sialomucin…))



(structure and composition of the tear film ( from Holly, F. J, and lemp. M.A: surv. Opthalmol. 22:69, 1977.)



(sagital section through upper eyelid, showing sebaceous and various accessory lacrimal glands… from Duke- Elder, W. S: Text-book of ophthalmology . vol. 5, St. Louis , 1952, The C.V. Mosby Co. )



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(orbicularis muscle,, sweat gland,, hair follicle,, gland of Zeis,,, Cilium,,, Gland of Moll,, pars marginalis of orbicularis muscle ,,,, pars subtarsalis of orbicularis muscle,,,, inferior arterial arcade,,,, Meibomian gland,,,Gland of Wolfring…. Conjunctival crypts,,,,, superior arterial arcade,,,,, Gland of Krause …..Muller's muscle ,,,,,, Levator palpebrae superioris ,,,,,fat ))))





Evaporation of tears

In the normal tear film the evaporation rate is low because of the protective oil surface… between 10% and 25% of the total tears secreted are lost by evaporation…in the absence of the protective oily layer, the rate of evaporation is increased 10 to 20 times… the tonicity of human tears is subject to a dynamic change because of the evaporation process and the rate of tear flow… when evaporation is prevented… the osmotic pressure of tears is equivalent to 0.9% sodium chloride solution….as tear flow increases , the effect of evaporation lessens…mastman and co-workers state that when tear flow decreases, as in Sjogren's syndrome,, the tears become markedly hypertonic (0.97% sodium chloride solution or more) and corneal dehydration results…. When the eyes are closed, there is no evaporation of tears, and the precorneal tear film is osmotic equilibrium with the cornea…with the eyes open, evaporation takes place, increasing the tonicity of the tear film and producing an osomotic gradient from the aqueous through the cornea to the tear film… this direction of flow will continue as long as evaporation maintains the hypertonicity of the tear film…..





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(physical properties and chemical composition of human tears and plasma)



Composition of tears

Data comparing physical properties and chemical composition of tears with those of blood plasma are summarized in tables



Average glucose in tears , 2.5 to 4.1 mg\ 100 ml, represents 40 % of the total reducing substance, whereas it makes up 80% of the total reducing substance in blood…. Changes in the concentrations of glucose in tears and blood show a close correlation… its possible to make a semiquantative determination of hyperglycemia with various commercially available reagent test strips moistened with tears… such a method is useful for a comatose patient or one from whom, for some reason , a urine sample can not be obtained….



When the concentrations of potassium and chlorides are greater in tears than in plasma, so that means these are secreted by the lacrimal gland… the total amount of urea in tears is small compared to plasma, but it increases with tear flow, although the concentration drops as tear secretion increases…



Alterations in the physical-chemical properties of tears in contact lens wearers provide some insights into the ability of the eye to tolerate these foreign bodies… when polymethy 1 methacrylate (PMMA, "hard") contact lenses are first worn, chloride ions in tears decrease, but return to normal in 5 or 6 days, correlating well with the visual adaptation period for these lenses….





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Protein content of tears

The protein content of tears differs in several respects from that of blood plasma, lending additional evidence that tear fluid is a true secretion …in human tears the protein fractions, determined by various electrophoretic techniques, are albumin, globulin fractions, and Iysozyme… Albumin represents about 60% of the total protein in tears, as it does in plasma… the remainder is divided equally between globulin and Iysozyme fractions