CRANIAL NERVES AND SENSE-ORGANS 241 



among domestic animals the hunting hound stands first, for he 

 can scent the quarry at a distance of 1-2 kilometres. 



(b) Vision. 



In broad terms it may be said that those animals possess 

 the most useful eyes in whom the preservation of the individual 

 and species is most dependent on acuity of vision (Schleich). 

 " The further an animal is wont to roam, the more perfect 

 must be its orientation-in-space mechanism." The efficiency 

 of the visual organs is inseparably bound up with the very 

 existence of the animal. 



As we were able to trace out step by step the development 

 of the eye from a simple pigment-enclosing nerve-fibre up to 

 the complicated structure of the vertebrate eye, so the visual 

 sense itself may be traced from a simple power of distinguishing 

 light and darkness up to the highly differentiated vision of 

 Vertebrates and man. In medusa, echinoderms and turbellaria 

 we see the pigment-spot with its nerve acquire a lens as a light- 

 refracting medium. In rotifers and leeches the disc-like pig- 

 ment-spot is provided with a number of crystal cones and retinal 

 cells as light-receptors. 



In the higher Mollusca (Helicid) with a cornea, lens and 

 retina, Lubbock believes that vision amounts to no more than 

 distinguishing light from darkness. Important differences 

 exist between the compound and simple eyes of insects 

 and crustaceans. The former, according to Miiller, have a 

 structure which may be likened to a mosaic picture. The 

 simple eyes of Mollusca and the accessory eyes of insects and 

 crustaceans on the other hand correspond to the camera-like 

 eye of Vertebrates where the retina receives an inverted image 

 of the object ; with this difference though, that in the verte- 

 brate eye the optic nerve pierces the retina and is distributed 

 over its anterior aspect, so that the rods and cones are ar- 

 ranged on the posterior surface and the light is reflected by 

 these after passing through the retina, while in the simple eyes 

 of the lower animals (with the exception of certain species of 

 onchidium) the optic nerve expands over the posterior surface of 

 the retina and the pigment lies in front of the rods, not behind 

 them, as in the human eye. 



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