Anatomy and Physiology: Vision Chapter

• Chapter 10 discusses vision

Eye Components

• Allow for an accurate image

Photoreceptors

• Located in a single layer of cells in the retina
• Detect images
• Generate visual nerve impulses

Eyeball Structure

• Consists of three layers: outer fibrous, middle vascular, and inner nervous

Outer Fibrous Layer

• Cornea is transparent letting light into the eye, it contains regularly arranged collagen fibers and many pain receptors, but no blood vessels.
• The sclera is the white part of the eye.
• The limbus is the junction of the cornea and sclera

Middle Vascular Layer (Uvea)

• Choroid: Lies between the sclera and retina, contains pigment and blood vessels.
• Tapetum lucidum: A highly reflective area in rear of eye.
• Iris: Pigmented muscular diaphragm that controls the amount of light entering the posterior eye.
• Pupil is the opening in the center of iris.
• Ciliary Body: Ring-shaped structure behind the iris made of tiny muscles which adjust lens to allow near and far vision.

Inner Nervous Layer

• Retina lines the back of the eye and is one component of the fundus.
• Retina contains rods and cones, the sensory receptors for vision

Compartments of the Eyeball

• The eye is separated into two major compartments, the aqueous compartment and the vitreous compartment.

Aqueous Compartment

• Located in front of the lens, it is subdivided by the iris into the anterior and posterior chambers.
• Contains a clear, watery fluid called the aqueous humor.
• The aqueous humor is produced in the posterior chamber by cells of the ciliary body
• It drains via the canal of Schlemm.

Vitreous Compartment

• Located behind the lens and ciliary body, it fills the whole back of the eyeball.
• Contains a clear, gelatinous fluid called the vitreous humor

Lens

• The lens is made of soft, translucent layers of fibers that are elastic and biconvex.
• The front surface is in contract with aqueous humor and back of the lens is in contact with vitreous humor
• It helps in focusing a clear image on the retina via accommodation

Retina

• Optic Disc is the "blind spot" of the eye.
• Site where nerve fibers on inside of retina converge and leave the eye to form the optic nerve
• Photoreceptor cells are neurons with dendrites modified into sensory receptors for light
  • Rods are more sensitive to light
  • Cones are more sensitive to color and detail

Image Formation

• Four refractive media in the eye help in forming as clear upside-down image on the retina
  • The four components are the cornea, aqueous humor, lens and vitreous humor
• The brain inverts the image, so the conscious mind sees everything right-side up

Extraocular Structures

• Conjunctiva: Thin, moist, transparent membrane that covers front portion of eyeball (bulbar conjunctiva) and lines interior surfaces of eyelids (palpebral conjunctiva).
• Conjunctival Sac: Space between bulbar and palpebral portions of conjunctiva.
• Eyelids: Upper and lower folds of skin, lined by conjunctiva.
• Lateral and Medial Canthus: Corners where the eyelids come together.
• Tarsal Glands (Meibomian Glands): Tiny openings found along eyelid margin that produce a waxy substance to prevent tears from overflowing onto the face.
• Eyelashes are present on upper lid in most animals.
• Third Eyelid (Nictitating Membrane): T-shaped plate of cartilage covered by conjunctiva, located medially between eyelids and eyeball.
  • It has no muscle attachment, causing passive movements.
  • Lymph nodules and accessory lacrimal gland are on the ocular surface

Lacrimal Apparatus

• The lacrimal apparatus produces, secretes and drains tears away from the surface of the eye.
• Lacrimal glands serve as the primary tear source.
• The tear drainage system includes lacrimal puncta, lacrimal sacs, and the nasolacrimal duct

Eye Muscles

• Small, skeletal muscles that attach to the sclera
• Allow for a wide range of movements.

Tears

• Liquid film that moistens and protects the surface of the eye.
• Three main layers of tears:
  • Inner mucous layer: From cells in conjunctiva; contains antibacterial substances.
  • Middle tear layer: From lacrimal glands and accessory lacrimal glands of the third eyelid; keeps the cornea moist.
  • Outer oily layer: From tarsal or meibomian glands; reduces evaporation of underlying tear layer and prevents tears from flowing over the lid margin.