Exam Essentials:
• Common Exit (Ox): All three exit via Foramen Orbitorotundum (fusion unique to ruminants)
• CN III (Oculomotor): Motor to Dorsal/Ventral/Medial Rectus, Ventral Oblique, Levator Palpebrae; Parasympathetic to sphincter pupillae (constriction)
• CN IV (Trochlear): Only nerve exiting dorsal brainstem; longest intracranial course (trauma vulnerable); innervates Dorsal Oblique (intorsion)
• CN VI (Abducens): Innervates Lateral Rectus (abduction) and Retractor Bulbi (protection)
• Lesion Signs: CN III = Ventrolateral strabismus + Ptosis + Mydriasis; CN VI = Medial strabismus (cross-eye)

The Oculomotor Trio

These three nerves function as the motor supply to the extraocular muscles. In the Ox, they are anatomically linked by their common exit through the Foramen Orbitorotundum, creating both efficiency and vulnerability—a single lesion can paralyze the entire globe.

Cranial Nerve III: The Oculomotor Nerve

Origin: Midbrain (ventral surface).
Exit (Ox): Foramen Orbitorotundum.
Target Muscles:

• Dorsal Rectus (elevation)
• Ventral Rectus (depression)
• Medial Rectus (adduction)
• Ventral Oblique (elevation, abduction)
• Levator Palpebrae Superioris (lifts upper eyelid)

Parasympathetic Component: Pre-ganglionic fibers travel to the Ciliary Ganglion. Post-ganglionic fibers (Short Ciliary Nerves) innervate the Sphincter Pupillae (pupil constriction) and Ciliary muscles (accommodation). This is why CN III lesions cause mydriasis (dilated pupil).

Cranial Nerve IV: The Trochlear Nerve

Origin: Midbrain (dorsal surface).
Unique Feature: It is the ONLY cranial nerve to exit the dorsal aspect of the brainstem. It has the longest intracranial course, making it vulnerable to trauma (especially in polioencephalomalacia or lead poisoning).
Target Muscle: Dorsal Oblique.
Function: Rotates the globe medially (intorsion).

Cranial Nerve VI: The Abducens Nerve

Origin: Medulla.
Exit (Ox): Foramen Orbitorotundum.
Target Muscles:

• Lateral Rectus: Abducts the eye (moves laterally)
• Retractor Bulbi: Retracts the globe into the orbit (protective reflex when eating or during corneal stimulation)

Clinical Signs of Paralysis

CN III Lesion:

• Ventrolateral Strabismus: Eye looks "down and out" because Lateral Rectus (VI) and Dorsal Oblique (IV) are unopposed
• Ptosis: Drooping of upper eyelid (Levator paralysis)
• Mydriasis: Dilated pupil (loss of parasympathetic constriction)

CN VI Lesion:

• Medial Strabismus: Eye deviates inward (cross-eyed) due to unopposed Medial Rectus
• Inability to retract globe: Loss of corneal reflex protection