246 THE EYE. 



retina, which is the expansion of the optic or 

 seeing nerve. This nerve passes through the 

 coats of the eye, and immediately divides itself 

 into a half circular net work, covering and lining 

 the whole of the inner surface of its back part. 



It is from the retina that we receive impressions 

 of light, and see the objects around us. 



The ball of the eye is of a roundish shape, and 

 furnished with six muscles, by means of which it 

 can be turned in every direction. 



This delicate and curious organ, the eye, is very 

 carefully protected. It is placed in a bony cavity 

 called the orbit, and provided with two moveable 

 outside curtains, known under the name of eye- 

 lids. These guard it from dust, keep the front 

 bright and clear, and spread the tears over the 

 whole surface of the eyeball, so that it may be 

 always moist, and easily moveable. In these 

 offices the eyelids are assisted by the eyelashes 

 and the eyebrows. 



To enable us to close our eyes when we go to 

 sleep, or when we are pained by an excess of 

 light, the lids are provided with muscles, and can 

 perform very rapid motions. 



Let us now consider how admirably our eyes 

 are fitted for vision. It is light which renders 

 things visible to us, for we cannot see in a dark 

 room, or in a very dark night. Now what we 

 call light is a succession of rays, proceeding from 

 any luminous body, which rays, after striking 

 upon objects, are reflected, or thrown back. When 



