THE SPECIAL SENSES 291 



ated near the inner angle of the eye, called the nasal duct. 

 This duct is shown in Fig. 63, 2, and is connected with each 

 lid by delicate tubes, which are indicated by dotted lines in 

 the figure ; the asterisk marks the little opening in the lower 

 lid, by which the tears enter the nasal duct. By gently turn- 

 ing the inner part of that lid downward, and looking in a mir- 

 ror, this small " lachrymal point " may be seen in your own 

 eye. In old people, these points become turned outward, and 

 do not conduct the tears to the nasal cavity, thus causing an 

 overflow of tears upon the face 



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, as it thus secures the perfect 

 lubrication of the surface of the eye, and cleanses it from the 

 smaller particles of dust 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. 



59. The Eyeball. The eyeball, or 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. 64, N) is united to it, resembling the junction of 

 the stem with the fruit. In its long diameter that is, 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, trans- 

 parent media, called humors, and three investing coats, or 



68. The watery fluid passing over the eyeball ? Design of the arrangement ? Winking ? 



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

 60. Of what is the eyeball composed ? State how. 



