THE PERCEPTION OF SPACE 



693 



(with the exception of the occurrence of reflex corrective movements 

 for convergence in a few species) voluntary moveinents are incoordinated 

 in the sense that the eyes move independently of each other. In Mammals, 

 and in Mammals alone, the ocular mov^ements are coordinated in the 

 sense that the movements of both eyes are conjugated with a consider- 

 able degree of exactitude. In the former case the movements are 

 generally staccato and quick ; in the latter they tend to be deliberate. 

 In the former case neither eye moves symmetrically or synchronously 

 with its fellow for not only may the eyes move in different directions at 

 the same time, but one may move while the other remains fixed. In 

 the first case there is a complete decussation of the optic nerve fibres 

 at the chiasma (with the known exception of a few fibres in some 

 snakes) ^ and each retina is projected in its entirety onto different 

 hemispheres of the cerebral cortex ; in the latter case there is a partial 

 decussation at the chiasma and both retinae are projected onto each 

 hemisphere of the cortex. 



Among FISHES spontaneous ocular movements are relatively rare 

 and as a general rule the visual field is changed or a moving object is 

 followed by movements of the body while the eyes remain still. 

 Spontaneous movements occur, however, in several species of active 

 and lively pelagic fishes particularly those ^^'ith a fovea, ^ for in these 

 tj^es fine ocular movements are essential if an area specialized for visual 

 acuity is to be usefully employed for fixation. But in these fishes, 

 apart from temporary convergence of the temporal fovea upon prey in 

 some species, the eyes (and fovese) are used uniocularly, and even those 

 t}^es which have a temporal fovea quite frequently use it for uniocular 

 fixation as well as for convergence (the blenny, Blennius ; the sea- 

 bass, Serranus ; the Hawaiian wrasse, Julis ; the weever, Trachinus, 

 etc.) ; they are, indeed, the only Vertebrates which can employ a 

 temporal fovea uniocularly (Walls, 1942). Exceptionally sluggish 

 fishes such as the sea-horse, Hvppocatwpus, and the cling-fish, 

 LejMdogaster. show quick, darting and wholly dissociated movements 

 of the eyes resembling those of the chameleon ; while bottom-fishes 

 such as the flounders (Pleuronectidse) and the soles (Soleidse), when 

 they lie half-buried in the sand, explore the surrounding water by 

 independent movements of their pediculated eyes. Rochon-Duvigneaud 

 (1943) observed small independent movements of the eyes in tjrpes 

 such as the carp, Cyprinus, and the European wrasse, Labrus, as they 

 lay immobile on the bottom of an aquarium. 



A greater degree of movement is sometimes seen in atypical fishes 

 with eyes adapted for aerial vision. Thus in the Indian mullet, Mugil, 

 which swims along the surface feeding upon algae and caddis-fly larvae, 

 1 p. 392. 2 p. 309. 



Serranus 



Hippocampus 



Sole 



