VISUAL ANGLES AND FIELDS 291 



vision within it will be disaxssed in detail farther on. Suffice it to say at 

 this point that two eyes are better than one, and that vertebrates in gen- 

 eral have seemingly striven to enlarge their binocular fields at the ex- 

 pense of their uniocular ones (uniocular being used here to denote the 

 part of a monocular field which is not overlapped by that of the other 

 eye). Animals which have clung to strong laterality have done so in 

 obedience to powerful factors, such as defenselessness (e.g., rabbits) or 

 total absence of cover in the environment (e.g., pelagic fishes), which 

 make the retention of periscopy vitally important. The various degrees 

 of partial frontality are compromises between the urge for binocularity 

 and the need for periscopy. 



In most groups of vertebrates the predaceous habit is a very common 

 specialization; so, the associated tendency toward frontality is likewise 

 common. Remembering that the visual field of a single eye is roughly 

 constant at 170 or so, we may consider the angular width of the binoc- 

 ular field to be quite directly related to the angle between the two optic 

 axes, which in itself will depend upon the position of the eyes in the head. 



Extent of the Binocular Field — There are very few vertebrates in- 

 deed which are known for certain to have no binocular field whatever. 

 The lampreys, the hammerhead sharks and a few large-headed teleosts, 

 such chunky amphibians as Cryptobranchus, the penguins of the genus 

 Spheniscus, and the larger whales constitute these exceptions. In some 

 other animals, as the chameleons and probably some fishes, there is no 

 binocularity when the eyes are at rest but it can be created by convergent 

 eye movements. Wherever the eyes are mobile, there exists the theoretical 

 possibihty of widening the binocular field by convergence of the optic 

 axes; but as we shall see, this possibility has been realized only in forms 

 which have developed an area centralis with or without a fovea, for only 

 such forms have any ability to move the eyes at will. 



The extents of the static binocular and uniocular fields have been 

 estimated for many animals by different means at various times. Over 

 a century ago, the positions of the eyes of a great number of vertebrates 

 were judged by Johannes Miiller from the angle between the planes of 

 the two orbital rims. Miiller assumed the optic axes to be perpendicular 

 to these planes. In 1877, Grossman and Mayerhausen also published a 

 long list of figures, based upon the divergence of the axes of the two 

 corneae. In modern times these patient researches have had to be dis- 

 carded, for the optic axis is neither normal to the plane of the orbit 



