C, The Cornea. A, The Aqueous Humor. I, The Iris. P, The Pupil. L, The Crystalline Lens. H, The Ligament of the Lens. B, The Ciliary Process. V, The Cavity containing the Vitreous Humor. S, The Sclerotic. Ch, The Choroid. R, The Retina. N, The Optic Nerve. DD, The Eyelids. X, The Levator Muscle of the Upper Lid. Y, The Upper Straight Muscle of the Eye. Z, The Lower Straight Muscle.] [Sidenote: 58. The watery fluid passing over the eyeball? Design of the arrangement? Winking?] 58. Thus we observe that the gland which forms the tears is placed at the outer part of the eye, while their means of exit is at the inner angle of the eye; which fact renders it necessary that this watery fluid shall pass over the surface of the eyeball before it can escape. This arrangement cannot be accidental, but evinces design, for it thus secures the perfect lubrication of the surface of the eye, and cleanses it from the smaller particles of dust {203} which may enter it, in spite of the vigilance of the lids and lashes. The act of winking, which is generally unconsciously 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. [Sidenote: 59. Describe the shape of the eyeball. Its structure.] 59. THE EYEBALL.--The remarkable optical instrument 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 projects 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. [Sidenote: 60. Of what is the eyeball composed? State how.] 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 transparent, so that the rays of light pass freely through it, as through a clear window-pane. [Sidenote: 61. The shape of the cornea? Its structure? The "white of the eye?"]