CRANIAL NERVES AND SENSE-ORGANS 



245 



nerve, whose unchanged fibres are insensitive to light, and 

 since the membranous expansion of these fibres forms the 

 innermost layer of the retina certain animals in whom the 

 fibres are less transparent find them a hindrance to distinct 

 vision. 



The extent of the visual field is determined entirely by the 

 position of the eye. In man and apes, with their eyes almost 

 entirely enclosed in a bony orbit and the almost parallel direc- 

 tion of the optic axes, the visual field is so limited that it does 

 not even include the whole of space lying in front of the eyes 

 and extends to none of that which lies behind. In the majority 

 of animals with their eyes directed to the front and sides there 

 is not the same restric- 

 tion of the field, which 

 is so extensive as to in- 

 clude objects lying be- 

 hind the observer ; but 

 this increase in the visual 

 field is obtained at the 

 expense of acuteness of 

 central vision. The per- 

 ception of movements, 

 however, is chiefly ap- 

 preciated at the periph- 

 ery of the retina, and 

 according to Berlin's ob- 

 servations the astigmat- 

 ism of animals' eyes tends to increase this faculty. It is essential 

 for certain creatures (fishes, beasts of prey, ruminants and horses, 

 etc.) to see in the dimmest light, and the tapetum serves to assist 

 admirably in achieving this; the tapetum (Fig. 123) consists 

 of a variegated pigmented layer of cells covering parts of the 

 periphery of the retina so constructed as to reflect on to the 

 retina the feeblest rays of light. 



As regards estimation of depth, man, and his nearest ally 

 the ape, stand far behind the higher Vertebrates, especially 

 the larger mammals. The wider separation of the eyes gives 

 these animals a better equipment for judging distances. In the 

 horse, for instance, the distance between the eyes is double that 



FIG. 123. Ophthalmoscopic appearance of fun- 

 dus of horse, a, optic papilla ; c, artery ; 

 d, vein ; b, choroidal ring ; T, tapetum. 



