Middle Layer of the Eye

Middle Layer

• The choroid has a rich capillary bed that nourishes photoreceptors of the eye and provides blood supply to the entire eye.
• It has a high concentration of light-absorbing pigment melanin.
• The ciliary body is a muscle that extends from the choroid in front of the eye, encircles the lens, and consists of a muscular component important for adjusting the refractive power of the lens.
• Ciliary processes have a vascular component that produces fluid (aqueous humor) that fills the front of the eye.

Iris and Pupil

• The iris is the colored part of the eye that regulates the entry of light into the eye.
• The pupil is the central opening of the iris, and its size is controlled neuronally by two sets of muscles with opposing actions, allowing the size of the pupil to be adjusted.

Intraocular Fluid Compartments

Aqueous Humor

• Aqueous humor is a freely flowing and transparent fluid found between the cornea and lens.
• It is produced by the ciliary epithelium through active secretion and by ciliary processes in the anterior chamber.
• It supplies the cornea and lens with nutrients (e.g., glucose, amino acid) and removes waste material.
• Aqueous humor is continually being formed and reabsorbed.
• It regulates intra-ocular pressure (15 mmHg) and is normally drained through a network of trabeculae into the canal of Schlemm, a venous channel at the junction between the iris and the cornea.

Vitreous Humor

• Vitreous humor occupies the space between the lens and retina.
• It is a transparent, jelly-like fluid that makes up 80% of the eye's volume and helps maintain the shape of the eyeball.
• It contains phagocytic cells that remove blood and debris that might otherwise interfere with light transmission.
• It is produced by retinal glial cells and ciliary epithelial cells.

Cornea

• The cornea is about 11 mm in diameter and 1 mm in thickness.
• It bulges forward and has no blood vessels.
• It obtains oxygen supply through diffusion from air and surrounding structures.
• It is a transparent window of the eye, composed of connective tissue with a thin layer of epithelium and collagenous fibers.
• It is supplied by the trigeminal nerve.
• The cornea has two main functions: light refraction (40 diopters) and protection against harmful agents (e.g., foreign bodies, dust).

Corneal Reflex

• The corneal reflex is a protective reflex that shuts the eye by its lids when the cornea is touched.
• Components of the corneal reflex include:
  • Receptor: corneal receptor
  • Afferent nerve: ophthalmic division of trigeminal nerve → trigeminal ganglion → trigeminal nucleus in pons
  • Center: superior colliculus
  • Efferent: Facial nerve
  • Effector: orbicularis oculi muscle
  • Result: shutting of the eyelids

Lens

• The lens is an avascular, biconvex structure enclosed in an elastic capsule.
• It lies directly behind the iris and pupil.
• It is a flexible, transparent body with a naturally rounded shape, held in place by a circular lens ligament (zonule).
• The zonule is attached to the thickened anterior part of the choroid, the ciliary body.
• The ciliary body contains circular muscle fibers and longitudinal muscle fibers that attach near the corneoscleral junction.
• The lens provides an important refractive media of the eye (20 diopters) and is under physiologic control.
• It enables the eye to see near objects (accommodation) and protects the retina from harmful effects of ultraviolet rays entering the eye.

Lacrimal Apparatus

• The lacrimal apparatus consists of a tear-secreting gland and a series of ducts that carry tears into the nasal cavity.
• Tears exit the lacrimal gland through tiny tubules and flow downward and medially across the eye.
• Superior and inferior canaliculi collect tears → into lacrimal sac → nasolacrimal duct, which empties into the nasal cavity.
• Normal secretion of tear is less than 1 ml/day, with a pH value of 7.4.
• Tear is iso-osmotic with blood plasma and has functions including:
  • Moistening and lubricating the surface of the eye
  • Mechanical function
  • Lysosome action (anti-bacterial action)
  • Cleaning and nourishing the cornea
  • Maintaining the corneal surface moist

Extraocular Muscles

• The eye is moved within the orbit by six extraocular muscles.
• Four rectus muscles control eye ball movement in perpendicular axes.
• Two oblique muscles enable transitional rotation of the eye.
• Three cranial nerves are concerned with eyeball movements:
  • Oculomotor N: superior rectus, medial rectus, inferior rectus, and inferior oblique
  • Trochlear N: superior oblique
  • Abducens N: lateral rectus