Orbitalis muscle

The orbitalis muscle (also known as musculus orbitalis or Müller’s muscle of the orbit) is a vestigial smooth muscle located within the orbit. In humans, it is poorly developed and functionally insignificant.

This muscle forms a thin sheet of non-striated fibres that span the inferior orbital fissure. It is supplied by postganglionic sympathetic fibres arising from the superior cervical ganglion.

Although its function in humans is uncertain, the orbitalis muscle is thought to provide minimal support to the position of the eyeball, contributing slightly to orbital tone. In some animals, it plays a role in protrusion of the eyeball.

Clinical note

Interruption of sympathetic innervation—such as in Horner’s syndrome—may result in mild enophthalmos (posterior displacement of the eyeball), partly due to loss of orbitalis muscle tone.