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 system.

c. Sense Organ. The eyeball is the special sense organ which contains the receptor 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.

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