676 



THE EYE IN EVOLUTION 



ORBITAL/ 

 AXIS 



Fig. 810. — Diagram to Show the Relation between the Orbital Axis 

 AND Visual Axis in Man. 



estimated from the centre of the cornea. Unfortunately, however, this method 

 is also gravely at fault since the optic axis rarely coincides with the visual axis 

 — when the latter exists. Indeed, unless there is an area centralis of acute 

 vision through which an animal habitually orientates itself towards an object 

 and around which spatial orientation is centred, the whole concept of fixation 

 along a visual axis is meaningless ; only in those animals provided with an area 

 of acute vision is such a concept possible and in these the angle gamma between 

 the optic and fixation axes varies between 5° in man to 80° or 85° in some Fishes 

 or the rabbit with laterally placed eyes (Figs. 811 to 813). When, however, 



Figs. 811 and 812. — The Angle Gamma in Vertebrates. 



Fig. 811. — The small angle gamma of 

 the cat. 



Fig. 812. — The large angle gamma of 

 the rabbit. 



The angle y measures the deviation between the optic axis (O) and the fixation axis (F). 



