SENSE OF SIGHT. 217 



Ahe orbit, the eyebrows, the eyelashes, the eyelids, and the 

 lachrymal apparatus. The or&& is a four-sided vault, com- 

 posed of seven bones, and shaped like a pyramid or cone, 

 the apex pointing backwards. The eyeball and the orbit 

 seem exactly made for each other ; the one being globular, 

 the other rounded to receive it. At the bottom of the orbit 

 are holes for the passage of the blood-vessels and nerves. 

 Now this cavity is not scooped out of the skull, as a boy 

 would make a hole in a block of wood, but the seven bones, 

 which enter into its composition, are so curiously dove-tailed 

 together as just to leave this space for the reception of the 

 eye and its appendages. What mechanic could place 

 seven irregular, jagged bones together so as to form a smooth, 

 polished cavity, for such a delicate organ as the eye to roll 

 in ! So securely is the eye protected by its bony house or 

 encasement, that it is a very rare thing for it to be injured, 

 except by some sharp pointed object. 



42. The eye then is surrounded on every side with bone, 

 except the front ; now, what have we here to serve as a 

 means of defence ? It would not have answered our pur- 

 pose to have had it entirely shut up in a box of bones, or 

 even to have left a little hole just to peep through. As we 

 cannot draw our heads into a shell when danger threatens, 

 like a snail or turtle, we are furnished with a couple of 

 moveable curtains or eyelids, placed before the eyes, which 

 we can let down or raise up just when we please. If the 

 eyes are " the windows of the soul," then I would call the 

 lids a pair of inimitable window shutters, for they exclude 

 the light whenever its presence would be disagreeable to us. 



43. The eyelids are composed of a thin skin, externally ; 

 internally of cellular membrane, its cells filled with a soft 

 semi-transparent fluid ; while their inner surface, in contact 

 with the eye, is lined with a delicate mucous membrane, 

 continuous with the skin, called conjunctiva) Besides these 

 there are beneath the skin, two sets of muscular fibres ; one 

 called orbicular t running round the lid horizontally, whose 



19 



