SENSE ORGANS. 



157 



patch, which has a bright, iridescent metallic luster. It 

 is called the tapetum. It is a modification of the choroid 

 coat for the purpose of reflection of light. The use of 

 it is not well understood, but it is believed to double the 

 impression of feeble light by making it pass twice through 

 the retina. It is this that causes the shining of the eyes 

 in the dark, if a bright light is present. 



Fovea. — There is in all mammals a central area, which 

 is a little more sensitive ; but a true fovea, with its three 

 characteristics (explained on page 121), is not found in 

 any mammal below man, except the anthropoid apes. 



Birds. — The iris in birds is very various in color, 

 most commonly the normal brown, but sometimes yel- 

 low, as in birds of prey, sometimes scarlet-red (summer 

 duck), and sometimes porcelain-white (white-eyed vireo). 



Sclerotic Bones. — In all birds and many reptiles 

 we find a series of bony plates in the front part of the 



sclerotic and radiating 



sb from the margin of the 



iris. These are beveled 



on the margins, and fit 



Fig. 95. — Eye of an owl : on, optic 

 nerve ; c, cornea ; sb, sclerotic bones. 



Fig. 96. — Sclerotic bones sepa- 

 rated and viewed in perspective. 



together in such wise as to slide a little over one an- 

 other. By appropriate muscles these may be made to 

 squeeze the ball so as to adapt it to clear vision of very 

 near objects (Figs. 95 and 96). 



Nictitating Membrane.— Birds have in the inner 

 corner of the eye a fold of the conjunctiva which may 



