Anatomy of the Eye in Animals

Fundus and Tapetum

• Choroidal vessels are visible throughout the fundus.
• In some diurnal animals, melanocytes absorb light that has passed by photoreceptors without stimulating them.
• Nocturnal and most domestic mammals have a reflective material called tapetum lucidum, which enhances dark-adapted vision under dim light.

Iris and Pupil

• The iris controls the amount of light that passes through the pupil.
• In the dark, the pupil will dilate (mydriasis).
• In the light, the pupil will contract (miosis).
• The iris contains dilator and sphincter muscles.
• Pupillary dilator muscle:
  • Radially arranged
  • Opposes the action of the sphincter
  • Contraction results in pupillary dilation (mydriasis)
  • Dilation reflects the general state of sympathetic tone (e.g., pain, fear, anger)
• Pupillary sphincter muscle:
  • Circularly arranged near the pupillary margin
  • Contraction results in decreased pupillary size (miosis)
  • Innervated by parasympathetic fibers

Lens and Accommodation

• Behind the iris is the lens.
• The lens is suspended by suspensory ligaments.
• These fibers are attached to the ciliary body.
• The ciliary body is a muscular structure near the base of the iris.
• The ciliary body helps with accommodation of the lens.
• Increase or decrease in tension on the lens makes the lens curvature more or less convex.
• This allows the lens to focus on near or far objects.

Vitreous Humor and Retina

• Behind the lens is a chamber filled with vitreous humor.
• Vitreous humor is a gelatinous fluid.
• It gives the spherical shape of the eye.
• Contains phagocytic cells.
• Behind the vitreous humor lies the retina.
• The retina is where light is transformed into electrical activity of neurons.

Optic Disc and Optic Nerve

• The retina is interrupted at a point where axons of the retina ganglion cell layer leave the eye on their way to the brain – optic disc or blind spot.
• The axons leaving the eye at the optic disc give rise to the optic nerve (CN II).
• There are more axons in both optic nerves than in all the dorsal roots of the spinal cord.

Photoreceptor Cells

• There are about 130 million photoreceptor cells in the retina.
• There are two types of photoreceptor cells: cone cells and rod cells.
• Each type consists of two portions: outer segment and inner segment.
• Outer segment is the photosensitive region.
• Inner segment is the metabolic region of the photoreceptor.
• Rod cells have rhodopsin, which is responsible for the perception of shades of gray.
• Rhodopsin has a low threshold of excitability, making it easy to stimulate by low-intensity light.
• Rod cells are essential for night vision.
• Cone cells have color pigments or cone pigments, which are responsible for the perception of color.
• There are three different types of cones in primates, each with a different type of color pigment.
• Most mammals have only two types of cones (dichromatic).

Visual System

• Color is the brain's interpretation of differences in the wavelengths of light.
• The visual system involves the activation of rhodopsin, leading to the activation of a G protein, transducin, and the hydrolysis of cGMP to 5'-GMP.
• This leads to hyperpolarization, reducing the release of glutamate, and ultimately, the excitation of bipolar cells.
• The ratio of rods to cone photoreceptor cells, as well as the ratio of photoreceptors cells to ganglion cells, affects the acuity of visual images.
• The area centralis provides the highest visual acuity in the retina.

Eye and Camera

• The eye is optically equivalent to the usual photographic camera.
• The eye catches the light reflected by the objects and guides its passage until the image is formed.
• The lens works by converging the light rays to a certain focal point on the retina.
• The images formed on the retina are real, inverted, and smaller than the object.

This quiz covers the structure and function of the eye in animals, including the choroidal vessels and the tapetum lucidum. Learn about the unique features of diurnal and nocturnal animals' eyes!