THE PERCEPTION OF SPACE 675 



tendency may be said to be carried to its extreme in the tubular eyes 

 of some abyssal fishes provided with a lateral accessory retina to 

 overcome the marked deficiency in the field which would result from 

 the use of the main retina alone. ^ In addition, the anatomical 

 configuration of the orbits and skull is frequently modified to eliminate 

 as far as possible any obstruction to the vital frontal field, the most 

 dramatic instance of which is the deep groove running nasally in the 

 cheek of certain tree-snakes in wliich the eye is set so that it has an 



Fig. 809. — -The Goliath Heron, Ahuea goliatu. 



To show the deep groove in the skull and bill to allow the accurate 

 fixation of prey by the frontally directed eyes. 



uninterrupted view straight ahead (Fig. 807) or the groove in the 

 side of the bill of the heron so that it can see accurately to fixate its 

 prey (Fig. 809). 



The first to investigate the extent of the binocular field in the various 

 classes of Vertebrates was Johannes Miiller (1826) who measured the angles 

 between the planes of the orbital margins in 190 vertebrate types, making the 

 unjustified asstimption that the visual axis was perpendicular to this. These 

 measurements were repeated by Leuckart (1875) and Grossmann and Mayer- 

 hausen (1877) and their absurdity soon became obvious. Thus although there 

 is little difference between the optic and orbital axes in most Fishes, there is 

 more in the horse, more still in the cat, W'hile in man the optic axes are almost 

 parallel and the orbital axes diverge by 45° (Fig. 810). A similarly painstaking 

 and elaborate investigation w^as therefore carried out by the last authors who 

 measured the apparent divergence of the eyes as indicated by the optic axis 



1 p. 323, Fig. 380. 



