EYE 



2118 



EYE 



another eating an orange or lemon feels that 

 he actually tastes the fruit itself. Our first im- 

 pressions of people are said to be, in great 

 part, formed by what we see in their eyes. 



Anatomy of the Eye. The eye is spoken of 

 as a ball, or globe; accurately speaking, how- 

 ever, it is neither a globe nor a ball, since its 

 vertical and transverse diameters measure 

 slightly more than its anterior and posterior 

 diameter, the former being approximately 

 about one inch, the latter about nine-tenths 

 of an inch. The eye is in reality a hollow ball 

 filled with a liquid, a solid, and a semi-solid 



m 



CROSS SECTION OF THE EYE 



(o) Optic nerve 



<p) Pupil 



(r) Retina 



( s ) Sclera 



(v) Vitreous humor 



(a) Aqueous humor 



(c) Cornea 



(ch) Choroid 



(i) Iris 



(I) Lens 



(TO) Ciliary muscle 



body. The walls forming the ball are arranged 

 in several layers, which are superimposed. 

 The outer wall, or layer, is made up of two 

 tissues which merge into one another. The 

 posterior five-sixths of this wall is formed by 

 a firm fibrous tissue called the sclerotic, which 

 one sees as the white of the eye. It gives form 

 and protection to the ball. Joining the scle- 

 rotic in front, and forming the remaining sixth, 

 is a transparent portion, the cornea, which 

 projects from the sclerotic as a watch crys- 

 tal projects from a watch. The cornea is really 

 the window of the eye. Within the sclerotic 

 is another layer, the choroid, made up prin- 

 cipally of blood vessels which are the chief 

 source of nourishment for the tissues of the 

 eyeball. This layer, as it runs forward, merges 

 into another tissue, the ciliary body, and this 

 in turn merges into the iris. The ciliary body 

 contains the muscle used in focusing the eye, 

 and to it also are attached the ligaments which 

 hold the lens in position. The iris is per- 



forated at about its center, forming the black 

 spot, or pupil. By means of muscles in the 

 iris substance, the pupil can be made to dilate 

 or contract grow smaller or larger and in 

 this way the amount of light permitted to 

 enter the eye is regulated. The iris is vari- 

 ously colored in different individuals, forming 

 the blue, brown or gray eyes, etc., the colors 

 depending upon the amount and general dis- 

 tribution of pigment in the iris substance. A 

 large number of infants at birth are blue-eyed. 

 In many, however, after some weeks have 

 passed the color of the iris changes, owing to 

 the development of the iris pigment, and the 

 eye becomes darker. In albinos the iris, in 

 common with the hair and skin, has no pig- 

 ment, which permits a reflection of the deeper 

 blood vessels in the interior of the eye to be 

 seen, causing such eyes to present a pink ap- 

 pearance. 



Within the choroid lies the inner layer, the 

 retina. This layer is formed by the special 

 nerve of sight, the optic nerve, which, after 

 leaving the brain, directs its course to the 

 back of the eyeball, which it penetrates, and 

 on reaching the level of the choroid spreads 

 / out on its surface as a nerve carpet or screen, 

 forming the retina. The point at which the 

 optic nerve enters the eyeball is known as the 

 blind spot (Mariotte's), so that it is a curious 

 fact that each eye contains a small area which 

 is blind. Another small area in the retina of 

 each eye, called the macula (macula lutea 

 yellow spot), is the area where vision is most 

 distinct. On the interior of the eyeball, im- 

 mediately behind the cornea and in front of 

 the iris and pupil, is a space called the ante- 

 rior chamber, which is filled with a liquid 



DIAGRAM FOR EXPERIMENT 

 The above illustration can be used to prove the 

 existence of a blind spot in the eye. Close the 

 left eye ; look squarely at the white cross. Move 

 the page varying distances from the eye ; at one 

 point the right side of the drawing will become 

 entirely black, the white spot disappearing. The 

 explanation is that the white spot has come into a 

 direct line with the blind spot. 



called the aqueous humor. Just back of the 

 iris is a double-convex, transparent, solid body, 

 the crystalline lens, which is the essential 

 structure in the function of accommodation, 

 and is also the structure in which the disease 

 known as cataract develops. Behind the lens 

 is a large cavity, the vitreous chamber, filled 



