Page 90 - 84 human physiolofy part-1

Chapter 5

extraocular muscles have their origin in the back of the orbit in a fibrous ring called the annulus
of Zinn. Four of these then course forward through the orbit and insert onto the globe on its
anterior half (i.e., in front of the eye's equator). These muscles are named after their straight
paths, and are called the four rectus muscles, or four recti. They insert on the globe at 12, 3, 6,
and 9 o'clock, and are called the superior, lateral, inferior and medial rectus muscles. (Note that
lateral and medial are relative to the subject, with lateral toward the side and medial toward the
midline, thus the medial rectus is the muscle closest to the nose).

Eye Movement

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. 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. 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.

Try This 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.

How we see an object

• The light rays enter the eye through the cornea (transparent front portion of eye to focus the
light rays)
• Then, light rays move through the pupil, which is surrounded by Iris to keep out extra light
• Then, light rays move through the crystalline lens (Clear lens to further focus the light
rays )
• Then, light rays move through the vitreous humor (clear jelly like substance)
• Then, light rays fall on the retina, which processes and converts incident light to neuron
signals using special pigments in rod and cone cells.
• These neuron signals are transmitted through the optic nerve,
• Then, the neuron signals move through the visual pathway - Optic nerve > Optic Chiasm >
Optic Tract > Optic Radiations > Cortex
• Then, the neuron signals reach the occipital (visual) cortex and its radiations for the brain's
processing.

90 | Human Physiology