THE SPECIAL SENSES. 203 



which may enter it, in spite of the vigilance of the lids 

 and lashes. The act of winking, which is generally un- 

 consciously performed, and which takes place six or more 

 times in a minute, assists this passage of the tears across 

 the eye, and is especially frequent when the secretion is 

 most abundant. 



59. The Eyeball. The remarkable optical instru- 

 ment called the eyeball, or the globe of the eye, upon 

 which sight depends, is, as the name indicates, spherical in 

 shape. It is not a perfect sphere, since the front part pro- 

 jects somewhat beyond the rest, and at the posterior part 

 the optic nerve (Fig. 48, N) is united to it, resembling the 

 junction of the stem with a fruit. In its long diameter, 

 that is, the horizontal or from side to side, it measures a 

 little more than an inch; in other directions it is rather 

 less than an inch. In structure, the ball of the eye is firm, 

 and its tense round contour may in part be felt by pressing 

 the fingers over the closed lids. 



60. The eyeball is composed chiefly of three internal, 

 transparent media, called humors; and three investing 

 coats, or tunics. The former are the aqueous humor, Fig. 

 48, A, the crystalline lens L, and the vitreous humor v. 

 Of these the lens alone is solid. The three coats of the 

 eyeball are called the sclerotic s, the choroid CH, and the 

 retina R. This arrangement exists in respect to five-sixths 

 of the globe of the eye, but in the anterior one-sixth, these 

 coats are replaced by the cornea c, which is thin and trans- 

 parent, so that the rays of light pass freely through it, as 

 through a clear window-pane. 



61. In shape, the cornea is circular and prominent, 

 resembling a miniature watch-glass, about -fa of an inch 

 thick. In structure, it resembles horn (as the name signi- 

 fies), or the nail of the finger, and is destitute of blood- 



59. Describe the shape of the eyeball. Its structure. 



GO. Of what is the eyeball composed * State how. 



6 1 . The shape of the cornea ? Its structure ? The " white of the eye ?' 



