EYE 



2119 



EYE 



with the vitreous humor, a substance some- 

 what of the consistency of thin jelly. The 

 eyeball itself is in great part enclosed in a 

 membranous sac, the capsule. The anterior 

 portion of the sclerotic is also covered by a 

 delicate membrane, the conjunctiva, which is 

 reflected from the sclerotic to the inner sur- 

 face of the lids. 



The eyeball rests in a bony cavity called the 

 socket, or orbit, upon a cushion of fat which 

 protects it from jars or shocks. It has at- 

 tached to it at various points muscles which 

 move it in the different directions. A gland, 

 the lachrymal, is located under the upper lid 

 at its outer portion, and this manufactures the 

 tears. A number of pipe lines or ducts lead 

 from this gland and open on the inner surface 

 of the upper lid. During the act of winking 

 these ducts are squeezed, and in this way the 

 delicate cornea of the eye is sprayed with the 

 tears, and its surface kept moist and brilliant. 

 The tears are carried away partly by evapora- 

 tion and partly by drainage canals located at 

 the inner corner of the eye, one on the mar- 

 gin of each lid, which lead by a duct or chan- 

 nel into the low r er portion of the interior of 

 the nose. 



The lids are formed of loose tissue, embed- 

 ded in which are plates of stiff material called 

 tarsal cartilages, which give them form and 

 firmness. The upper lid is very movable; the 

 lower lid moves but little. The eyelashes are 

 inserted in the margin of each lid, those of 

 the upper curving upward, those of the lower 

 downward, in order that they may not inter- 

 lace when the eye is closed. The opening be- 

 tween the lids is called the commissure. A 

 large or small eye depends not upon the size 

 of the eyeball itself, but upon the width and 

 breadth of this commissure which permits a 

 greater or less amount of the eyeball to be 

 seen. The eyelids and eyelashes are a great 

 protection to the eye; they safeguard it from 

 flying bodies, dust and wind, shade it from 

 the light, and by the act of winking sweep it 

 of dust and distribute the tears and keep its 

 surface moist. The eyebrows shade the eyes 

 and prevent dust and perspiration from enter- 

 ing them. 



How We See. Light is a form of energy 

 caused by vibration of the ether. It eminates 

 from a luminous body as waves, called rays. 

 When an object is seen, the rays of light from 

 this object strike first the cornea of the eye, 

 then pass through it and through the anterior 

 chamber, pupil, lens and vitreous humor. 



These media, as they are called, refract, or 

 bend, the rays to such a degree that they are 

 all collected into a small point, and come 

 finally to a focus or meeting place on the 

 retina, which is the receiving nerve screen of 

 the eye. The different rays from the object 

 really form upon the retina a miniature pic- 

 ture of the object looked at, and this picture 

 is relayed to the optic nerve, which in turn 

 carries it to the center of sight in the brain, 

 where it is finally interpreted as the picture 

 of the object itself. Each eye conveys a sep- 

 arate picture of the object looked at to the 

 sight centers in the brain, and these pictures 

 are inverted, or upside down. In the sight 

 centers, however, by a process little under- 

 stood, the two pictures are fused or merged 

 into one single, distinct image of the object 

 looked at, and the inverted position is cor- 

 rected; so through these agencies the final 

 result of the visual act is that in normal eyes 

 only one object is seen, and this in its correct, 

 upright position. 



Accommodation. If you look at something 

 very near to your eye, objects at a distance 

 then become indistinct; if, on the other hand, 

 you gaze at something far away, objects close 

 at hand become hazy. This is so because na 

 lens can focus on the same screen at the same 

 instant rays of light coming from a near ob- 

 ject and rays of light coming from a distant 

 one. In order that a clear, well-defined, dis- 

 tinct picture of an object be seen it is neces- 

 sary that the rays of light coming from this 

 object be brought to a definite, precise focus 

 on the retina and that all other objects not 

 within this particular distance be more or less 

 excluded from consideration. 



In a camera the adaptation, or focusing, for 

 near and distant objects is brought about by 

 moving the lens of the camera forward and 

 backward the appropriate distance. In the 

 human eye, however, it is not possible to alter 

 in this way the distance by moving the lens 

 of the eye backward and forward, but the ad- 

 justment is brought about by the human lens 

 changing its shape or curvature to suit the 

 varying distances. This function of the lens 

 is designated as that of accommodation, and 

 is brought about by a highly-complex nerve- 

 muscle impulse-adjustment. 



We Cannot Always Believe What We See. In 

 spite of the wonderful structure of our eyes 

 they would be untrustworthy if intelligence 

 were not applied to correct their mistakes. 

 Are the things we see outside of our eyes or 



