This section is from the book "The Human Body: An Elementary Text-Book Of Anatomy, Physiology, And Hygiene", by H. Newell Martin. Also available from Amazon: The Human Body.
The Visual Apparatus consists of nervous tissues immediately concerned in giving rise to sensations, supported, protected, and nourished by other parts. Its essential parts are, (1) the retina, a thin membrane lying in the eyeball and containing microscopic elements which are so acted upon by light as to stimulate (2) the optic nerve; this nerve ends (3) in a part of the brain (visual centre) which when stimulated arouses in our consciousness a feeling or sensation of sight. The visual centre may be excited in very many ways, and quite independently of the optic nerve or the retina; as is frequently seen in delirious persons, in whom inflammation or congestion of the brain excites directly the visual centre and gives rise to visual hallucinations.
Usually, however, the cerebral visual centre is only excited through the optic nerve, and the optic nerve only by light acting upon the retina. The eyeball, containing the retina, is so constructed that light can enter it, and so placed and protected in the body that as a general thing no other form of energy can act upon it so as to stimulate the retina. Under exceptional circumstances we may have sight-sensations when no light reaches the eye; anything which stimulates the retina, so long as it is connected by the optic nerve with the cerebral visual centre, will cause a sight-sensation. A severe blow on the eye, even in complete darkness, will cause the sensation of a flash of light; the compression of the eyeball excites the retina, the retina excites the optic nerve, the optic nerve the visual nerve-centre, and the result is a sight-sensation.*
Of what does the visual apparatus consist? What are its essential parts? What happens when the visual centre is stimulated? Is it only stimulated by the agency of light? Illustrate.
How is the visual centre usually excited? Why is light the form of energy which most often stimulates the retina? Give an example of the production of a sight-sensation in the absence of light. What happens in the nervous system when a man " sees sparks" on receiving a blow in the eye?
The eyeball is lodged in a bony cavity, the orbit, open in front. Each orbit is a pyramidal chamber containing connective tissue, blood-vessels, nerves, and much fat; the fat forms a soft cushion on which the back of the eyeball rolls.
The Eyelids are folds of skin, strengthened by cartilage and moved by muscles. Opening along the edge of each eyelid are from twenty to thirty minute glands, called the Meibomian follicles. Their secretion is sometimes abnormally abundant, and then appears as a yellowish matter along the edges of the eyelids, which often dries in the night and causes the lids to be glued together in the morning. The eyelashes are curved hairs, arranged in one or two rows along each lid and helping to keep dust from falling into the eye; and, when the lids are nearly closed, to protect it from a dazzling light.
In what is each eyeball lodged? What does the orbit contain?
What are the eyelids? The Meibomian glands? Why are the eyelids sometimes stuck together in the morning? What are the uses of the eyelashes?
* The fact that right-sensations may be aroused quite independently of all light acting upon the eye is paralleled by similar phenomena in regard to other senses, and is of fundamental psychological and metaphysical importance. That a blow on the closed eye gives rise to a vivid light-sensation, even in the absence of all actual light, proves that our sensation of light is quite a different thing from light itself. The visual sensory apparatus, it is true, is so constructed and protected that of all the forces of nature, light is the one which far most frequently stimulates it But as regards the peculiarity in the quality of the sensation which leads us to classify it as " a visual sensation," that peculiarity has nothing to do with any property of light. The visual nerve-centre when stimulated causes a sight-sensation, whether it has been excited by light, or by a blow, or by electricity. Similarly the auditory brain-centre gives us a sound-sensation when stimulated by actual external sound-waves, or by a blow on the ear, or by disease of the auditory organ. One kind of energy, light, excites more often than any other the visual nerve-apparatus; another, sound, the auditory nerve-apparatus; a third, pressure, the touch nerve-organs. Hence we come to associate light with visual sensations and to think of it as something like our sight-feelings; and to imagine sound as something like our auditory sensations; and so forth. As a matter of fact both light and sound are merely movements of ether or air; it is our own stimulated nerve-centres which produce visual and auditory sensations; the ethereal or aerial vibrations merely act as the stimuli which arouse the nervous apparatus.