SENSE ORGANS. 1 Q l 



The difficulty of converging on a near point becomes 

 greater ; the common field of view is more restricted, 

 until in fishes the eyes are completely on the side of 

 the head ; the optic axes diverge one hundred and eighty 

 degrees ; convergence on a point is impossible ; each eye 

 has its own field of view, which do not overlap to make 

 a common field, and therefore they can not have binocu- 

 lar vision. 



All mammals (except perhaps whales) probably en- 

 joy binocular vision in various degrees of perfection. 

 Birds also probably are similarly endowed (although 

 their eyes are so widely divergent), but this is by virtue 

 of a peculiar structure, to be spoken of under the next 

 head. 



Fovea. — This is not only the most sensitive spot of 

 the retina, but it is the center about which the corre- 

 sponding points of the two retina are symmetrically 

 arranged. It is undoubtedly necessary for binocular 

 vision in its highest perfectio?i. Now, this pitlike spot is 

 found among mammals only in man and the anthropoid 

 apes. Mammals generally have indeed a central area 

 (which may become a tapetum), about the center of which 

 corresponding points are symmetrically arranged, but 

 no true fovea. It is probable that in them the advan- 

 tages of accurate observation of a single thing is sacri- 

 ficed to the much greater advantages of somewhat dis- 

 tinct vision over a wide field. 



In birds the fovea again appears, and yet their optic 

 axes are so widely divergent as to make it impossible to 

 converge these axes on a point (see Fig. 100). Never- 

 theless, birds seem to have binocular vision, but this is 

 by virtue of another fovea. In other words, among all 

 animals birds are peculiar in having two fovea in each eye, 

 one monocular and the other binocular. The monocular 

 ones, a a', are axial and are the more distinct; the binoc- 



