Musculus retractor bulbi
The retractor bulbi is a cone-shaped muscle surrounding the optic nerve. It originates from the orbital periosteum around the optic foramen and inserts on the posterior sclera of the eyeball. Its action is to retract the eyeball into the orbit; this retraction passively pushes the third eyelid (nictitating membrane) across the eye for protection. It is unique to many non-primate mammals. It is innervated by the abducens nerve (cranial nerve VI).
Origin: Orbital periosteum around the optic foramen
Insertion: Posterior sclera of the eyeball
Action: Retracts the eyeball into the orbit; this passively pushes the third eyelid across the eye for protection
Innervation: Abducens nerve (CN VI)
Relevant in evaluations of ocular retraction and third-eyelid function. Isolated clinical problems are uncommon. It can be assessed as part of the corneal or palpebral reflex.
In tetanus (caused by Clostridium tetani toxin), the retractor bulbi goes into spasm, causing prolapse of the third eyelid. This is one of the earliest and most characteristic signs of tetanus in hors
## Overview Prolapse or prominence of the third eyelid is a useful diagnostic sign. Bilateral prominence triggered by stimulation suggests tetanus. Unilateral prominence indicates eye pain (such as a
Persistent inability to retract the eye or third-eyelid issues after trauma should be evaluated.
In tetanus (caused by Clostridium tetani toxin), the retractor bulbi goes into spasm, causing prolapse of the third eyelid. This is one of the earliest and most characteristic signs of tetanus in horses. Signs include: Third eyelid flashing across the eye, especially when the horse is startled or the head is elevated (flicking the ear or tapping under the jaw triggers it); progressing to stiff gait, sawhorse stance, lockjaw, and death without treatment. Tetanus is preventable with vaccination.
## Overview Prolapse or prominence of the third eyelid is a useful diagnostic sign. Bilateral prominence triggered by stimulation suggests tetanus. Unilateral prominence indicates eye pain (such as a corneal ulcer). Bilateral persistent prominence can occur with dehydration or sedation. Retraction of the globe makes the third eyelid more visible. ## What It Indicates Tetanus, ocular pain, dehydration, or sedation, depending on laterality and triggers. ## Diagnosis Observation in context. Examine the eye and assess for tetanus or dehydration. ## Treatment Directed at the underlying cause. ## Recovery and Prognosis Depends on the cause. ## Prevention Depends on etiology. Signs include: visible third eyelid covering part of the eye; bilateral and stimulus-triggered in tetanus; unilateral with eye pain; bilateral and persistent with dehydration
Relevant in evaluations of ocular retraction and third-eyelid function. Isolated clinical problems are uncommon. It can be assessed as part of the corneal or palpebral reflex.
Persistent inability to retract the eye or third-eyelid issues after trauma should be evaluated.
See the Retractor Bulbi on our interactive 3D horse model. Open 3D Model