Orbitalis Muscle
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The orbitalis muscle is one of those ‘hidden anatomy’ muscles most people never hear about. It is a tiny smooth muscle sheet deep in the orbit, sometimes called Müller’s orbital muscle — confusing, because ‘Müller’s muscle’ is also used for the superior tarsal muscle of the eyelid.
Function
Uncertain in humans. It may provide subtle orbital tone or slight support around the posterior orbit; in some animals, similar orbital smooth muscle can alter orbital volume or assist eyeball protrusion.
Origin
Periorbita/periosteum near the infraorbital groove and inferior orbital fissure region at the back of the orbit.
Insertion
Blends with the periorbita/periosteum as a thin smooth muscle layer spanning the inferior orbital fissure; no distinct tendon-like insertion.
Innervation
Sympathetic fibers (postganglionic sympathetic innervation associated with the oculosympathetic pathway).