126 THE STRUCTURE AND LIFE OF BIRDS chap. 



be the only light in the room. Get some one to hold 

 it by the side of your eye, and with the help 

 of a lens to focus the rays upon it. Look at the wall, 

 which must be uniformly coloured. The shadows of 

 the blood vessels which ramify in the retina in front of 

 the rods and cones will be distinctly visible. Not only 

 is it the hindmost layer on which light makes itself 

 felt, but the rods and cones look backward. In the 

 invertebrate eye the retina looks forward, and its front 

 surface is the sensitive one. In this important point 

 the pineal body is the eye of an invertebrate. 1 Behind 

 the retina is a deep layer of dark pigment, called the 

 Choroid (CH) ; in this the rays after passing the sensitive 

 cells are absorbed. Were they reflected from one part 

 of the retina to another, any clearness of vision would 

 be impossible. But it would be rash to say that the 

 pigment exists for the sole purpose of preventing 

 reflection. This coloured layer is continued in front, and 

 forms the round curtain called the Iris, and, besides this, 

 where the sclerotic or white of the eye passes into the 

 transparent cornea, it sends out a number of muscular 

 frills, which lie behind the iris and which are separate 

 from it except that they spring from the same point ; 

 for the Iris, like these frills or, as they are called, ciliary 

 processes (CP), arises, as I have said, from the choroid 

 and is attached to the sclerotic at its margin close to 

 the cornea. It is these ciliary processes, consisting of 

 striated or voluntary muscle, which enable the eye to 



1 See Grenadier's Sehorgan der Arthropoden. Sir John 

 Lubbock, by an oversight, has stated in his Senses of Animals 

 that the pigment lies in front of the sensitive cells (retina) in 

 the eyes of vertebrates. This of course cannot be so. 



