Certain stimuli are received by sense organs distributed throughout the entire body. These are referred to as the general senses.
Certain other stimuli are received by pairs of receptor organs located in the head. These are the special senses.
The cells of the retina include special photoreceptor cells in the form of cones and rods. The light ray stimulus chemically changes the visual chemical of the cones and rods.
The cones of the retina are for acute vision and also receive color information. The cones tend to be concentrated at the rear of the eyeball.
Light received by the rods is perceived in terms of black and white. The rods are sensitive to less intensive light than the cones.
The two optic nerves enter the cranial cavity and join in a structure known as the optic chiasma. Leading from the optic chiasma on either side of the brainstem is the optic tract.
There are a number of protective reactions to visual input - the visual reflexes.
When an unexpected visual stimulus occurs within the visual field, the individual's response will often include movement and other types of reaction. This is a part of the startle reflex.
When there is a change in the amount of light entering the eyeball, the size of the pupil will change. This is the pupillary reflex. In the blink reflex, the eyelids automatically move over the exterior surface of the eyeball.
Helping to fill the orbit are a number of structures associated with the eyeball; these are the adnexa. Among these other structures is the lacrimal apparatus. The lacrimal gland is located in the upper outer corner in front.