Chapter III - Visual Apparatus

Related Parts of Eyes
In many species, the eyes are inset in the portion of the skull known as the orbits or eye sockets. This placement of the eyes helps to protect them from injury.

Eyebrows
In humans, the eyebrows redirect flowing substances (usually rainwater) away from the eye. Water in the eye can alter the refractive properties of the eye and blur vision. It can also wash away the tear fluid - along with it the protective lipid layer - and can alter corneal physiology, due to osmotic differences between tear fluid and freshwater. This is made apparent when swimming in freshwater pools, as the osmotic gradient draws 'pool water' into the corneal tissue, causing edema, and subsequently leaving the swimmer with "cloudy" or "misty" vision for a short period thereafter. It can be reversed by irrigating the eye with hypertonic saline.

Eyelids
Eyelids wipe the eye and prevent dehydration. They spread tear fluid on the eyes, which contains substances which help fight bacterial infection as part of the immune system. Some aquatic animals have a second eyelid in each eye, which refracts the light and helps them see clearly both above water and below it. Most creatures will automatically react to a threat to its eyes (such as an object moving straight at the eye or a bright light) by covering the eyes, and/or by turning the eyes away from the threat. Blinking the eyes is, of course, also a reflex.

Eyelashes
Eyelashes prevent fine particles from entering the eye. Fine particles can be bacteria, but also simple dust, which can cause irritation of the eye, and lead to tears and subsequent blurred vision.

Eye Movements

Extraocular muscles
Each eye has six muscles that control its movements: the lateral rectus, the medial rectus, the inferior rectus, the superior rectus, the inferior oblique, and the superior oblique. When the muscles exert different tensions, a torque is exerted on the globe that causes it to turn. This is an almost pure rotation, with only about one millimeter of translation (Carpenter, 1988). Thus, the eye can be considered as undergoing rotations about a single point in the center of the eye.

Rapid eye movement
Rapid eye movement typically refers to the stage during sleep during which the most vivid dreams occur. During this stage, the eyes move rapidly. It is not in itself a unique form of eye movement.

Saccades
Saccades are rapid refocussing actions of the eyes. Many animals are able to quickly look at a point in space (prompted by memory, peripheral vision or an audio cue) without actively looking at anything in between. The eyes simply jerk into a new position. Saccades move the eye at up to 900°/s in adult humans, and take roughly 250 milliseconds to be initiated by the neural network.

Microsaccades
Even when looking intently at a single spot, the eyes drift around. This ensures that individual photosensitive cells are continually stimulated in different degrees. Without changing input, these cells would otherwise stop generating output. Microsaccades move the eye no more than a total of 0.2° in adult humans.

Vestibulo-ocular reflex

The vestibulo-ocular reflex (VOR) is a reflex eye movement that stabilizes images on the retina during head movement by producing an eye movement in the direction opposite to head movement, thus preserving the image on the center of the visual field. For example, when the head moves to the right, the eyes move to the left, and vice versa. Since slight head movements are present all the time, the VOR is very important for stabilizing vision.

Smooth pursuit movement
The eyes can also follow a moving object around. This is less accurate than the vestibulo-ocular reflex as it requires the brain to process incoming visual information and supply feedback. Following an object moving at constant speed is relatively easy, though the eyes will often make saccadic jerks to keep up. The smooth pursuit movement can move the eye at up to 100°/s in adult humans.

Optokinetic reflex
The optokinetic reflex is a combination of a saccade and smooth pursuit movement. When, for example, looking out of the window in a moving train, the eyes can focus on a 'moving' tree for a short moment (through smooth pursuit), until the tree moves out of the field of vision. At this point, the optokinetic reflex kicks in, and moves the eye back to the point where it first saw the tree (through a saccade).

Accommodation
Accommodation is the process by which the eye increases optical power to maintain a clear image (focus).

The accommodation reflex is a reflex action of the eye, in response to focusing on a near object, then looking at distant object (and vice versa). It is dependent on cranial nerve II (afferent limb of reflex), higher centres and cranial nerve III.

A near object (for example, a computer screen) appears large in the field of vision, and the eye receives light from wide angles. When focusing on a near object, the pupil constricts in order to prevent diverging light rays from hitting the periphery of the retina and resulting in a blurred image. As the pupil constricts, the lens becomes more spherical to allow for the diverging light rays.

Animals with compound eyes have a wide field of vision, allowing them to look in many directions. To see more, they have to move their entire head or even body.

The visual system in the brain is too slow to process that information if the images are slipping across the retina at more than a few degrees per second. Thus, for humans to be able to see while moving, the brain must compensate for the motion of the head by turning the eyes. Another complication for vision in frontal-eyed animals is the development of a small area of the retina with a very high visual acuity. This area is called the fovea, and covers about 2 degrees of visual angle in people. To get a clear view of the world, the brain must turn the eyes so that the image of the object of regard falls on the fovea. Eye movements are thus very important for visual perception, and any failure to make them correctly can lead to serious visual disabilities. To see a quick demonstration of this fact, try the following experiment: hold your hand up, about one foot (30 cm) in front of your nose. Keep your head still, and shake your hand from side to side, slowly at first, and then faster and faster. At first you will be able to see your fingers quite clearly. But as the frequency of shaking passes about one hertz, the fingers will become a blur. Now, keep your hand still, and shake your head (up and down or left and right). No matter how fast you shake your head, the image of your fingers remains clear. This demonstrates that the brain can move the eyes opposite to head motion much better than it can follow, or pursue, a hand movement. When your pursuit system fails to keep up with the moving hand, images slip on the retina and you see a blurred hand. Having two eyes is an added complication, because the brain must point both of them accurately enough that the object of regard falls on corresponding points of the two retinas; otherwise, double vision would occur. The movements of different body parts are controlled by striated muscles acting around joints. The movements of the eye are no exception, but they have special advantages not shared by skeletal muscles and joints, and so are considerably different.

The Orbit