THE NERVOUS SYSTEM AND SPECIAL FUNCTIONS. 297 



with the number of vertebrae. The sympathetic system 

 is made up of a double cord, running on each side of the 

 vertebrae lengthwise. 



Sight. 



The sense of sight is amazingly developed in birds, 

 seemingly far beyond the power of vision in man, unless 

 it is aided by some faculty which does not come to the 

 aid of the sense in man. The function of accommoda- 

 tion is marvelously developed. You have seen a hawk 

 swoop down with a rush, and yet not fail to reach the 

 prey sought. Such rapidity of change in the function of 

 accommodation in the human eye would be a serious tax. 

 Humming birds fly with more rapidity of motion than 

 the human eye can follow, yet they avoid obstacles and 

 find the flower. You and I have considerable difficulty 

 in locating angleworms, even with diligent digging, before 

 we start off fishing; but a robin will sit on the tree bough, 

 apparently looking about him, and all at once will dart 

 down and complacently pull up a fine, fat worm. The 

 bald eagle up in the blue sees an osprey fishing, and is 

 thereby spared the trouble of getting a meal for himself. 

 The osprey 's dash out of the depths of the same aerial 

 ocean to the water surface, just beneath which a fish is 

 swimming, is even more wonderful. 



Structurally, the eye consists of the corneal window, 

 the crystalline lens, the iris, the conjunctiva, and the cavity 

 filled by the aqueous humor. Two eyelids are present as in 

 man, and there is also a third, the nictitating membrane; 

 this third eyelid moves across the ball of the eye, at right 

 angles with the other lids, from the inner corner to the 

 outer. Owls, which do not see well in the daytime, sit 

 with this membrane across the eyeball. Threaten to hit 



