692 THE EYE IN EVOLUTION 



are found throughout the entire vertebrate phylum in all animals 

 wherein ocular movements occur, so that in postural attitudes the 

 eyes, so far as is possible, maintain the same position in space, while 

 the head revolves around them (Figs. 840 to 842). It is important to 

 remember that the static postural reflexes are not associated with 

 vision and for this reason they occur in the blind and in the decerebrate 

 animal. 



These ocular movements are precisely correlated with the bodily movements ; 

 their regularity is seen when their excursion is plotted diagrammatically in the 

 form of a graph. Fig. 843 illustrates such a graph taken from Benjamins (1920) 

 on fish : positive values indicate deviations of the anterior pole of the cornea 



O 41 90 lis ISO 225 270 325 360 



Fig. 843. — Rotation or the Eye of the Perch about the Visual Axis 

 IN Response to Rotation of the Body about a Transverse Axis. 



Positive values indicate deviations of the anterior pole of the eye 

 towards the belly ; negative towards the back. 



The ordinates indicate the angle through which the anterior pole of the eye 

 has rotated. The abscissiB indicate the position of animal in degrees 

 (Benjamins). 



towards the belly, negative values towards the back ; the ordinates represent 

 the angle through which the eye has rotated. 



The ocular movements take place with extreme rapidity, their latent 

 periods being measured in milliseconds in contrast to the slow postural adjust- 

 ments of the head and limbs, de Kleijn established that rabbits can compensate 

 head movements of 100° about a bitemporal axis with ocular movements of a 

 rapidity of this order. In the pigeon comi^ensation is perfect only up to move- 

 ments of 10° and the eyes move back either by a slow drift or a quick flip so that 

 during flight accurate fixation of the next landing point is rapidly possible 

 (Whitteridge, 1956). In man compensation is complete during head movements 

 of up to 30° and it is interesting that during the excursion visual acuity is not 

 affected (Merton, 1956). 



VOLUNTARY OCULAR MOVEMENTS. Voluntary movements of the 

 eyes are a later evolutionary development adopted in the interests of 

 vision as a perceptual process. Within the vertebrate phylum an 

 important evolutionary step is seen. In all Vertebrates below Mammals 



