438 Animal Biology 



upper lid, the lower, which is fused with the third eyelid or nictitating 

 membrane (L. nictare, to beckon). The lens permits objects to be seen 

 at definite distances, especially moving objects. The pupil contracts and 

 regulates the amount of light which enters. The sensitive retina within 

 the eye is stimulated by light and transfers the impulses to the optic nerve 

 which carries them to the brain to give the sensation of sight. The eyes 

 lie in orbits (sockets) at the side of the skull and are moved by six eye 

 muscles known as external and internal recti, the superior and inferior 

 recti, and the superior and inferior oblique muscles. 



The Number, Name, Origin, Distribution, and Function of the Cranial 



Nerves of Vertebrates 



*The XI and XII pairs are not present in fishes and amphibia. 



The tympanic membrane of the outer ear communicates with the inner 

 ear by a bony columella which vibrates with the sound stimuli received 



