248 THE HUMAN SPECIES 



Rivers found the colour-sense very deficient in Papuans, al- 

 though it was exceptionally well developed in Eskimos, who 

 were very accurate in naming the colours. 



In all Vertebrates, including man, the eye is innervated, as 

 regards vision, from the optic nerve ; sensation is supplied by 

 the first division of the fifth nerve ; the motor supply to the 

 muscles is as follows : levator palpebrae superioris, superior- 

 internal and inferior recti and the inferior oblique from the 

 third (oculomotor) nerve, the superior oblique from the fourth 

 (trochlear) nerve, and the external rectus from the sixth (ab- 

 ducens) nerve. 



Other Vertebrates, excepting man and apes, have an ad- 

 ditional muscle, retractor bulbi, which receives a double inner- 

 vation from the oculomotor and abducens nerves. 



Probably the nuclei of the second (optic) and third (oculo- 

 motor) nerves are closely associated in the corpora quadrigemina, 

 for stimulation of the optic nerve by light entering the eye will 

 transmit an impulse to the oculomotor nerve in the corpora 

 quadrigemina. 1 



In those animals whose optic nerve only partially decussates 

 at the chiasma (man, ape, dog, cat) the two oculomotor nerves 

 must be connected in the brain by intercentral fibres, since 

 light falling upon one eye alone causes reflex contraction of 

 both pupils ; in animals whose optic nerve decussates com- 

 pletely the pupil reflex is confined to the eye illuminated 

 (ungulates, rabbits, owls). 



All Vertebrates are not capable of the same perfect move- 

 ments of the eyes as man and other mammals ; in birds, and 

 still more in reptiles, the movements are as a rule very feeble, 

 movement of the head taking their place. One notable excep- 

 tion is found among reptiles, namely, the chameleon, whose 

 eyes move quite independently of one another and can execute 

 the widest excursions in opposite directions. 



(c) Sense of Hearing. 



The same difficulty which besets the investigation of the 

 sense of smell in the lower animals confronts us when we try 

 to determine whether, and how much, they hear. Frequently 



1 Munk-Schultz, loc. cit., p. 497 et seq. 



