Orbit

Cone shaped cavity of the skull which houses the eye, made up of bones, ligaments, and the soft tissues of the pterygopalatine fossa

Bony Orbit

Portion of the orbit consisting of bone: frontal, lacrimal, presphenoid, palatine, and zygomatic bones

Orbital ligament

ligament bridging the bony prominences of the frontal and zygomatic bones in carnivores, since they have an incomplete bony orbit

Optic canal

 transmits the optic nerve and hte internal ophthalmic artery (branch of the rostral cerebral artery

Orbital fissure

Ventral to teh optic canal in carnivores, which transmits CN III, IV, VI, and ophthalmic division of CN V. As well as an anastomotic artery that links the internal carotid a to the external ophthalmic artery

Supraorbital foramen

A hole in the equine and ruminant orbit through which the supraorbital nerve, artery and vein travel

supraorbital nerve

continuation of the frontal nerve, a sensory branch of the ophthalmic nerve

foramen orbitorotundum

In ruminants: combined orbital fissure and round foramen transmitting CN III, IV, entire V, and VI. A number of small branches of the maxillary artery enter the skull through it to participate in the formation of the rete mirabile

lacrimal bulla

unique featuer of the ruminant orbit in its ventral floor, which is a caudal extension of the maxillary sinus

Ocular adnexa

The adjacent structures of the eye such as the lacrimal apparatus, the extraocular muscles and the eyelids, eyelashes, eyebrows and the conjunctiva.

Palpebrae

eyelids: upper = palpebra superior, and lower = palpebra inferior, both of which meet at the medial and lateral canthi

palpebral fissure

gap between the margins of the palpebrae, whose size is determined by the mm of the eyelids (m. orbicularis oculi and m. levator palpebrae superioris)

tarsal plate

layer of dense fibrous connective tissue within the palpebrae that affords them a certain amount of rigidity

cilia

eyelashes present on the palpebra superior of dogs, horses, cattle, pigs, and sheep (lesser number found on palpebra inferior of herbivores). cats have no cilia on either eyelid

Tarsal glands (Meibomian glands)

• row of large modified sebaceous glands associated with the lid margin, embedded in the tarsal plate and opening into a shallow furrow immediately deep to the mucocutaneous junction of the lid. produce oil later of the tear film
• commonly become infected (tarsitis), neoplastic (tarsal adenoma), or impacted with their secretions (chalazion).

Conjunctiva

mucous membrane that lines the inside of the lids and the surface of the eyeball (excluding the cornea)

Palpebral conjunctiva

conjunctiva on the deep side of the palpebrae

Bulbar conjunctiva

conjunctiva as it reflects back onto the surface of the eyeball

conjunctival fornix

angle made by the conjunctival reflection

conjunctival sac

space between the eyelids and the surface of the eye (normally potential except for a small amount of mucus and tears

conjunctivitis

inflammation of the conjunctiva associated with increased secretion of mucus, swelling and marked reddening.

Nictitating membrane

Third eyelid: fold of mucus membrane arising from the ventromedial aspect of hte conjunctival sac. Rigidity given by a T-shaped cartilage, and it functions to smooth the precorneal tear film, to produce a portion of the precorneal tear film, and to protect the cornea

Protrusion of third eyelid in Horner's Syndrome

Sympathetic tone to the smooth mm of the periorbita surrounding the globe helps keep eyeball placed well forward in the orbit. Also, some fascicles of periorbital smooth mm attach at the base of the 3rd eyelid and actively keep it retracted.

Gland of the third eyelid

seromucoid gland at the base of the third eyelid that contributes about 50% precorneal tear film

Cherry eye

prolapse of the gland of hte third eyelid, where the gland loses its fibrous attachment to deeper structures and everts into the conjunctival sac

Lacrimal system

consists of lacrimal gland, gland of the third eyelid, accessory lacrimal glands, precorneal tear film, mucous thread, lacrimal puncta and canaliculi, nasolacrimal duct, and nasal punctum

lacrimal gland

• Gland found within the dorsolateral portion of the orbit (deepto the bone and/or orbital lig), where it communicates with the superior conjunctival fornix via numerous microscopic ducts (serous secretion).
• Together with gland of the third eyelid, major contributor to the precorneal tear film

Lacrimal nerve

Sensory innervation to the lacrimal gland (branch of the ophthalmic division of CN V)

Major petrosal nerve

• parasympathetic motor innervation to the lacrimal gland (branch of a facial n.)
• parasympathetic preganglionic neurons synapse in the pterygopalatine ganglion, and from there, postganglionic fibers join the lacrimal nerve to arrive in the gland, where activity stimulates secretion.

Accessory lacrimal glands

Diffuse glands of the conjunctiva and the eyelids that make a small contribution ot the aqueous layer of hte tears

precorneal tear film

• layer of moisture that covers the anterior surface of the globe
• 3 distinct layers
  • oil layer
  • aqueous layer
  • mucoid layer

oil layer of precorneal tear film

• oils and phospholipids that serve as oily barrier vs evaporation of the tear film (high surface tension prevents overflow of the precorneal tear film
• produced predominantly by Meibomian glands

Aqueous layer of the precorneal tear film

• Thick middle layer of precorneal tear film
• generatede primarily by the lacrimal gland and the gland of the 3rd eyelid
• Functions to:
  • Rinse away foreign debris from the conjunctival sac
  • Lubricate the conjunctival and corneal surfaces so that eyelids slide freely across the surface of the globe
  • Provide a medium for diffusion of O2 and migration of inflammatory cells to the avascular cornea
  • Provide an optically smooth surface to the cornea
    

Mucoid layer

Innermost layer of the precorneal tear film consisting of the muco-proteinaceous secretions of the conjunctiva's many goblet cells. Amphipathic mucoproteins in this layer bind the hydrophilic aqueous layer to the hydrophobic corneal tissues.

Mucous threads

linear accumulations of mucus normally found int he superior and inferior fornices, and are secretionso fhte goblet cells and debris collected from the conjunctival sac. Accumulate and migrate down tot he nasal canthus.

Lacrimal caruncle

Slight mucosal elevation within conjunctival sac at the nasal canthus of the eye

Lacrimal punctum

slit-like openings on the margins of the 2 palpebrae at the medial canthus, that opens into the lacrimal canaliculi

Lacrimal canaliculi

short ducts that join one another at a small sac (lacrimal sac) residing within the lacrimal fossa of the lacrimal bone

Lacrimal sac

Origin of the nasolacrimal duct

Nasolacrimal duct

Travels through the lacrimal and maxillary bones to open into the nasal cavity and/or nasal vestibule at the nasal punctum

Lacrimal lake

Fromed by the accumulation of the precorneal tear film in the inferior conjunctival fornix. Majority of this is removed from the conjunctival sac via the nasolacrimal duct

Conjugate eye movements

muscles always operate in coordinated pairs, not only within single orbit, but within counterparts in the opposite eye. Orchestrated by the MLF and vestibular nuclei.

Strabismus

failure of the eyees to maintain parallel visual axes

3 Fascial Layers of the Muscluar in orbit

• Muscular fasciae (epimyseal investments of extraocular eye muscles)
• Bulbar sheath: thin capsule enveloping the eyeball from the edge of the cornea to the optic n. 
• Periorbita: CT cone that surrounds the eyeball and its muscles, neres, and vessels. Contains circular smooth muscles that acts to compress its contents and places the eyeball forward in the orbit (sympathetic tone)

Retrobulbar fat

Adipose tissue that cushions orbital contents

3 Tunics of the globe

• Fibrous tunic: sclera and cornea. superficial to the sclera is the bulbar sheath, which is attached to teh outermost layer of the sclera, the episclera (highly vascular)
• Vascular tunic (Uvea): composed of choroid, ciliary body, and iris
• Nervous tunic (Retina):

Determinants of corneal transparency

1. lack of vascular elements and cells
2. lack of pigment
3. relative dehydration of the tissue
4. a smooth optical surface
5. highly regular, laminar pattern of collagen fibers that reduces light scatter

Five layers of the cornea

1. precorneal tear film
2. anterior epithelium + basement membrane (endowed with nociceptors from teh long ciliary nerves - brr of ophthalmic division of V)
3. Stroma: 90% of corneal thickness (few fibrocytes and keratocytes and a thick layer of collagen)
4. Descemet's membrane (exaggerated basement membrane of the endothelium (fine collagen).
5. Endothelium: actively pumps water from stroma to the anterior chamber

Area cribrosa

sieve-like holes in the sclera pirced by the axons of CN II

Descemetocele

bulging of the Descemet's membrane when corneal injuries penetrate the full thickness of the stroma

Posterior uvea

choroid: pigmented vascular layer of the posterior portion of the eye

Tepetum lucidum

• Tepetum is the deepest layer of the choroid, which is only separated from the retina by a thin capillary layer and basal lamina. 
• carnivores have cellular, herbivores have collagenous
• reflective surface designed to bounce incoming photons back onto the retina and enhance these spp' sensitivity to low levels of light
• At the dorsal part of the posterior globe

Tepetal reflex

Reflection of light from the tepetum

Ciliary body

• anterior continuation of the uvea and lies immediately posterior to the iris
• circumferential thickening of the vascular tunic, and gives rise to ciliary processes to which the lens is attached by suspensory ligaments
• bulk of ciliary body consists of CT, capillary network, and ciliary muscle (circumferential smooth muscle fibers that act like a sphincter as they contract and reduces tension on suspensory ligg allowing lens to assume spherical shape)
• Ciliary mm innervated by parasympathetic fibers of short ciliary nerves, which are brr of CN III
• Capillaries of ciliary body produces aqueous humor, and is BP and carbonic anhydrase dependent
  

Iris

• Most anterior portion of the uvea and only part of vascular tunici normally visible in the living animal
• divides the anterior segment of the eye in to anterior and posterior chambers.
• Controls amt light entering posterior poart of the eye by changing size of pupil using 2 sets of sm mm

M. constrictor pupillae

• reduces diameter of the pupil (miosis)
• array of sm mm that encircle the pupil
• Constricts in response to parasympathetic input from short ciliary nerves of CN III (postganglionic parasympathetic from the ciliary ganglion)
• Preganglionic fibers arise from parasympathetic nucleus of CN III of mesencephalon

m. dilator pupillae

• dilation of pupil
• constriction of radially oriented smooth mm fibers
• receives sympathetic innervation via postganglionic fibers arising from the cranial cervical ganglion
• When congracts, increases diameter of the pupil (mydriasis)

corpora nigra

nodular mass along margin of pupil that are visible in the anterior chamber

Persistent pupillary membranes

When pupillary membrane that stretcheds across the anterior chamber during development fails to atrophy

Filtration angle

Where anterior surface of the iris meets the fibrous tunic, and the site at which the aqueous produced by the ciliary body is reabsorbed back into the circulation

Pectinate ligaments

Series of fine fibers that bridge where teh iris and sclera approach one another in the anterior chamber. Deep to these is the trabecular meshwork, ad labyrinth of fluid passages. As aqueous moves through the spaces within meshwork, it makes its way tot he scleral venous plexus, with thich the anterior chamber is continuous

Glaucoma

• increased intraocular pressure that can rapidly result in blindness as the increased pressure ofhte aqueous is transferred through the vitreous body to teh photosensitive retina

pars iridica retinae

pigmented epithelium attached to the posterior surface of the iris (gives iris its color)

pars ciliaris retinae

Part of nervous tunic that reflects over the surface of hte ciliary body

pars optica retinae

light sensitive, extends from just posterior to the ciliary body to the posterior pole of the globe

Fundus

part of teh pars optica retinae that can be viewed through the pupil wiht an ophthalmoscope

Pigment epithelium

• simple cuboidal epithelium with lots of melanin granules just deep to the choroid (no pigment underneath the tepetum)

scotopic vision

vision in dim light