690 THE EYE IN EVOLUTION 



the essential purpose of which is compensatory in nature, tending to 

 maintain the visual field as far as possible in its normal orientation. 



(2) Voluntary movements made spontaneously for the purpose of 

 changing the visual field to allow the deliberate exploration of space. 

 While the involuntary movements tend to maintain constancy in the 

 visual field, voluntary movements are designed to achieve its variation. 



(3) Reflex corrective movements associated with fixation and 

 fusion. 



It is interesting that apart from retraction and elevation (move- 

 ments associated with the contact reflex of the cornea and with 

 swallowing^), no ocular movements have been seen in Anurans ; nor 

 have they in Crocodilians but these reptiles have received little 

 study in this respect ; while in many Birds the eyes are immobile 

 and even reflex involuntary movements are often largely undertaken 

 by the unusually flexible neck.^ 



INVOLUNTARY OCULAR MOVEMENTS. We havc already seen that 

 the primary function of vision is to control the movements of the 

 animal ; indeed, the primitive photokineses and phototaxes of the 

 lower Invertebrates survive in the fundamental postural reflexes of the 

 Vertebrates. The early aquatic Vertebrates (Cyclostomes, Fishes, 

 Urodeles and larval anuran Amphibians) were provided with an 

 elaborate system of lateral line organs attuned to respond to vibrations 

 in a watery medium associated with a labyrinth designed to subserve 

 a postural mechanism. When Vertebrates left the water for land the 

 lateral organs disappeared to be replaced by a new organ, the cochlea, 

 designed to respond to vibrations in the new medium (air), but the 

 labyrinth was still retained and was associated with proprioceptive 

 impulses from the neck and limbs. The stimuli from the lateral organs 

 and the labyrinths were carried to the tegmentum and the tectum where 

 they were associated with visual stimuli ; the stimuli from the more 

 lately developed organs took a similar course, and in the mid-brain an 

 important group of centres became aggregated controlling the reflexes 

 concerned with the acquirement and maintenance of posture and 

 associating them with the eyes.^ The mechanism involved is elaborate 

 and has been elucidated in a classical series of researches by the great 

 Dutch physiologist, Rudolf Magnus, and his associate, de Kleijn, whose 

 work was inspired by Sherrington's analysis of the phenomena of 

 decerebrate rigidity.^ To the basic concepts advanced by these workers 

 little fundamental has yet been added. 



The primary function of the ocular movements was therefore 



1 p. 345. 2 p_ 695_ 



3 See Figs. 712-.5. 



* See Sherrington, 1904^6 ; Magnus, 1924. 



