320 THE AXIHAL KINGDOM. 



membrane full of blood-vessels called the " choroid," this on 

 its inner side is covered with a black substance called the 

 " black pigment," and within this (lining the ball) is the 

 nervous coat or retina, it is the optic nerve so expanded as 

 to form a lining to the membranes of the ball of the eye for 

 the greater part of its extent. The ball of the eye is filled 

 with a transparent fluid, somewhat glutinous, called the 

 "vitreous humour;" stretched across the front part of the 

 ball of the eye is a membrane having a round hole in its 

 centre, called the ''iris," coated (like the choroid) with 

 black pigment on its back surface ; this membrane (the iris) 

 is the coloured part of the eye, and its transparency or 

 opacity determines the colour of the eye. The pupil is the 

 hole in its centre, it looks black because the blaok pigment 

 which covers the inside of the ball of the eye (for the retina 

 is transparent) is seen through it ; behind the iris and in 

 front of the vitreous humour, is the crystalline lens, it is 

 just the shape of a very thick lens, but is more convex behind 

 than in front, it is quite transparent, and of a density almost 

 approaching to solidity. The front fifth of the ball of the 

 eye is covered in by a dense transparent substance called 

 the cornea, of the form of a watch-glass and quite trans- 

 parent, between this and the crystalline lens there is a 

 small space called the anterior chamber, filled with a 

 clear watery fluid called the aqueous humour. The eye is 

 moved by six muscles, four straight and two oblique, these 

 turn it in every direction with the greatest accuracy ; the 

 front of the eye is defended by the eye-lids, which by wink- 

 ing and being moistened by the tears, keep the eye always 

 moist and free from dust. 



The eyes of insects and several other classes of animals 

 are called compound eyes, and the image they produce must 

 resemble objects depicted in mosaic, or made up of small 

 spots of distinct colours ; they consist of hundreds of small 

 tubes, radiating and forming part of a sphere on each side 

 of the head, each of these tubes admits the light from objects 

 at which they point. Some of the animals still lower in the 

 scale have simple spots of transparent membrane with an 

 expansion of optic nerve beneath them ; in such creatures 



