Retractor bulbi

The retractor bulbi muscle is a striated muscle belonging to the muscles of the eyeball. It originates in the apex of the orbit, and projects rostrally, dividing into four bands that surround the optic nerve before inserting into the posterior region of the eyeball. It retracts the eyeball into the orbit.

Configuration: Orbital striated muscle

Origin: Apex of the orbit

Insertion: Posterior region of eyeball

Relations: Posterior to the eyeball, surrounding the optic nerve, surrounded by the rectus muscles.

Action: Retraction of the eyeball

Vascularization: Muscular branches of the external ophthalmic artery, ophthalmic plexus

Innervation: Abducens nerve

Description:

The retractor bulbi muscle is one of the striated muscles of the orbit.

It originates at the apex of the orbit near the optic canal, attaching to the periorbita.

Muscle fibers then travel rostrally towards the eyeball. They diverge to form four more or less separated subdivisions, cone-shaped around the optic nerve. These subdivisions are inserted into the posterior portion of the eyeball, either in a continuous circle of insertion, or in various separate insertions. The retractor bulbi is in itself surrounded by the cone formed by the recti muscles of the eyeball.

It is vascularized by the muscular branches of the external ophthalmic artery, and drained by veins forming part of the ophthalmic plexus.

It is innervated by the abducens nerve, which arrives through the orbital fissure close to the origin of the muscle.

Variations:

In equines, the retractor bulbi muscle forms a continuous muscular cone with a single medial opening for the optic nerve. It inserts close to the equator of the eyeball.

In bovines, the muscle forms a complete cone composed of three stipes, medial, dorsolateral and ventrolateral, which insert into the equatorial region of the sclera, and a fourth deep stipe that inserts dorsally to the optic nerve near the posterior pole of the eyeball.

In sheep, four distinct bundles form the muscular cone and insert at the equator, but a few fibers separate from the main stripes and insert dorsally to the optic nerve near the posterior pole of the eyeball.

In pigs, the four bundles are only slightly separated, forming a complete cone that inserts near the equator of the eyeball.

In dogs, the cone is divided into four bundles, which are inserted 5 mm posterior to the insertions of the recti muscles. In some individuals, the stripes may join to form a complete cone.

In cats, the muscle is poorly developed, consisting of four distinct stripes that insert into the postequatorial region of the eyeball.

In rabbits, the muscle forms an almost complete cone, but is short and weak. It inserts far posteriorly to the equator of the eyeball.

In humans, this muscle does not exist.