Accessory Structures of the Eye

Lacrimal Apparatus

The lacrimal apparatus is responsible for producing and draining tears, which serve to protect, moisten, and lubricate the eye. Tears are secreted by the lacrimal glands and emptied into the nasal cavity.

• Functions: Protects, moistens, and lubricates the eye

• Lacrimal secretions (tears) contain:

  • Dilute salt solution

  • Mucus

  • Antibodies

  • Lysozyme (an enzyme that destroys bacteria)

Extrinsic Eye Muscles

Six extrinsic eye muscles attach to the outer surface of the eye, allowing for a wide range of eye movements. These muscles are essential for coordinated and precise movement of the eyeball.

• Six muscles attach to the outer surface of the eye

• Produce eye movements

Structure of the Eye

Layers of the Eyeball

The wall of the eyeball is composed of three main layers, each with distinct functions and structures. Additionally, the interior of the eye is filled with fluids known as humors.

• Fibrous layer: Outside layer

• Vascular layer: Middle layer

• Sensory layer: Inside layer

• Humors: Fluids that fill the interior of the eyeball

The Fibrous Layer

The fibrous layer is the outermost layer of the eye, providing structure and protection. It consists of the sclera and the cornea.

• Sclera:

  • White connective tissue layer

  • Seen anteriorly as the "white of the eye"

• Cornea:

  • Transparent, central anterior portion

  • Allows for light to pass through

  • Repairs itself easily

  • The only human tissue that can be transplanted without fear of rejection

The Vascular Layer

The vascular layer is the middle layer of the eye, rich in blood vessels and pigment. It nourishes the eye and prevents light from scattering within the eyeball.

• Choroid: Blood-rich nutritive layer in the posterior of the eye

  • Pigment prevents light from scattering

• Modified anteriorly into two structures:

  1. Ciliary body: Smooth muscle attached to the lens by the ciliary zonule (suspensory ligament)

  2. Iris: Regulates the amount of light entering the eye

     • Pigmented layer that gives eye color

     • Pupil: Rounded opening in the iris