THE EYE 357 



The Orbit. The human eye is peculiarly well protected in its 

 bony socket. Nothing but a well-aimed thrust of a comparatively 

 sharp instrument can injure it. Ordinary blows are sure to be re- 

 ceived by the prominent cheek-bone below or the edge of the 

 frontal above. It is also protected from shocks by a cushion of 

 soft material. The cavity of the orbit is conical in shape, and 

 the space not occupied by the eye and its appendages is filled with 

 loose tissue containing fat. 



The Eyeball. The eyeball is in the shape of a sphere, with 

 the segment of a smaller sphere grafted upon it, making the diam- 

 eter from before backward a little greater than the lateral diam- 

 eter, which is about one inch. The eye is apparently set in a slit 

 in the skin of the face, but really this is not the case, for the skin 

 of the eyelids turns over their edge and becomes here a thin, trans- 

 parent, smooth, and exceedingly sensitive mucous membrane, the 

 conjunctiva, which lines the lids and extends over the front part 

 of the eyeball itself so that the eye is really behind the skin. 



The Sclerotic Coat. The eye is a globular receptacle filled 

 with transparent fluids. The walls of the receptacle are three in 

 number, closely attached to each other, and called respectively 

 the outer or first, the middle or second, and the inner or third 

 tunic. The outer tunic is in two parts, one opaque, the other 

 transparent. That part which covers about five-sixths of the 

 eye and is of a pearly white color is called the sclerotic coat. The 

 part of it which we can see is called the "white of the eye." It 

 is a very tough, dense membrane, rigid enough to give shape to 

 the eyeball, yet elastic and yielding to pressure. The muscles 

 of the eye are inserted into it; and it is perforated at the back 

 part for the entrance of the optic nerve. The existence of nerves 

 in it is doubtful, and its blood-vessels are few in number. The 

 veins we see in the white of the eye when it is " blood-shot" are 

 in the conjunctiva, which is so transparent that we do not see 

 it except when its vessels are filled to excess with blood. 



The Cornea. The cornea is perfectly transparent and fits into 

 the sclerotic coat like the crystal of a watch in its case. In out- 

 line it is almost circular. The conjunctiva covers it in front. 

 It contains numerous nerves but no blood-vessels. The cornea 



