318 THE HUMAN BODY. 



the optic nerve the visual nerve-centre, and the result is a 

 sight-sensation. * 



The Eye-Socket, The eyeball is lodged in a bony cav- 

 ity, 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 eye- 

 lid are from twenty to thirty minute glands, called the 

 Meibomian follicles. Their secretion is sometimes abnor- 

 mally 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 



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 sight-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 metaphj-sical 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 con- 

 structed 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 pecu- 

 liarity 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 8omething 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. 



