Veterinary Ophthalmology: Anatomy

• Thorough ophthalmic anatomy knowledge is key for veterinary ophthalmologists to differentiate normal from abnormal structures during examinations and surgeries.
• The eye retains similar basic parts across vertebrate evolution, but clinically relevant differences exist among species.
• This chapter focuses on canine, feline, equine, livestock, and avian ophthalmic anatomy with selected exotic species in Section IV and additional literature available.

Orbit

• The orbit is a bony fossa protecting the eye and separating it from the cranial cavity.
• It provides pathways for blood vessels and nerves.
• Orbit size, shape, and position relate to visual activity and feeding behavior.
• Domestic carnivores' orbital axes are rostrolateral, at approximately 10° (cats) and 20° (dogs) from midline, enhancing binocular vision for predation.
• Equine and ruminant orbits are more lateral at about 40° (horses) and 50° (cattle) from midline, enhancing monocular vision for predator detection.
• Rabbit eyes extend 85° from midline, also found in lizards, some snakes, and fish, while reduced binocular vision can cause eyes to protrude, expanding the visual axis.
• Vertebrate orbits are either enclosed, fully encompassed by bone, or open/incomplete, only partially surrounded by bone.
• Horses, sheep, cattle, and goats have enclosed orbits while pigs, dogs, and cats have open orbits.
• Herbivorous prey species require enclosed orbits for protection; open orbits allow carnivores to open jaws widely for consumption.
• The bony orbit typically consists of five to seven bones.
• The canine orbit includes five, and sometimes six, bones, the supraorbital ligament (frontal to zygomatic bone), and periosteum, with a rim formed by frontal, lacrimal, and zygomatic bones.
• Canine orbits have lateral support from the supraorbital ligament and an incomplete floor partially formed by sphenoid and palatine bones.
• In felines, extended frontal and zygomatic bone processes result in a shortened supraorbital ligament.
• Enclosed orbits result from the union of the zygomatic process and frontal bone, intervening with the temporal bone in horses.
• Orbits have foramina and fissures for blood vessel and nerve passage between the cranial cavity/alar canal and orbit.
• Constant foramina in domestic animals include rostral alar, ethmoidal, lacrimal, orbital, ovale, optic, rotundum, and supraorbital foramina.
• The pterygopalatine region has maxillary, caudal palatine, and sphenopalatine foramina.
• The orbital foramen is elongated in most domestic animals besides horses, so it is called the orbital fissure, but it is fused with foramen rotundum as "foramen orbitorotundum" in cattle.

Orbital Fascia

• Orbital fascia has three components.
• The orbital fascia is thin, tough connective tissue lining the orbit and enveloping all structures.
• Components include:
  • Periorbita
  • Tenon's capsule/fascia bulbi
  • Extraocular muscle (EOM) fascial sheaths.
• The periorbita is conically shaped.
• The periorbita is a fibrous membrane lining the orbit and enclosing the globe, EOMs, blood vessels, and nerves, attaching to orbital bones as periosteum.
• Canine periorbita does not always fuse with the periosteum of the frontal and sphenoid bones, remaining distinct, especially toward the orbital ligament.
• The periorbita is thicker laterally.
• Cranially in its dorsolateral aspect, the periorbita separates and surrounds the lacrimal gland.
• At the periorbital rim, septa divides, with one part becoming continuous with the periosteum of the facial bones and another merging with eyelids and becoming continuous with tarsal plates.
• Smooth muscle within the periorbital tissue of carnivores facilitates globe movement and has been observed along lateral walls of orbits, portions of the roof and floor of the orbit, and next to the periosteal lining of the orbital bones.
• Tenon's capsule (fascia bulbi) is connective tissue on the outer sclera, separated by Tenon's space—a narrow cleft filled with loose connective tissue.
• Tenon's capsule attaches near the corneoscleral junction and becomes continuous with EOM fasciae.
• EOM fascial sheaths consist of dense, fibrous membranes loosely attached to muscles via delicate trabeculae of connective tissue.
  • EOM fascial sheaths are continuous/reflections of Tenon's capsule, but they are not always considered part of it.
• Canine muscular fasciae are 3-layered.
  • Superficial thick layer extends caudally from the orbital septum.
  • Middle layers consist of the superficial/deep sheets that attach to the outer corneal junction, and then a deep layer sits next to the EOMs, separating recti muscles from rectractor oculi muscles.
• Separated by orbital fat, orbital fascia contains three sheets, filling the orbit and acting as a cushioning agent and protector for the eye.
• Amount of orbital fat changes within individuals and species, with coloration from white to yellow from lutein, beta Carotene, retinol, carotene.
• Birds and many reptiles have very little.

Extraocular Muscles (EOMs)

• They suspend the globe in the orbit, provide ocular motility, and there are four rectus muscles are the dorsal, ventral, medial, lateral recti, originating from the orbital apex and inserting around the limbus.
  • The EOMs' movement is in direction of names/directions.
• Dorsal/superior oblique originates from the medial orbital apex, goes dorsomedially through a trochlea near the medial canthus, acutely turns, goes dorsolaterally to the globe, medially and ventrally (intorsion).
• Ventral/inferior oblique originates ventrolaterally from a palatine one anterolateral to each other, crossing ventral recti, division the musculature at the tendons for their destination.
  • Muscles move in a dorsal, medial and directional rotation (extortion).
• EOM of birds vary among species such as hawks producing forces corresponding to globe size.
• Retractor oculi/bulbi originates from and continues around to form a cone shape through all layers into to insert in all layers of the posterior recti.
  • Rectract the globe or retract the outer recti muscles around.