THE SENSE-ORGANS 393 



following the same path constrict the pupil (contract the 

 sphincter and relax the radial muscles of the iris). The pupil 

 is dilated by impulses in fibres coming from the sympathetic 

 system of the neck. 



In the lower vertebrates, the eyes are on each side of the 

 head, and there is little, if any, overlap in the two fields of 

 vision. In these forms, the decussation or crossing- over of 

 the fibres at the optic chiasma is complete : the fibres from 

 an eye run to the opposite side of the brain. In the higher 

 vertebrates, on the other hand, it is common for the fields of 

 vision of the two eyes to overlap considerably, and even to 

 coincide. In these cases both eyes can be brought to bear 

 on a single object, which enables the animal to estimate 

 distance. This is of importance in arboreal animals which 

 have to gauge the strength of their efforts in leaping from branch 

 to branch. This binocular vision is present in the monkeys 

 and man, in the owls, and to a varying extent in other animals. 



The possession of binocular vision is a great advantage, 

 but it robs the animal of vision over a large radius around it, 

 which it would have if its eyes diverged widely on each side 

 of the head. It is found as a rule that the more timid mammals 

 have widely divergent axes of vision, amounting to nearly two 

 right angles in the case of the rabbit. The rabbit therefore 

 can see objects almost everywhere all round it ; it uses its 

 eyes qualitatively to warn it of the approach of enemies. The 

 axes of vision of the lion, on the other hand, are almost parallel ; 

 it sacrifices a large field of vision for the advantage of using its 

 eyes quantitatively in estimating distance and spatial relations. 



In mammals with binocular vision, it is important that the 

 movements of the two eyes should be co-ordinated so that their 

 axes of vision remain more or less parallel with one another. 

 In other animals each eye can be moved separately, and this 

 faculty is extremely developed in Chamaeleo. 



The fibres from the eyes of mammals such as the rabbit 

 decussate almost completely at the optic chiasma. In the 

 monkeys and man, on the other hand, the decussation of the 

 fibres is incomplete. Fibres from the lateral portion of the 

 retina of each eye do not cross-over, but go to the same side 



