Veterinary Gross Anatomy-II

Muscles of the orbital region /eyeball

MUSCLES OF THE EYE BALL (OCCULAR MUSCLES)

The muscles are enclosed in superficial and deep fasciae. They are seven in number as four straight, one retractor and two oblique muscles. They are as follows

• Rectus superioris (dorsalis)
• Rectus inferioris (ventralis)
• Rectus medialis
• Rectus lateralis
• Retractor oculi
• Oblique superior
• Oblique inferior

Straight muscles

• The straight muscles are named according to their position as Rectus superioris (dorsalis),Rectus inferioris (ventralis),Rectus medialis and Rectus lateralis.
• They also named as per their action as levator, depressor, abductor and adductor oculi.
• These are flat muscular bands arise around the optic foramen, diverge and pass along the corresponding aspects of the eye-ball forward and terminate in fine aponeurotic tendons to be inserted to the fore part of the sclera in front of the equator.
• Action: The superior rectus elevates the eyeball or turns it upwards, the inferior rectus depresses the eyeball or turns it downwards, the external rectus abducts the eyeball or turns it outwards and the internal rectus adducts the eyeball or turns it inwards. The four recti acting together will retract the eyeball.

Posterior muscle

• The posterior muscle - retractor oculi is enclosed within the recti muscles and envelops the optic nerve. It is made of four bundles, which arise around the optic foramen and inserted to the posterior part of the sclera behind the recti. The bundles alternate with the recti. Its action is to retract the eyeball into the orbit.

Oblique muscles

• The oblique muscles are two in number-superior and inferior.
• The oblique superior is long and differs from others in having an interrupted course. It has a narrow muscular band and a thin aponeurotic termination. It arises from back of the orbit, at about the ethmoidal foramen and passes forward along the internal wall of the orbit, lying medial to the internal rectus and gains at the level of supraorbital process, a fibro-cartilaginous pulley. It then reflects around this pulley outwards and forwards and passes between the eyeball and dorsal rectus and terminates between the dorsal and lateral recti about half an inch behind the margin of the cornea.
• The oblique inferior muscle is shorter and thicker than the superior and entirely muscular. It arises from the depression behind the lacrimal fossa, curves around the rectus ventralis and is inserted into the sclera near and partly beneath the lateral rectus.
• Action: The superior oblique muscle elevates and rotates the eyeball while the inferior oblique depresses and draws the eyeball inwards.
• Blood supply: Ophthalmic artery.
• Nerve supply: Oculomotor nerve supplies all the muscles except the superior oblique, lateral rectus and the retractor. The trochlear nerve supplies the superior oblique and the abducent nerve supplies the lateral rectus and retractor.