244 



THE HUMAN SPECIES 



images may be upon the individual retina ; we judge the 

 magnitude of objects and distances by the relation they bear 

 to the size of our own body. 



All travellers have been impressed by the fact that there 

 is no very great difference in the acuity of vision between 

 savage and civilised races ; the former have the advantage, 

 however, when it comes to seeing at a distance, or in the dark. 

 The retina is not equally sensitive in all parts. In all 

 Vertebrates there is a point in the retina where visual acuity is 

 greatest ; this is known as the yellow spot, a minute depression 



over which the nerve- 

 cells of the retina are 

 specially modified (m, 

 Fig. 122). 



Apes are almost 

 identical with men in 

 this detail ; certain 

 birds, however, are 

 exceptional in having 

 two such spots instead 

 of one (H. Miiller). 

 So that while for man 

 and all other Verte- 

 brates there are two 

 points in the field of 

 vision which are seen 

 most distinctly, in these 

 birds there are four such points, one for seeing with each eye 

 separately, and two for the two eyes acting together, which 

 coincide in binocular vision, so that actually there are three 

 points for all practical purposes. 



The position of the yellow spot enables those creatures the 

 axes of whose eyes are directed forwards to see most distinctly 

 what lies in front of them (birds, apes and men), but in all 

 other Vertebrates whose eyes are set laterally on the head this 

 does not hold good. 



In contradistinction to the yellow spot where vision is most 

 acute, there is in the eyes of all Vertebrates a " blind spot," cor- 

 responding to the optic papilla, or point of entrance of the optic 



FIG. 122. Ophthalmoscopic appearance of human 

 fundus. a, papilla of optic nerve ; c, artery ; 

 d, vein ; m, macula lutea (yellow spot) ; 

 t, temporal side ; n, nasal side ; b, choroidal 

 ring. 



