The fascial sheath of the eyeball is a membranous socket for the eyeball. It is deficient in front over the cornea. It is separated from the eyeball by soft, semi-fluid areolar tissue which allows the eyeball to slide freely in the sheath. The free anterior margin of the sheath fuses with the ocular conjunctiva close to
Fig. 11.12 Horizontal section through the left orbit to show arrangement of the fascial sheath of the eyeball (blue) and check ligaments.
[See also Fig. 8.13A.]
Nasal bone Floor of frontal sinus Upper end of lacrimal sac Lacrimal part of orbicularis oculi Medial palpebral ligament Cavity of nose Ethmoidal sinuses Medial check ligament
Sheath of medial rectus Retina and choroid Sclera
Sheath of optic nerve
Orbital fat
Anterior clinoid process Optic nerve and canal Fatty fibrous tissue
Temporal fascia Tendinous fibres of temporalis Sheath of inferior oblique Sheath of lateral rectus Zygomatic bone Lateral check ligament Lateral palpebral ligament Back of lower eyelid Orbicularis oculi Palpebral fissure Back of upper eyelid Tarsus of upper eyelid
Fascial sheath of eyeball
Fascial sheath of eyeball
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Ligaments in the orbit
lacrimal and zygomatic bones. It is broadest be- neath the eyeball where it is attached to the fascial sheath and helps to support the eyeball and steady the fascial sheath. (2) The check ligaments are strong bands which pass from the sheaths around the lateral and medial rectus muscles to the zygo- matic and lacrimal bones respectively, close to the attachments of the suspensory ligament [Fig.
11.12]. These ligaments limit the movement of the
Table 11.1 Yoked muscles
Direction of gaze Muscle acting in left eye Muscle acting in right eye
To the left Lateral rectus Medial rectus
To the right Medial rectus Lateral rectus
To the left and up Superior rectus Inferior oblique
To the right and up Inferior oblique Superior rectus
To the left and down Inferior rectus Superior oblique
To the right and down Superior oblique Inferior rectus
CLINICAL APPLICATION 11.2 Paralysis of extraocular muscles Paralysis of extraocular muscles can occur due to injury of
the nerve supplying it, or to the brainstem in the region from where the nerves arise. Injury can be due to trauma, com- pression by a tumour, or ischaemia. Typically, the individual would present with diplopia (double vision) and a squint. In order to determine which muscle is paralysed, each muscle is isolated by function and tested. For instance, to test the function of the superior oblique muscle, the subject is asked to adduct the eye and look down. In the adducted position, the axis of the eyeball is in line with the oblique muscles, and the inferior rectus will not act as a depressor. Any downward gaze a patient is able to achieve is solely by the action of the superior oblique.
Study question 1: how would you test the remaining ex- traocular muscles of the eyeball? (Answer: to test the lateral
rectus: ask the patient to look laterally. Medial rectus: ask the patient to look medially. Superior rectus: ask the patient to abduct and then elevate the eyeball. Inferior rectus: ask the patient to abduct and then depress the eyeball. Infe- rior oblique: ask the patient to adduct and then elevate the eyeball.)
Paralysis of the levator palpebrae superioris causes ptosis (drooping of the eyelid).
Study question 2: describe the features of oculomo- tor paralysis. (Answer: the paralysed eye would be turned down and out, due to unopposed action of the lateral rectus and superior oblique. The patient would also have ptosis due to paralysis of the levator palpebrae superioris, and a dilated pupil due to paralysis of the sphincter pupillae [Chapter 12].)
CLINICAL APPLICATION 11.1 Conjugate eye movements Movements of the eye and pupil occur constantly to bring
the image of the object of visual interest to focus on the retina (fovea). Conjugate eye movement is the motor co- ordination of the eyes which allows both eyes to fix on a single object. Yoked muscles are contralaterally paired ex- traocular muscles which work together to direct the gaze in
a particular direction. The movement of the two eyes should be matched accurately to avoid diplopia—double vision.
Table 11.1 shows the yoked muscles for each direction of gaze. It must be remembered that when one set of muscles contract to act, the antagonists of those muscles will relax to allow movement.
lateral and medial recti. Similar checks are present on the superior and inferior rectus muscles.
The zygomatic and infra-orbital nerves, though contents of the orbit, lie outside the orbital perios- teum. They are most conveniently dissected later with the maxillary sinus.
See Clinical Applications 11.1 and 11.2 for the practical implications of the anatomy discussed in this chapter.
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chapter 12
The eyeball
The eyeball lies in the anterior part of the orbit, enclosed in a fascial sheath which separates it from the orbital muscles and fat. It is about 2.5 cm in diameter but is not perfectly spherical. The ante- rior clear part—the cornea—has a smaller radius of curvature than the rest of the globe and protrudes from the anterior surface of the eyeball.
For a satisfactory dissection of the eyeball, use one which has been hardened for a few days in for- maldehyde. You will need four of these to do all the dissections described here.