ORBIT

I. BONES OF ORBIT ‑ bones are rigidly linked together to form a stable socket to permit precise movements of eye.

A. Boundaries (Atlas Fig. 7.36A)

1. Roof ‑ Frontal bone (anterior cranial fossa is superior to roof)(Atlas Fig. 7.76A)

2. Floor ‑ Maxillary bone (maxillary sinus is inferior to floor)(Atlas Fig. 7.71)

3. Medial wall ‑ Maxillary, Lacrimal, Ethmoid, Frontal

and Sphenoid bones (nasal cavity is medial to medial wall of orbit)(Atlas Fig. 7.76A)

4. Lateral wall ‑ Zygomatic bone and sphenoid bone (greater wing).

B. Foramina ‑ openings which transmit nerves and vessels to structures in orbit (eye, extraocular muscles and lacrimal gland (tears); also IMPORTANTLY, orbit serves as passageway for nerves that are sensory to face, scalp and nasal cavity. (Atlas Fig. 7.36A) - see Foramina handout

C. Lining of orbit ‑ periosteum of bones of orbit is called Periorbita.

II. EYELIDS ‑ layered, moveable structures which protect eye, keep cornea (outermost layer) of eye moist.(Atlas Fig. 7.39C, 7.37B)

A. Layers

1. Skin ‑ contains eyelashes (cilia), openings of sebaceous glands and sweat glands.

2. Subcutaneous layer ‑ connective tissue containing sebaceous glands; obstruction of sebaceous glands called a sty (hordeolum).

3. Orbicularis oculi muscle ‑ skeletal muscle which surrounds eyelid; closes eyelids; innervated by Facial nerve (VII); damage to facial nerve paralyzes muscle; patient unable to close eyelids and spread tears over cornea; can result in corneal damage.

4. Orbital septum, tarsal plate and Levator Palpebrae Superioris muscle.

a. Orbital septum ‑ fascial layer inside eyelid, is continuous with connective tissue lining orbit (periorbita).

b. Tarsal plate ‑ dense fibrous connective tissue, located deep to orbital septum; forms 'skeleton' of eyelid; contains tarsal glands (obstruction called a chalazion).

c. Levator palpebrae superioris muscle ‑ muscle composed of both smooth and skeletal muscle components; origin ‑ Tendinous ring (see below); insertion ‑ skin and tarsal plate of upper lid; action ‑ opens eyelids; innervation ‑ skeletal part by Oculomotor nerve (III), smooth part by Sympathetics. (Atlas Fig. 7.39).

5. Conjunctiva ‑ membrane covering inner side of eyelid; conjunctiva continues as a layer over sclera of eye and fuses to cornea; reflection of conjunctiva from eyelid to eye called Superior and Inferior fornices of conjunctiva; very sensitive.

III. LACRIMAL APPARATUS ‑ tears are constantly produced in lacrimal gland, drain to nasal cavity via lacrimal duct. (Atlas Fig. 7.37B,C)

A. Lacrimal gland ‑ located in superolateral orbit (Atlas Fig. 7.31A, 7.38); have numerous ducts (about 12) which open through conjunctiva; produce tears; tears circulate over conjunctiva and wash out dirt; drain through lacrimal puncta (openings) in medial part of upper and lower eyelids (you can see these on yourself in a mirror); puncta drain to lacrimal sac which drains via lacrimal duct to Inferior meatus of nasal cavity (this is why you blow your nose when you are crying)

B. Innervation of lacrimal gland ‑ Parasympathetics from Facial nerve (VII) via a complicated pathway in which fibers hitch‑hike with branches of the Trigeminal nerve (V) (more in Cranial nerve II lecture).

IV. FASCIAL SHEATH OF EYEBALL ‑ thin fascial membrane surrounding eye (also called Tenon's capsule); thickenings of sheath attach to bones and form Medial and Lateral Check ligaments which prevent excess movement of eye.

V. EXTRAOCULAR MUSCLES

A. Origins ‑ all take origin from Tendinous ring (except Inferior Oblique which has origin on floor of orbit); Tendinous ring is ring of connective tissue surrounding opening of Optic canal and Superior Orbital Fissure.(Atlas Fig. 7.40A)

B. Actions and Innervation (Atlas Figures with Tables 7.7, 7.8)

Muscle Nerve Action

Medial rectus III Adduct eye

Lateral rectus VI Abduct eye

Inferior rectus III Adduct, lower and laterally rotate eye

Superior rectus III Adduct, raise and medially rotate eye

Superior oblique IV Abduct, lower and medially

rotate eye

Inferior oblique III Abduct, raise and laterally

rotate eye.

VI. EYE

A. Structure of eyeball ‑ described as three layers (Atlas Figs. 7.41, 7.42)

1. Fibrous layer

a. Sclera ‑ tough, smooth fibroelastic layer surrounding eye (continuous anteriorly with cornea); functions to protect eye and maintain shape; provides attachment of extraocular muscles; pierced by nerves and vessels of eye.

b. Cornea ‑ avascular, transparent layer covering anterior eye; important in focusing light; irregularities in cornea responsible for astigmatism.

2. Vascular layer

a. Choroid ‑ highly vascular, pigmented membrane; provides nutrients and oxygen to other layers of eye.

b. Ciliary body ‑ attaches to suspensory ligament of lens; hold lens taut; contains ciliary muscles.

i. Ciliary muscles ‑ smooth muscles attached to suspensory ligament of lens; contraction of muscles produces relaxation of suspensory ligament; causes lens to thicken for near vision (accommodation); innervation ‑ Parasympathetics from Ciliary ganglion (nerve III) cause contraction of ciliary muscles (parasympathetics travel in Short Ciliary nerves).

c. Iris ‑ pigmented, contractile layer surround pupil (opening); controls amount of light entering eye; contains two muscles

i. Constrictor pupillae ‑ circular smooth muscle which constricts iris; innervated by Parasympathetics (from Ciliary ganglion of III).

ii. Dilator pupillae ‑ radial smooth muscle which dilates pupil; innervated by sympathetics.

3. Retina ‑ contains photosensitive rods and cones and many neurons which process visual information; artery ‑ Central artery of retina (branch of Ophthalmic artery), has no anastomoses (occlusion results in blindness).

VII. CILIARY GANGLION ‑ parasympathetic ganglion of Oculomotor nerve (III)(Atlas Fig. 7.39A)

A. Contains ‑ parasympathetics for Ciliary muscles and Sphincter Pupillae; parasympathetics travel in Short Ciliary nerves.

B. Nerves passing to back of eye (in addition to Optic Nerve)

1. Short Ciliary nerves - parasympathetics from III to Ciliary muscles and constrictor pupillae

2. Long ciliary nerves ‑ sensory branches of Ophthalmic division (V1) of Trigeminal nerve which innervate cornea.

VIII. NERVE DAMAGE

A. Abducens nerve (VI) ‑ damage causes Medial Strabismus (cross‑eyed).

B. Trochlear nerve (IV) ‑ damage results in inability to turn eye down and out.

C. Oculomotor nerve (III) ‑ damage causes ptosis (drooping lid from paralysis of skeletal component of Levator palpebrae superioris), Lateral Strabismus (wall‑eyed, from damage to Medial rectus), dilated pupil (from paralysis of Constrictor pupillae) and diplopia (double vision)