114 



VERTEBRATE PHOTORECEPTORS 



found a fovea lateralis (Figure 79), and these forms exhibit 

 definite monocular vision. The absence of the fovea lateralis 

 in fresh-water fishes is apparently associated with a lack 

 of eye muscle movements. In forms higher than the fishes, 

 the fovea lateralis (temporalis), wherever it exists, presum- 

 ably involves binocular vision. Some snakes are described 

 as possessing a lateral fovea, and reference has already been 

 made to their presence in certain rapacious and other birds 

 (e.g. haw^ks, swallow, and tern). 



Fig. 79. Diagrams showing fovea lateralis in three salt-water fishes, a, Hip- 

 pocampus guttulatus; b, Serranus scriba; c, Coris julius. Fo, fovea lateralis. 

 (Photographed from Kahmann, 1936. v. Graefe's Arch. f. Ophth., Bd. 135) 



Whereas it is seen that foveal development and the ability 

 for marked movements of the eyes seem to go together, 

 conjugate movements, binocular vision, and partial decussa- 

 tion of the optic tracts are not necessarily implied. In this 

 connection Kahmann points out that the presence of a fovea 

 in so many animals, among which a binocular use of the eyes 

 is never made, shows us how much monocular vision pre- 

 dominates. 



