126 THE STRUCTURE AND LIFE OF BIRDS cuap. 
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 Grenacher’s Sehorgan der Arthropoden. Sir John 
Lubbock, by an oversight, has stated in his Sezses of Animals 
that the pigment lies in front of the sensitive cells (retina) in 
the eyes of vertebrates. This of course cannot be so. 
