a. Shape. Normally the bulbus oculi is a spherical bulb-like structure. Its
anterior surface, transparent and more curved, is known as the cornea of the bulbus
b. Wall of the Bulbus Oculi. The bulbus oculi is a hollow structure. Its wall is
made up of three layers known as coats or tunics.
(1) Sclera. The outermost layer is white and very dense fibrous connective
tissue (FCT). It is known as the sclera, scleral coat, or fibrous tunic. Its anterior portion
is called the cornea. As already mentioned, the cornea is transparent and more curved
than the rest of the sclera. The fixed curvature of the cornea enables it to serve as the
major focusing device for the bulbus oculi.
(2) Choroid. The middle layer of the wall of the bulbus oculi is known as the
choroid, the choroid coat, or the vascular tunic. This layer is richly supplied with blood
vessels. It is also pigmented with a black material. The black color absorbs the light
rays and prevents them from reflecting at random.
(3) Retina. The inner layer of the wall of the bulbus oculi is known as the
retina, retinal coat, or internal tunic. The actual photoreceptor elements are located in
the retina at the back and sides of the bulbus oculi. These elements are the rods and
cones. They constitute the nervous portion of the retina. In the anterior part of the
bulbus oculi, the retina continues as a non-nervous portion.
Internal Structures of the Bulbus Oculi.
The nervous retina.
(a) The photoreceptors of the nervous portion of the retina (Figure 5-2)
contain chemicals known as visual pigments (rhodopsin). The cones are more
concentrated in the center at the back of the bulbus oculi. The cones can perceive
colors and are used for acute vision. However, cones require more intense light than do
rods. The rods are distributed more toward the sides of the nervous retina. Although
the rods are capable of perceiving less intense light, rods perceive only black and white.
(b) If you look directly at an object, light from the object will fall in the
small depression of the retina called the fovea centralis. The fovea centralis is at the
posterior end of the bulbus oculi, exactly opposite the centers of the cornea, pupil, and
lens. The fovea centralis is found in a small yellow area of the retina called the macula
lutea. The macula lutea is the area of the retina where vision is the sharpest.