HORIZONTAL RECTUS MUSCLES	Section 5 of 10
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• The medial and lateral rectus muscles originate from the annulus of Zinn.

• They travel anteriorly along the orbital walls and insert 5.5 mm and 6.9 mm from the limbus, respectively.

	VERTICAL RECTUS MUSCLES	Section 6 of 10
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• The superior and inferior rectus muscles originate from the annulus of Zinn.
• They travel anteriorly and laterally at an angle of 23° with the visual axis of the eye in primary position.
• They insert 7.7 mm and 6.5 mm from the limbus, respectively.

	OBLIQUE MUSCLES	Section 7 of 10
Author Information Nerve Supply Of The Extraocular Muscles Blood Supply Of The Extraocular Muscles Orbital Connections Of The Extraocular Muscles Horizontal Rectus Muscles Vertical Rectus Muscles Oblique Muscles Rectus Muscle Insertion Relationships Structure Of The Extraocular Muscles Bibliography

• The superior oblique muscle originates from the orbital apex above the annulus of Zinn and passes anteriorly along the superomedial orbital wall.
  • The tendon of the superior oblique muscle passes through the trochlea (which is located nasally at the superior orbital rim) and is reflected inferiorly, posteriorly, and laterally at an angle of 51° to the visual axis with the eye in primary position.

  • The tendon passes beneath the superior rectus muscle before inserting posterior to the equator on the superior and lateral aspect of the globe.

• The inferior oblique muscle originates from the maxillary bone behind the lacrimal fossa just posterior to the orbital rim. The muscle passes posteriorly and laterally in the orbit, forming an angle of 51° with the visual axis of the eye in primary position before passing beneath the inferior rectus muscle and inserting posterior to the equator on the inferior and lateral aspect of the globe.

	RECTUS MUSCLE INSERTION RELATIONSHIPS	Section 8 of 10
Author Information Nerve Supply Of The Extraocular Muscles Blood Supply Of The Extraocular Muscles Orbital Connections Of The Extraocular Muscles Horizontal Rectus Muscles Vertical Rectus Muscles Oblique Muscles Rectus Muscle Insertion Relationships Structure Of The Extraocular Muscles Bibliography

• A working knowledge of the relationships between the insertions of the rectus muscles is essential to perform effective strabismus surgery.
• The tendon of the medial rectus muscle inserts 5.5 mm posterior to the limbus along the medial aspect of the globe.
• Next most posterior in its insertion is the inferior rectus, which inserts 6.5 mm posterior to the inferior limbus.
• Continuing counterclockwise around the globe, the lateral rectus inserts 6.9 mm posterior to the lateral limbus, and the superior rectus inserts 7.7 mm from the superior limbus.
• An imaginary line connecting these insertion points creates a spiral known as the spiral of Tillaux.

	STRUCTURE OF THE EXTRAOCULAR MUSCLES	Section 9 of 10
Author Information Nerve Supply Of The Extraocular Muscles Blood Supply Of The Extraocular Muscles Orbital Connections Of The Extraocular Muscles Horizontal Rectus Muscles Vertical Rectus Muscles Oblique Muscles Rectus Muscle Insertion Relationships Structure Of The Extraocular Muscles Bibliography

The extraocular muscles differ histologically from other skeletal muscles in several respects.

• Eye muscle fibers are smaller, more densely innervated, have centrally placed nuclei within muscle fibers, and have more interstitial connective tissue.

• The muscle is composed of many fascicles surrounded by connective tissue called perimysium. The fascicles are bundles of muscle fibers, each of which is surrounded by endomysium.

• At the next level of resolution, each muscle fiber is composed of many hundreds of myofibrils, which are elongated structures composed of repeating elements called sarcomeres.
  • Each sarcomere is made up of interdigitating contractile elements, actin and myosin, that provide for muscle shortening by sliding relative to one another.

  • The “Z bands” that divide sarcomeres in the aggregate produce the striated appearance that characterizes skeletal and extraocular striated muscles.

• The most fundamental physiologic classification for extraocular and skeletal striated muscle is between "fast twitch" or “twitch” fibers and “slow” or “tonic” fibers.
  • Twitch fibers contract very quickly but do not sustain the contraction; tonic fibers contract in a more graded and sustained manner.

  • Early histologic studies differentiated fibrillenstruktur twitch–type fibers, with well-defined myofibrils and a well-developed, orderly arrangement of sarcomeres, from felderstruktur tonic fibrils that have a less well-organized myofibril structure and a more haphazard organization of sarcomeres.

  • Twitch fibers are more plentiful on the orbital side of the extraocular muscles, and tonic fibers predominate on the globe side.

• Cholinergic motor neurons supply both types of muscle fibers.

• The innervation to fibrillenstruktur fibrils is thick and heavily myelinated, with a single "en plaque" neuromuscular junction, whereas the innervation to felderstruktur fibrils is thin, with multiple grape-like clusters of neuromuscular junctions.

• One theory states that twitch fibers are important for rapid eye movements (eg, saccades) and the tonic fibers are more adapted to holding the eyes in eccentric positions after a saccade brings the eye to the desired position in the orbit.
  • Experimental evidence suggests that the situation is probably much more complex.

  • Both types of fibers contract during all types of fast and slow eye movements.