Orbits

MEDU3300 Human Structure II

Introduction

  • Course focusing on Human Structure II, specifically the orbit of the eye.

  • Led by Dr. Christopher See at the Faculty of Medicine, The Chinese University of Hong Kong.

Learning Objectives

  • Bony Boundaries and Foramina of the Orbit:

    • Identify the bony structures that compose the orbit and the foramina (openings) through which various nerves and vessels pass.

  • Relations to Neighbouring Regions:

    • Understand the anatomical relations of the orbit to adjacent skull regions.

  • Superficial Structures:

    • Understand key superficial structures associated with the eye, including the eyelids, lacrimal gland, and tear drainage system.

  • Nerve Supply:

    • Describe the sensory, motor, and autonomic nerve supply to the orbit and eye.

  • Blood Vessels:

    • Discuss major blood vessels of the eye, their functions, and clinical significance.

  • Extraocular Muscles:

    • Relate extraocular muscles to specific eye movements.

Anatomy of the Orbit

General Structure

  • The orbit is a pyramidal structure, not a perfect sphere:

    • Apex: points posteriorly.

    • Base: situated anteriorly.

Composed of Seven Bones:

  1. Larger External Bones:

    • Frontal

    • Sphenoid

    • Maxilla

    • Zygomatic

    • Palatine*

  2. Smaller Internal Bones:

    • Lacrimal

    • Ethmoid

Detailed Bone Structures

Notable Components:

  • Frontal Bone:

    • Contains the supraorbital notch (foramen) for the passage of nerves and vessels.

  • Zygomatic Bone:

  • Maxilla:

    • Houses the infraorbital foramen for nerve passage.

  • Sphenoid Bone:

    • Features lesser and greater wings, which contribute to the shape and structure of the orbit.

Foramina of the Orbit

  • Important openings in the orbit:

    • Superior Orbital Fissure: Allows passage of nerves and vessels into the orbit.

    • Optic Canal/Foramen: Another passageway for important neural and vascular structures.

Clinical Cases

Case 1: Exophthalmos (Bulging Eye)

  • Can be caused by autoimmune inflammation of orbital tissues (such as in Graves' disease), bleeding, or tumors.

Contents of the Orbit

  • Eyeball:

    • Related lecture on vision will be covered later in the semester.

  • Eyelids and Associated Structures:

  • Muscle Composition:

    • 8 Muscles: 4 Recti (straight) and 2 Obliques (angled).

    • Levator palpebrae superioris (raises the upper eyelid).

    • Superior Tarsal Muscle (smooth muscle contributing to eyelid function).

  • Cranial Nerves:

    • 5 key cranial nerves involved in sensory and motor functions:

    1. Optic (CN II): Responsible for vision.

    2. Oculomotor (CN III): Controls most eye movements.

    3. Trochlear (CN IV): Assists in eye movement.

    4. Abducens (CN VI): Controls lateral movement of the eye.

    5. Trigeminal (CN V): Provides sensory function to the face and eye.

  • Blood Supply:

    • Arterial supply from the ophthalmic artery; venous drainage via superior and inferior ophthalmic veins.

  • Orbital Fat:

    • Provides cushioning and stabilizes extraocular muscles in the orbit.

Arterial Supply of the Orbit

  • Ophthalmic Artery:

    • Has multiple branches, including:

    • Central Retinal Artery: Supplies the retina.

    • Long and Short Posterior Ciliary Arteries: Supply the eyeball.

    • Muscular Branches: Serve extraocular muscles.

    • Additional arteries include: Anterior and Posterior Ethmoidals, Supratrochlear, and Dorsal Nasal Arteries.

Venous Drainage of the Orbit

  • More complex anatomy with anastomoses connecting external (facial) veins to internal (pterygoid plexus and cavernous sinus) veins.

  • Central Retinal Vein:

    • Drains into the superior ophthalmic vein or cavernous sinus directly.

Nerves of the Orbit

Sensory and Motor Nerves:

  • Sensory:

    • Optic CN II: Carries vision signals.

    • Trigeminal (Ophthalmic Branch) CN V1 includes:

    • Frontal

    • Lacrimal

    • Nasociliary Nerves.

  • Motor:

    • Extraocular muscle innervation:

    • CN III (Oculomotor): Innervates Levator palpebrae superioris, Superior, Inferior, and Medial Recti, and Inferior Oblique.

    • CN IV (Trochlear): Innervates Superior Oblique.

    • CN VI (Abducens): Innervates Lateral Rectus.

Autonomic Innervation Overview

Parasympathetic Innervation:

  • Innervation to iris and ciliary body:

    • Via CN III through the ciliary ganglion.

  • Innervation to lacrimal gland:

    • Via CN VII through the pterygopalatine ganglion.

Sympathetic Innervation:

  • Sympathetic fibers originate from the superior cervical ganglion and travel along the internal carotid plexus to reach different structures of the eye.

Clinical Conditions Related to Eyelids

  • Discuss traumatic issues, muscle innervations affecting eyelid positions, such as ptosis (droopy eyelid).

  • Horner's Syndrome:

    • Defined: A triad of symptoms due to sympathetic trunk damage:

    1. Partial ptosis (due to denervation of the superior tarsal muscle).

    2. Miosis (pupil constriction due to denervation of dilator pupillae).

    3. Anhydrosis (absence of sweating on the ipsilateral side).

    • Can indicate serious conditions like Pancoast tumor, aortic aneurysm, or thyroid carcinoma.

Tear Drainage System

Structure and Function:

  • Tears are produced and drained through the lacrimal apparatus:

    • Tears swept by blinking from the eye margin towards the lacrimal punctum, where drainage occurs.

Extraocular Muscles Overview

  • CN III (Oculomotor), CN IV (Trochlear), and CN VI (Abducens) control specific muscle groups.

Actions of Extraocular Muscles:

  1. Rectus muscles: Pull in the direction of their names (e.g., Medial Rectus adducts the eye).

  2. Superior and Inferior Rectus: Also adduct and elevate/depress the eye.

  3. Superior and Inferior Oblique: Actions are reversed; they abduct the eye in opposite directions (e.g., Inferior Oblique elevates).

Clinical Testing of Extraocular Muscles

  • Conducted using an H-pattern tracking method to assess eye movements:

    • Superior Rectus, Inferior Oblique, Lateral Rectus, Medial Rectus, Inferior Rectus, Superior Oblique.

Conclusion

  • The orbit and its contents, including the extraocular muscles, their innervation, and pathologies, are key to understanding eye function and clinical conditions related to ocular health.