Section IX. THE SPECIAL SENSE OF VISION (SIGHT)
a. Stimulus. Rays of light stimulate the receptor tissues of the eyeballs (bulbus
oculi) to produce the special sense of vision. This includes both the sensation of vision
or seeing and a variety of reactions known as the light reflexes. The actual reception of
the light energy is a chemical reaction which in turn stimulates the neuron endings.
b. Optical Physics. To appreciate the functioning of the bulbus oculi, some
simple principles of optical physics must be understood.
(1) By means of a lens system, light rays are bent and brought to the focal
point for acute vision. This process is referred to as focusing.
(2) The focal length is the distance from the focal point to the center of the
lens. The amount of bending or focusing depends upon the exact curvatures of the lens
c. Sense Organ. The eyeball is the special sense organ which contains the re-
ceptor tissues. The eyeball is suspended in the orbit. The orbit is a skeletal socket of
the skull which helps protect the eyeball. Various structures associated with the
functioning of the eyeball are called the adnexa. The adnexa include the eyelids, the
lacrimal system, etc.
11-27. THE EYEBALL (FIGURE 11-10)
a. Shape. In the main, the eyeball is a spherical bulb-like structure. Its anterior
surface, transparent and more curved, is known as the cornea of the eyeball.
b. Wall of the Eyeball. The eyeball 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 FCT (fibrous
connective tissue). 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 eyeball.
(2) Choroid. The middle layer of the wall of the eyeball 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 light rays
and prevents them from reflecting at random.